Warn-by-default Lints
These lints are all set to the 'warn' level by default.
anonymous_parameters
array_into_iter
asm_sub_register
bad_asm_style
bare_trait_objects
bindings_with_variant_name
break_with_label_and_loop
clashing_extern_declarations
coherence_leak_check
confusable_idents
const_evaluatable_unchecked
const_item_mutation
dead_code
deprecated
deprecated_where_clause_location
deref_into_dyn_supertrait
deref_nullptr
drop_bounds
duplicate_macro_attributes
dyn_drop
ellipsis_inclusive_range_patterns
exported_private_dependencies
for_loops_over_fallibles
forbidden_lint_groups
function_item_references
illegal_floating_point_literal_pattern
improper_ctypes
improper_ctypes_definitions
incomplete_features
indirect_structural_match
inline_no_sanitize
invalid_doc_attributes
invalid_value
irrefutable_let_patterns
large_assignments
late_bound_lifetime_arguments
legacy_derive_helpers
mixed_script_confusables
named_arguments_used_positionally
no_mangle_generic_items
non_camel_case_types
non_fmt_panics
non_shorthand_field_patterns
non_snake_case
non_upper_case_globals
nontrivial_structural_match
opaque_hidden_inferred_bound
overlapping_range_endpoints
path_statements
private_in_public
redundant_semicolons
renamed_and_removed_lints
repr_transparent_external_private_fields
semicolon_in_expressions_from_macros
special_module_name
stable_features
suspicious_auto_trait_impls
temporary_cstring_as_ptr
trivial_bounds
type_alias_bounds
tyvar_behind_raw_pointer
uncommon_codepoints
unconditional_recursion
undefined_naked_function_abi
unexpected_cfgs
unfulfilled_lint_expectations
uninhabited_static
unknown_lints
unnameable_test_items
unreachable_code
unreachable_patterns
unstable_name_collisions
unstable_syntax_pre_expansion
unsupported_calling_conventions
unused_allocation
unused_assignments
unused_attributes
unused_braces
unused_comparisons
unused_doc_comments
unused_features
unused_imports
unused_labels
unused_macros
unused_must_use
unused_mut
unused_parens
unused_unsafe
unused_variables
warnings
where_clauses_object_safety
while_true
anonymous-parameters
The anonymous_parameters
lint detects anonymous parameters in trait
definitions.
Example
#![deny(anonymous_parameters)] // edition 2015 pub trait Foo { fn foo(usize); } fn main() {}
This will produce:
error: anonymous parameters are deprecated and will be removed in the next edition
--> lint_example.rs:4:12
|
4 | fn foo(usize);
| ^^^^^ help: try naming the parameter or explicitly ignoring it: `_: usize`
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(anonymous_parameters)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
This syntax is mostly a historical accident, and can be worked around
quite easily by adding an _
pattern or a descriptive identifier:
#![allow(unused)] fn main() { trait Foo { fn foo(_: usize); } }
This syntax is now a hard error in the 2018 edition. In the 2015
edition, this lint is "warn" by default. This lint
enables the cargo fix
tool with the --edition
flag to
automatically transition old code from the 2015 edition to 2018. The
tool will run this lint and automatically apply the
suggested fix from the compiler (which is to add _
to each
parameter). This provides a completely automated way to update old
code for a new edition. See issue #41686 for more details.
array-into-iter
The array_into_iter
lint detects calling into_iter
on arrays.
Example
#![allow(unused)] fn main() { #![allow(unused)] [1, 2, 3].into_iter().for_each(|n| { *n; }); }
This will produce:
warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
--> lint_example.rs:3:11
|
3 | [1, 2, 3].into_iter().for_each(|n| { *n; });
| ^^^^^^^^^
|
= warning: this changes meaning in Rust 2021
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
= note: `#[warn(array_into_iter)]` on by default
help: use `.iter()` instead of `.into_iter()` to avoid ambiguity
|
3 | [1, 2, 3].iter().for_each(|n| { *n; });
| ~~~~
help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
|
3 | IntoIterator::into_iter([1, 2, 3]).for_each(|n| { *n; });
| ++++++++++++++++++++++++ ~
Explanation
Since Rust 1.53, arrays implement IntoIterator
. However, to avoid
breakage, array.into_iter()
in Rust 2015 and 2018 code will still
behave as (&array).into_iter()
, returning an iterator over
references, just like in Rust 1.52 and earlier.
This only applies to the method call syntax array.into_iter()
, not to
any other syntax such as for _ in array
or IntoIterator::into_iter(array)
.
asm-sub-register
The asm_sub_register
lint detects using only a subset of a register
for inline asm inputs.
Example
#[cfg(target_arch="x86_64")]
use std::arch::asm;
fn main() {
#[cfg(target_arch="x86_64")]
unsafe {
asm!("mov {0}, {0}", in(reg) 0i16);
}
}
This will produce:
warning: formatting may not be suitable for sub-register argument
--> src/main.rs:7:19
|
7 | asm!("mov {0}, {0}", in(reg) 0i16);
| ^^^ ^^^ ---- for this argument
|
= note: `#[warn(asm_sub_register)]` on by default
= help: use the `x` modifier to have the register formatted as `ax`
= help: or use the `r` modifier to keep the default formatting of `rax`
Explanation
Registers on some architectures can use different names to refer to a subset of the register. By default, the compiler will use the name for the full register size. To explicitly use a subset of the register, you can override the default by using a modifier on the template string operand to specify when subregister to use. This lint is issued if you pass in a value with a smaller data type than the default register size, to alert you of possibly using the incorrect width. To fix this, add the suggested modifier to the template, or cast the value to the correct size.
See register template modifiers in the reference for more details.
bad-asm-style
The bad_asm_style
lint detects the use of the .intel_syntax
and
.att_syntax
directives.
Example
#[cfg(target_arch="x86_64")]
use std::arch::asm;
fn main() {
#[cfg(target_arch="x86_64")]
unsafe {
asm!(
".att_syntax",
"movq %{0}, %{0}", in(reg) 0usize
);
}
}
This will produce:
warning: avoid using `.att_syntax`, prefer using `options(att_syntax)` instead
--> src/main.rs:8:14
|
8 | ".att_syntax",
| ^^^^^^^^^^^
|
= note: `#[warn(bad_asm_style)]` on by default
Explanation
On x86, asm!
uses the intel assembly syntax by default. While this
can be switched using assembler directives like .att_syntax
, using the
att_syntax
option is recommended instead because it will also properly
prefix register placeholders with %
as required by AT&T syntax.
bare-trait-objects
The bare_trait_objects
lint suggests using dyn Trait
for trait
objects.
Example
#![allow(unused)] fn main() { trait Trait { } fn takes_trait_object(_: Box<Trait>) { } }
This will produce:
warning: trait objects without an explicit `dyn` are deprecated
--> lint_example.rs:4:30
|
4 | fn takes_trait_object(_: Box<Trait>) {
| ^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
= note: `#[warn(bare_trait_objects)]` on by default
help: use `dyn`
|
4 | fn takes_trait_object(_: Box<dyn Trait>) {
| +++
Explanation
Without the dyn
indicator, it can be ambiguous or confusing when
reading code as to whether or not you are looking at a trait object.
The dyn
keyword makes it explicit, and adds a symmetry to contrast
with impl Trait
.
bindings-with-variant-name
The bindings_with_variant_name
lint detects pattern bindings with
the same name as one of the matched variants.
Example
#![allow(unused)] fn main() { pub enum Enum { Foo, Bar, } pub fn foo(x: Enum) { match x { Foo => {} Bar => {} } } }
This will produce:
warning[E0170]: pattern binding `Foo` is named the same as one of the variants of the type `Enum`
--> lint_example.rs:9:9
|
9 | Foo => {}
| ^^^ help: to match on the variant, qualify the path: `Enum::Foo`
|
= note: `#[warn(bindings_with_variant_name)]` on by default
Explanation
It is usually a mistake to specify an enum variant name as an
identifier pattern. In the example above, the match
arms are
specifying a variable name to bind the value of x
to. The second arm
is ignored because the first one matches all values. The likely
intent is that the arm was intended to match on the enum variant.
Two possible solutions are:
- Specify the enum variant using a path pattern, such as
Enum::Foo
. - Bring the enum variants into local scope, such as adding
use Enum::*;
to the beginning of thefoo
function in the example above.
break-with-label-and-loop
The break_with_label_and_loop
lint detects labeled break
expressions with
an unlabeled loop as their value expression.
Example
#![allow(unused)] fn main() { 'label: loop { break 'label loop { break 42; }; }; }
This will produce:
warning: this labeled break expression is easy to confuse with an unlabeled break with a labeled value expression
--> lint_example.rs:3:5
|
3 | break 'label loop { break 42; };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(break_with_label_and_loop)]` on by default
help: wrap this expression in parentheses
|
3 | break 'label (loop { break 42; });
| + +
Explanation
In Rust, loops can have a label, and break
expressions can refer to that label to
break out of specific loops (and not necessarily the innermost one). break
expressions
can also carry a value expression, which can be another loop. A labeled break
with an
unlabeled loop as its value expression is easy to confuse with an unlabeled break with
a labeled loop and is thus discouraged (but allowed for compatibility); use parentheses
around the loop expression to silence this warning. Unlabeled break
expressions with
labeled loops yield a hard error, which can also be silenced by wrapping the expression
in parentheses.
clashing-extern-declarations
The clashing_extern_declarations
lint detects when an extern fn
has been declared with the same name but different types.
Example
#![allow(unused)] fn main() { mod m { extern "C" { fn foo(); } } extern "C" { fn foo(_: u32); } }
This will produce:
warning: `foo` redeclared with a different signature
--> lint_example.rs:9:5
|
4 | fn foo();
| --------- `foo` previously declared here
...
9 | fn foo(_: u32);
| ^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
|
= note: expected `unsafe extern "C" fn()`
found `unsafe extern "C" fn(u32)`
= note: `#[warn(clashing_extern_declarations)]` on by default
Explanation
Because two symbols of the same name cannot be resolved to two
different functions at link time, and one function cannot possibly
have two types, a clashing extern declaration is almost certainly a
mistake. Check to make sure that the extern
definitions are correct
and equivalent, and possibly consider unifying them in one location.
This lint does not run between crates because a project may have
dependencies which both rely on the same extern function, but declare
it in a different (but valid) way. For example, they may both declare
an opaque type for one or more of the arguments (which would end up
distinct types), or use types that are valid conversions in the
language the extern fn
is defined in. In these cases, the compiler
can't say that the clashing declaration is incorrect.
coherence-leak-check
The coherence_leak_check
lint detects conflicting implementations of
a trait that are only distinguished by the old leak-check code.
Example
#![allow(unused)] fn main() { trait SomeTrait { } impl SomeTrait for for<'a> fn(&'a u8) { } impl<'a> SomeTrait for fn(&'a u8) { } }
This will produce:
warning: conflicting implementations of trait `main::SomeTrait` for type `for<'a> fn(&'a u8)`
--> lint_example.rs:4:1
|
3 | impl SomeTrait for for<'a> fn(&'a u8) { }
| ------------------------------------- first implementation here
4 | impl<'a> SomeTrait for fn(&'a u8) { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a> fn(&'a u8)`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
= note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
= note: `#[warn(coherence_leak_check)]` on by default
Explanation
In the past, the compiler would accept trait implementations for identical functions that differed only in where the lifetime binder appeared. Due to a change in the borrow checker implementation to fix several bugs, this is no longer allowed. However, since this affects existing code, this is a future-incompatible lint to transition this to a hard error in the future.
Code relying on this pattern should introduce "newtypes",
like struct Foo(for<'a> fn(&'a u8))
.
See issue #56105 for more details.
confusable-idents
The confusable_idents
lint detects visually confusable pairs between
identifiers.
Example
#![allow(unused)] fn main() { // Latin Capital Letter E With Caron pub const Ě: i32 = 1; // Latin Capital Letter E With Breve pub const Ĕ: i32 = 2; }
This will produce:
warning: identifier pair considered confusable between `Ě` and `Ĕ`
--> lint_example.rs:5:11
|
3 | pub const Ě: i32 = 1;
| - this is where the previous identifier occurred
4 | // Latin Capital Letter E With Breve
5 | pub const Ĕ: i32 = 2;
| ^
|
= note: `#[warn(confusable_idents)]` on by default
Explanation
This lint warns when different identifiers may appear visually similar, which can cause confusion.
The confusable detection algorithm is based on Unicode® Technical
Standard #39 Unicode Security Mechanisms Section 4 Confusable
Detection. For every distinct identifier X execute
the function skeleton(X)
. If there exist two distinct identifiers X
and Y in the same crate where skeleton(X) = skeleton(Y)
report it.
The compiler uses the same mechanism to check if an identifier is too
similar to a keyword.
Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
const-evaluatable-unchecked
The const_evaluatable_unchecked
lint detects a generic constant used
in a type.
Example
#![allow(unused)] fn main() { const fn foo<T>() -> usize { if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T 4 } else { 8 } } fn test<T>() { let _ = [0; foo::<T>()]; } }
This will produce:
warning: cannot use constants which depend on generic parameters in types
--> lint_example.rs:11:17
|
11 | let _ = [0; foo::<T>()];
| ^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
= note: `#[warn(const_evaluatable_unchecked)]` on by default
Explanation
In the 1.43 release, some uses of generic parameters in array repeat expressions were accidentally allowed. This is a future-incompatible lint to transition this to a hard error in the future. See issue #76200 for a more detailed description and possible fixes.
const-item-mutation
The const_item_mutation
lint detects attempts to mutate a const
item.
Example
const FOO: [i32; 1] = [0]; fn main() { FOO[0] = 1; // This will print "[0]". println!("{:?}", FOO); }
This will produce:
warning: attempting to modify a `const` item
--> lint_example.rs:4:5
|
4 | FOO[0] = 1;
| ^^^^^^^^^^
|
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> lint_example.rs:1:1
|
1 | const FOO: [i32; 1] = [0];
| ^^^^^^^^^^^^^^^^^^^
= note: `#[warn(const_item_mutation)]` on by default
Explanation
Trying to directly mutate a const
item is almost always a mistake.
What is happening in the example above is that a temporary copy of the
const
is mutated, but the original const
is not. Each time you
refer to the const
by name (such as FOO
in the example above), a
separate copy of the value is inlined at that location.
This lint checks for writing directly to a field (FOO.field = some_value
) or array entry (FOO[0] = val
), or taking a mutable
reference to the const item (&mut FOO
), including through an
autoderef (FOO.some_mut_self_method()
).
There are various alternatives depending on what you are trying to accomplish:
- First, always reconsider using mutable globals, as they can be difficult to use correctly, and can make the code more difficult to use or understand.
- If you are trying to perform a one-time initialization of a global:
- If the value can be computed at compile-time, consider using const-compatible values (see Constant Evaluation).
- For more complex single-initialization cases, consider using a
third-party crate, such as
lazy_static
oronce_cell
. - If you are using the nightly channel, consider the new
lazy
module in the standard library.
- If you truly need a mutable global, consider using a
static
, which has a variety of options:- Simple data types can be directly defined and mutated with an
atomic
type. - More complex types can be placed in a synchronization primitive
like a
Mutex
, which can be initialized with one of the options listed above. - A mutable
static
is a low-level primitive, requiring unsafe. Typically This should be avoided in preference of something higher-level like one of the above.
- Simple data types can be directly defined and mutated with an
dead-code
The dead_code
lint detects unused, unexported items.
Example
#![allow(unused)] fn main() { fn foo() {} }
This will produce:
warning: function `foo` is never used
--> lint_example.rs:2:4
|
2 | fn foo() {}
| ^^^
|
= note: `#[warn(dead_code)]` on by default
Explanation
Dead code may signal a mistake or unfinished code. To silence the
warning for individual items, prefix the name with an underscore such
as _foo
. If it was intended to expose the item outside of the crate,
consider adding a visibility modifier like pub
. Otherwise consider
removing the unused code.
deprecated
The deprecated
lint detects use of deprecated items.
Example
#![allow(unused)] fn main() { #[deprecated] fn foo() {} fn bar() { foo(); } }
This will produce:
warning: use of deprecated function `main::foo`
--> lint_example.rs:6:5
|
6 | foo();
| ^^^
|
= note: `#[warn(deprecated)]` on by default
Explanation
Items may be marked "deprecated" with the deprecated
attribute to
indicate that they should no longer be used. Usually the attribute
should include a note on what to use instead, or check the
documentation.
deprecated-where-clause-location
The deprecated_where_clause_location
lint detects when a where clause in front of the equals
in an associated type.
Example
#![allow(unused)] fn main() { trait Trait { type Assoc<'a> where Self: 'a; } impl Trait for () { type Assoc<'a> where Self: 'a = (); } }
This will produce:
warning: where clause not allowed here
--> lint_example.rs:7:18
|
7 | type Assoc<'a> where Self: 'a = ();
| ^^^^^^^^^^^^^^
|
= note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
= note: `#[warn(deprecated_where_clause_location)]` on by default
help: move it to the end of the type declaration
|
7 - type Assoc<'a> where Self: 'a = ();
7 + type Assoc<'a> = () where Self: 'a;
|
Explanation
The preferred location for where clauses on associated types in impls is after the type. However, for most of generic associated types development, it was only accepted before the equals. To provide a transition period and further evaluate this change, both are currently accepted. At some point in the future, this may be disallowed at an edition boundary; but, that is undecided currently.
deref-into-dyn-supertrait
The deref_into_dyn_supertrait
lint is output whenever there is a use of the
Deref
implementation with a dyn SuperTrait
type as Output
.
These implementations will become shadowed when the trait_upcasting
feature is stabilized.
The deref
functions will no longer be called implicitly, so there might be behavior change.
Example
#![allow(unused)] #![deny(deref_into_dyn_supertrait)] #![allow(dead_code)] fn main() { use core::ops::Deref; trait A {} trait B: A {} impl<'a> Deref for dyn 'a + B { type Target = dyn A; fn deref(&self) -> &Self::Target { todo!() } } fn take_a(_: &dyn A) { } fn take_b(b: &dyn B) { take_a(b); } }
This will produce:
error: `dyn B` implements `Deref` with supertrait `(dyn A + 'static)` as output
--> lint_example.rs:19:12
|
19 | take_a(b);
| ^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(deref_into_dyn_supertrait)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
The dyn upcasting coercion feature adds new coercion rules, taking priority over certain other coercion rules, which will cause some behavior change.
deref-nullptr
The deref_nullptr
lint detects when an null pointer is dereferenced,
which causes undefined behavior.
Example
#![allow(unused)] fn main() { #![allow(unused)] use std::ptr; unsafe { let x = &*ptr::null::<i32>(); let x = ptr::addr_of!(*ptr::null::<i32>()); let x = *(0 as *const i32); } }
This will produce:
warning: dereferencing a null pointer
--> lint_example.rs:5:14
|
5 | let x = &*ptr::null::<i32>();
| ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
|
= note: `#[warn(deref_nullptr)]` on by default
Explanation
Dereferencing a null pointer causes undefined behavior even as a place expression,
like &*(0 as *const i32)
or addr_of!(*(0 as *const i32))
.
drop-bounds
The drop_bounds
lint checks for generics with std::ops::Drop
as
bounds.
Example
#![allow(unused)] fn main() { fn foo<T: Drop>() {} }
This will produce:
warning: bounds on `T: Drop` are most likely incorrect, consider instead using `std::mem::needs_drop` to detect whether a type can be trivially dropped
--> lint_example.rs:2:11
|
2 | fn foo<T: Drop>() {}
| ^^^^
|
= note: `#[warn(drop_bounds)]` on by default
Explanation
A generic trait bound of the form T: Drop
is most likely misleading
and not what the programmer intended (they probably should have used
std::mem::needs_drop
instead).
Drop
bounds do not actually indicate whether a type can be trivially
dropped or not, because a composite type containing Drop
types does
not necessarily implement Drop
itself. Naïvely, one might be tempted
to write an implementation that assumes that a type can be trivially
dropped while also supplying a specialization for T: Drop
that
actually calls the destructor. However, this breaks down e.g. when T
is String
, which does not implement Drop
itself but contains a
Vec
, which does implement Drop
, so assuming T
can be trivially
dropped would lead to a memory leak here.
Furthermore, the Drop
trait only contains one method, Drop::drop
,
which may not be called explicitly in user code (E0040
), so there is
really no use case for using Drop
in trait bounds, save perhaps for
some obscure corner cases, which can use #[allow(drop_bounds)]
.
duplicate-macro-attributes
The duplicate_macro_attributes
lint detects when a #[test]
-like built-in macro
attribute is duplicated on an item. This lint may trigger on bench
, cfg_eval
, test
and test_case
.
Example
#[test]
#[test]
fn foo() {}
This will produce:
warning: duplicated attribute
--> src/lib.rs:2:1
|
2 | #[test]
| ^^^^^^^
|
= note: `#[warn(duplicate_macro_attributes)]` on by default
Explanation
A duplicated attribute may erroneously originate from a copy-paste and the effect of it being duplicated may not be obvious or desirable.
For instance, doubling the #[test]
attributes registers the test to be run twice with no
change to its environment.
dyn-drop
The dyn_drop
lint checks for trait objects with std::ops::Drop
.
Example
#![allow(unused)] fn main() { fn foo(_x: Box<dyn Drop>) {} }
This will produce:
warning: types that do not implement `Drop` can still have drop glue, consider instead using `std::mem::needs_drop` to detect whether a type is trivially dropped
--> lint_example.rs:2:20
|
2 | fn foo(_x: Box<dyn Drop>) {}
| ^^^^
|
= note: `#[warn(dyn_drop)]` on by default
Explanation
A trait object bound of the form dyn Drop
is most likely misleading
and not what the programmer intended.
Drop
bounds do not actually indicate whether a type can be trivially
dropped or not, because a composite type containing Drop
types does
not necessarily implement Drop
itself. Naïvely, one might be tempted
to write a deferred drop system, to pull cleaning up memory out of a
latency-sensitive code path, using dyn Drop
trait objects. However,
this breaks down e.g. when T
is String
, which does not implement
Drop
, but should probably be accepted.
To write a trait object bound that accepts anything, use a placeholder trait with a blanket implementation.
#![allow(unused)] fn main() { trait Placeholder {} impl<T> Placeholder for T {} fn foo(_x: Box<dyn Placeholder>) {} }
ellipsis-inclusive-range-patterns
The ellipsis_inclusive_range_patterns
lint detects the ...
range
pattern, which is deprecated.
Example
#![allow(unused)] fn main() { let x = 123; match x { 0...100 => {} _ => {} } }
This will produce:
warning: `...` range patterns are deprecated
--> lint_example.rs:4:6
|
4 | 0...100 => {}
| ^^^ help: use `..=` for an inclusive range
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
= note: `#[warn(ellipsis_inclusive_range_patterns)]` on by default
Explanation
The ...
range pattern syntax was changed to ..=
to avoid potential
confusion with the ..
range expression. Use the new form instead.
exported-private-dependencies
The exported_private_dependencies
lint detects private dependencies
that are exposed in a public interface.
Example
pub fn foo() -> Option<some_private_dependency::Thing> {
None
}
This will produce:
warning: type `bar::Thing` from private dependency 'bar' in public interface
--> src/lib.rs:3:1
|
3 | pub fn foo() -> Option<bar::Thing> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(exported_private_dependencies)]` on by default
Explanation
Dependencies can be marked as "private" to indicate that they are not exposed in the public interface of a crate. This can be used by Cargo to independently resolve those dependencies because it can assume it does not need to unify them with other packages using that same dependency. This lint is an indication of a violation of that contract.
To fix this, avoid exposing the dependency in your public interface. Or, switch the dependency to a public dependency.
Note that support for this is only available on the nightly channel. See RFC 1977 for more details, as well as the Cargo documentation.
for-loops-over-fallibles
The for_loops_over_fallibles
lint checks for for
loops over Option
or Result
values.
Example
#![allow(unused)] fn main() { let opt = Some(1); for x in opt { /* ... */} }
This will produce:
warning: for loop over an `Option`. This is more readably written as an `if let` statement
--> lint_example.rs:3:10
|
3 | for x in opt { /* ... */}
| ^^^
|
= note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
|
3 | while let Some(x) = opt { /* ... */}
| ~~~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
|
3 | if let Some(x) = opt { /* ... */}
| ~~~~~~~~~~~~ ~~~
Explanation
Both Option
and Result
implement IntoIterator
trait, which allows using them in a for
loop.
for
loop over Option
or Result
will iterate either 0 (if the value is None
/Err(_)
)
or 1 time (if the value is Some(_)
/Ok(_)
). This is not very useful and is more clearly expressed
via if let
.
for
loop can also be accidentally written with the intention to call a function multiple times,
while the function returns Some(_)
, in these cases while let
loop should be used instead.
The "intended" use of IntoIterator
implementations for Option
and Result
is passing them to
generic code that expects something implementing IntoIterator
. For example using .chain(option)
to optionally add a value to an iterator.
forbidden-lint-groups
The forbidden_lint_groups
lint detects violations of
forbid
applied to a lint group. Due to a bug in the compiler,
these used to be overlooked entirely. They now generate a warning.
Example
#![forbid(warnings)] #![deny(bad_style)] fn main() {}
This will produce:
warning: deny(bad_style) incompatible with previous forbid
--> lint_example.rs:2:9
|
1 | #![forbid(warnings)]
| -------- `forbid` level set here
2 | #![deny(bad_style)]
| ^^^^^^^^^ overruled by previous forbid
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
= note: `#[warn(forbidden_lint_groups)]` on by default
Recommended fix
If your crate is using #![forbid(warnings)]
,
we recommend that you change to #![deny(warnings)]
.
Explanation
Due to a compiler bug, applying forbid
to lint groups
previously had no effect. The bug is now fixed but instead of
enforcing forbid
we issue this future-compatibility warning
to avoid breaking existing crates.
function-item-references
The function_item_references
lint detects function references that are
formatted with fmt::Pointer
or transmuted.
Example
fn foo() { } fn main() { println!("{:p}", &foo); }
This will produce:
warning: taking a reference to a function item does not give a function pointer
--> lint_example.rs:4:22
|
4 | println!("{:p}", &foo);
| ^^^^ help: cast `foo` to obtain a function pointer: `foo as fn()`
|
= note: `#[warn(function_item_references)]` on by default
Explanation
Taking a reference to a function may be mistaken as a way to obtain a
pointer to that function. This can give unexpected results when
formatting the reference as a pointer or transmuting it. This lint is
issued when function references are formatted as pointers, passed as
arguments bound by fmt::Pointer
or transmuted.
illegal-floating-point-literal-pattern
The illegal_floating_point_literal_pattern
lint detects
floating-point literals used in patterns.
Example
#![allow(unused)] fn main() { let x = 42.0; match x { 5.0 => {} _ => {} } }
This will produce:
warning: floating-point types cannot be used in patterns
--> lint_example.rs:5:5
|
5 | 5.0 => {}
| ^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #41620 <https://github.com/rust-lang/rust/issues/41620>
= note: `#[warn(illegal_floating_point_literal_pattern)]` on by default
Explanation
Previous versions of the compiler accepted floating-point literals in patterns, but it was later determined this was a mistake. The semantics of comparing floating-point values may not be clear in a pattern when contrasted with "structural equality". Typically you can work around this by using a match guard, such as:
#![allow(unused)] fn main() { let x = 42.0; match x { y if y == 5.0 => {} _ => {} } }
This is a future-incompatible lint to transition this to a hard error in the future. See issue #41620 for more details.
improper-ctypes
The improper_ctypes
lint detects incorrect use of types in foreign
modules.
Example
#![allow(unused)] fn main() { extern "C" { static STATIC: String; } }
This will produce:
warning: `extern` block uses type `String`, which is not FFI-safe
--> lint_example.rs:3:20
|
3 | static STATIC: String;
| ^^^^^^ not FFI-safe
|
= help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
= note: this struct has unspecified layout
= note: `#[warn(improper_ctypes)]` on by default
Explanation
The compiler has several checks to verify that types used in extern
blocks are safe and follow certain rules to ensure proper
compatibility with the foreign interfaces. This lint is issued when it
detects a probable mistake in a definition. The lint usually should
provide a description of the issue, along with possibly a hint on how
to resolve it.
improper-ctypes-definitions
The improper_ctypes_definitions
lint detects incorrect use of
extern
function definitions.
Example
#![allow(unused)] fn main() { #![allow(unused)] pub extern "C" fn str_type(p: &str) { } }
This will produce:
warning: `extern` fn uses type `str`, which is not FFI-safe
--> lint_example.rs:3:31
|
3 | pub extern "C" fn str_type(p: &str) { }
| ^^^^ not FFI-safe
|
= help: consider using `*const u8` and a length instead
= note: string slices have no C equivalent
= note: `#[warn(improper_ctypes_definitions)]` on by default
Explanation
There are many parameter and return types that may be specified in an
extern
function that are not compatible with the given ABI. This
lint is an alert that these types should not be used. The lint usually
should provide a description of the issue, along with possibly a hint
on how to resolve it.
incomplete-features
The incomplete_features
lint detects unstable features enabled with
the feature
attribute that may function improperly in some or all
cases.
Example
#![allow(unused)] #![feature(generic_const_exprs)] fn main() { }
This will produce:
warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
--> lint_example.rs:1:12
|
1 | #![feature(generic_const_exprs)]
| ^^^^^^^^^^^^^^^^^^^
|
= note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
= note: `#[warn(incomplete_features)]` on by default
Explanation
Although it is encouraged for people to experiment with unstable features, some of them are known to be incomplete or faulty. This lint is a signal that the feature has not yet been finished, and you may experience problems with it.
indirect-structural-match
The indirect_structural_match
lint detects a const
in a pattern
that manually implements PartialEq
and Eq
.
Example
#![deny(indirect_structural_match)] struct NoDerive(i32); impl PartialEq for NoDerive { fn eq(&self, _: &Self) -> bool { false } } impl Eq for NoDerive { } #[derive(PartialEq, Eq)] struct WrapParam<T>(T); const WRAP_INDIRECT_PARAM: & &WrapParam<NoDerive> = & &WrapParam(NoDerive(0)); fn main() { match WRAP_INDIRECT_PARAM { WRAP_INDIRECT_PARAM => { } _ => { } } }
This will produce:
error: to use a constant of type `NoDerive` in a pattern, `NoDerive` must be annotated with `#[derive(PartialEq, Eq)]`
--> lint_example.rs:11:9
|
11 | WRAP_INDIRECT_PARAM => { }
| ^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/62411>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(indirect_structural_match)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
The compiler unintentionally accepted this form in the past. This is a future-incompatible lint to transition this to a hard error in the future. See issue #62411 for a complete description of the problem, and some possible solutions.
inline-no-sanitize
The inline_no_sanitize
lint detects incompatible use of
#[inline(always)]
and #[no_sanitize(...)]
.
Example
#![feature(no_sanitize)] #[inline(always)] #[no_sanitize(address)] fn x() {} fn main() { x() }
This will produce:
warning: `no_sanitize` will have no effect after inlining
--> lint_example.rs:4:1
|
4 | #[no_sanitize(address)]
| ^^^^^^^^^^^^^^^^^^^^^^^
|
note: inlining requested here
--> lint_example.rs:3:1
|
3 | #[inline(always)]
| ^^^^^^^^^^^^^^^^^
= note: `#[warn(inline_no_sanitize)]` on by default
Explanation
The use of the #[inline(always)]
attribute prevents the
the #[no_sanitize(...)]
attribute from working.
Consider temporarily removing inline
attribute.
invalid-doc-attributes
The invalid_doc_attributes
lint detects when the #[doc(...)]
is
misused.
Example
#![allow(unused)] #![deny(warnings)] fn main() { pub mod submodule { #![doc(test(no_crate_inject))] } }
This will produce:
error: this attribute can only be applied at the crate level
--> lint_example.rs:5:12
|
5 | #![doc(test(no_crate_inject))]
| ^^^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #82730 <https://github.com/rust-lang/rust/issues/82730>
= note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(warnings)]
| ^^^^^^^^
= note: `#[deny(invalid_doc_attributes)]` implied by `#[deny(warnings)]`
Explanation
Previously, there were very like checks being performed on #[doc(..)]
unlike the other attributes. It'll now catch all the issues that it
silently ignored previously.
invalid-value
The invalid_value
lint detects creating a value that is not valid,
such as a null reference.
Example
#![allow(unused)] fn main() { #![allow(unused)] unsafe { let x: &'static i32 = std::mem::zeroed(); } }
This will produce:
warning: the type `&i32` does not permit zero-initialization
--> lint_example.rs:4:27
|
4 | let x: &'static i32 = std::mem::zeroed();
| ^^^^^^^^^^^^^^^^^^
| |
| this code causes undefined behavior when executed
| help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
|
= note: references must be non-null
= note: `#[warn(invalid_value)]` on by default
Explanation
In some situations the compiler can detect that the code is creating an invalid value, which should be avoided.
In particular, this lint will check for improper use of
mem::zeroed
, mem::uninitialized
, mem::transmute
, and
MaybeUninit::assume_init
that can cause undefined behavior. The
lint should provide extra information to indicate what the problem is
and a possible solution.
irrefutable-let-patterns
The irrefutable_let_patterns
lint detects irrefutable patterns
in if let
s, while let
s, and if let
guards.
Example
#![allow(unused)] fn main() { if let _ = 123 { println!("always runs!"); } }
This will produce:
warning: irrefutable `if let` pattern
--> lint_example.rs:2:4
|
2 | if let _ = 123 {
| ^^^^^^^^^^^
|
= note: this pattern will always match, so the `if let` is useless
= help: consider replacing the `if let` with a `let`
= note: `#[warn(irrefutable_let_patterns)]` on by default
Explanation
There usually isn't a reason to have an irrefutable pattern in an
if let
or while let
statement, because the pattern will always match
successfully. A let
or loop
statement will suffice. However,
when generating code with a macro, forbidding irrefutable patterns
would require awkward workarounds in situations where the macro
doesn't know if the pattern is refutable or not. This lint allows
macros to accept this form, while alerting for a possibly incorrect
use in normal code.
See RFC 2086 for more details.
large-assignments
The large_assignments
lint detects when objects of large
types are being moved around.
Example
let x = [0; 50000];
let y = x;
produces:
warning: moving a large value
--> $DIR/move-large.rs:1:3
let y = x;
- Copied large value here
Explanation
When using a large type in a plain assignment or in a function argument, idiomatic code can be inefficient. Ideally appropriate optimizations would resolve this, but such optimizations are only done in a best-effort manner. This lint will trigger on all sites of large moves and thus allow the user to resolve them in code.
late-bound-lifetime-arguments
The late_bound_lifetime_arguments
lint detects generic lifetime
arguments in path segments with late bound lifetime parameters.
Example
struct S; impl S { fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {} } fn main() { S.late::<'static>(&0, &0); }
This will produce:
warning: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
--> lint_example.rs:8:14
|
4 | fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {}
| -- the late bound lifetime parameter is introduced here
...
8 | S.late::<'static>(&0, &0);
| ^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #42868 <https://github.com/rust-lang/rust/issues/42868>
= note: `#[warn(late_bound_lifetime_arguments)]` on by default
Explanation
It is not clear how to provide arguments for early-bound lifetime parameters if they are intermixed with late-bound parameters in the same list. For now, providing any explicit arguments will trigger this lint if late-bound parameters are present, so in the future a solution can be adopted without hitting backward compatibility issues. This is a future-incompatible lint to transition this to a hard error in the future. See issue #42868 for more details, along with a description of the difference between early and late-bound parameters.
legacy-derive-helpers
The legacy_derive_helpers
lint detects derive helper attributes
that are used before they are introduced.
Example
#[serde(rename_all = "camelCase")]
#[derive(Deserialize)]
struct S { /* fields */ }
produces:
warning: derive helper attribute is used before it is introduced
--> $DIR/legacy-derive-helpers.rs:1:3
|
1 | #[serde(rename_all = "camelCase")]
| ^^^^^
...
2 | #[derive(Deserialize)]
| ----------- the attribute is introduced here
Explanation
Attributes like this work for historical reasons, but attribute expansion works in
left-to-right order in general, so, to resolve #[serde]
, compiler has to try to "look
into the future" at not yet expanded part of the item , but such attempts are not always
reliable.
To fix the warning place the helper attribute after its corresponding derive.
#[derive(Deserialize)]
#[serde(rename_all = "camelCase")]
struct S { /* fields */ }
mixed-script-confusables
The mixed_script_confusables
lint detects visually confusable
characters in identifiers between different scripts.
Example
#![allow(unused)] fn main() { // The Japanese katakana character エ can be confused with the Han character 工. const エ: &'static str = "アイウ"; }
This will produce:
warning: the usage of Script Group `Japanese, Katakana` in this crate consists solely of mixed script confusables
--> lint_example.rs:3:7
|
3 | const エ: &'static str = "アイウ";
| ^^
|
= note: the usage includes 'エ' (U+30A8)
= note: please recheck to make sure their usages are indeed what you want
= note: `#[warn(mixed_script_confusables)]` on by default
Explanation
This lint warns when characters between different scripts may appear visually similar, which can cause confusion.
If the crate contains other identifiers in the same script that have
non-confusable characters, then this lint will not be issued. For
example, if the example given above has another identifier with
katakana characters (such as let カタカナ = 123;
), then this indicates
that you are intentionally using katakana, and it will not warn about
it.
Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
named-arguments-used-positionally
The named_arguments_used_positionally
lint detects cases where named arguments are only
used positionally in format strings. This usage is valid but potentially very confusing.
Example
#![deny(named_arguments_used_positionally)] fn main() { let _x = 5; println!("{}", _x = 1); // Prints 1, will trigger lint println!("{}", _x); // Prints 5, no lint emitted println!("{_x}", _x = _x); // Prints 5, no lint emitted }
This will produce:
error: named argument `_x` is not used by name
--> lint_example.rs:4:20
|
4 | println!("{}", _x = 1); // Prints 1, will trigger lint
| -- ^^ this named argument is referred to by position in formatting string
| |
| this formatting argument uses named argument `_x` by position
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(named_arguments_used_positionally)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use the named argument by name to avoid ambiguity
|
4 | println!("{_x}", _x = 1); // Prints 1, will trigger lint
| ++
Explanation
Rust formatting strings can refer to named arguments by their position, but this usage is potentially confusing. In particular, readers can incorrectly assume that the declaration of named arguments is an assignment (which would produce the unit type). For backwards compatibility, this is not a hard error.
no-mangle-generic-items
The no_mangle_generic_items
lint detects generic items that must be
mangled.
Example
#![allow(unused)] fn main() { #[no_mangle] fn foo<T>(t: T) { } }
This will produce:
warning: functions generic over types or consts must be mangled
--> lint_example.rs:3:1
|
2 | #[no_mangle]
| ------------ help: remove this attribute
3 | / fn foo<T>(t: T) {
4 | |
5 | | }
| |_^
|
= note: `#[warn(no_mangle_generic_items)]` on by default
Explanation
A function with generics must have its symbol mangled to accommodate
the generic parameter. The no_mangle
attribute has no effect in
this situation, and should be removed.
non-camel-case-types
The non_camel_case_types
lint detects types, variants, traits and
type parameters that don't have camel case names.
Example
#![allow(unused)] fn main() { struct my_struct; }
This will produce:
warning: type `my_struct` should have an upper camel case name
--> lint_example.rs:2:8
|
2 | struct my_struct;
| ^^^^^^^^^ help: convert the identifier to upper camel case: `MyStruct`
|
= note: `#[warn(non_camel_case_types)]` on by default
Explanation
The preferred style for these identifiers is to use "camel case", such
as MyStruct
, where the first letter should not be lowercase, and
should not use underscores between letters. Underscores are allowed at
the beginning and end of the identifier, as well as between
non-letters (such as X86_64
).
non-fmt-panics
The non_fmt_panics
lint detects panic!(..)
invocations where the first
argument is not a formatting string.
Example
#![allow(unused)] fn main() { panic!("{}"); panic!(123); }
This will produce:
warning: panic message contains an unused formatting placeholder
--> lint_example.rs:2:9
|
2 | panic!("{}");
| ^^
|
= note: this message is not used as a format string when given without arguments, but will be in Rust 2021
= note: `#[warn(non_fmt_panics)]` on by default
help: add the missing argument
|
2 | panic!("{}", ...);
| +++++
help: or add a "{}" format string to use the message literally
|
2 | panic!("{}", "{}");
| +++++
Explanation
In Rust 2018 and earlier, panic!(x)
directly uses x
as the message.
That means that panic!("{}")
panics with the message "{}"
instead
of using it as a formatting string, and panic!(123)
will panic with
an i32
as message.
Rust 2021 always interprets the first argument as format string.
non-shorthand-field-patterns
The non_shorthand_field_patterns
lint detects using Struct { x: x }
instead of Struct { x }
in a pattern.
Example
struct Point { x: i32, y: i32, } fn main() { let p = Point { x: 5, y: 5, }; match p { Point { x: x, y: y } => (), } }
This will produce:
warning: the `x:` in this pattern is redundant
--> lint_example.rs:14:17
|
14 | Point { x: x, y: y } => (),
| ^^^^ help: use shorthand field pattern: `x`
|
= note: `#[warn(non_shorthand_field_patterns)]` on by default
Explanation
The preferred style is to avoid the repetition of specifying both the field name and the binding name if both identifiers are the same.
non-snake-case
The non_snake_case
lint detects variables, methods, functions,
lifetime parameters and modules that don't have snake case names.
Example
#![allow(unused)] fn main() { let MY_VALUE = 5; }
This will produce:
warning: variable `MY_VALUE` should have a snake case name
--> lint_example.rs:2:5
|
2 | let MY_VALUE = 5;
| ^^^^^^^^ help: convert the identifier to snake case: `my_value`
|
= note: `#[warn(non_snake_case)]` on by default
Explanation
The preferred style for these identifiers is to use "snake case",
where all the characters are in lowercase, with words separated with a
single underscore, such as my_value
.
non-upper-case-globals
The non_upper_case_globals
lint detects static items that don't have
uppercase identifiers.
Example
#![allow(unused)] fn main() { static max_points: i32 = 5; }
This will produce:
warning: static variable `max_points` should have an upper case name
--> lint_example.rs:2:8
|
2 | static max_points: i32 = 5;
| ^^^^^^^^^^ help: convert the identifier to upper case: `MAX_POINTS`
|
= note: `#[warn(non_upper_case_globals)]` on by default
Explanation
The preferred style is for static item names to use all uppercase
letters such as MAX_POINTS
.
nontrivial-structural-match
The nontrivial_structural_match
lint detects constants that are used in patterns,
whose type is not structural-match and whose initializer body actually uses values
that are not structural-match. So Option<NotStructuralMatch>
is ok if the constant
is just None
.
Example
#![deny(nontrivial_structural_match)] #[derive(Copy, Clone, Debug)] struct NoDerive(u32); impl PartialEq for NoDerive { fn eq(&self, _: &Self) -> bool { false } } impl Eq for NoDerive { } fn main() { const INDEX: Option<NoDerive> = [None, Some(NoDerive(10))][0]; match None { Some(_) => panic!("whoops"), INDEX => dbg!(INDEX), }; }
This will produce:
error: to use a constant of type `NoDerive` in a pattern, the constant's initializer must be trivial or `NoDerive` must be annotated with `#[derive(PartialEq, Eq)]`
--> lint_example.rs:9:47
|
9 | match None { Some(_) => panic!("whoops"), INDEX => dbg!(INDEX), };
| ^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #73448 <https://github.com/rust-lang/rust/issues/73448>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(nontrivial_structural_match)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Previous versions of Rust accepted constants in patterns, even if those constants' types
did not have PartialEq
derived. Thus the compiler falls back to runtime execution of
PartialEq
, which can report that two constants are not equal even if they are
bit-equivalent.
opaque-hidden-inferred-bound
The opaque_hidden_inferred_bound
lint detects cases in which nested
impl Trait
in associated type bounds are not written generally enough
to satisfy the bounds of the associated type.
Explanation
This functionality was removed in #97346, but then rolled back in #99860 because it caused regressions.
We plan on reintroducing this as a hard error, but in the mean time, this lint serves to warn and suggest fixes for any use-cases which rely on this behavior.
Example
trait Trait {
type Assoc: Send;
}
struct Struct;
impl Trait for Struct {
type Assoc = i32;
}
fn test() -> impl Trait<Assoc = impl Sized> {
Struct
}
{{produces}}
In this example, test
declares that the associated type Assoc
for
impl Trait
is impl Sized
, which does not satisfy the Send
bound
on the associated type.
Although the hidden type, i32
does satisfy this bound, we do not
consider the return type to be well-formed with this lint. It can be
fixed by changing impl Sized
into impl Sized + Send
.
overlapping-range-endpoints
The overlapping_range_endpoints
lint detects match
arms that have range patterns that
overlap on their endpoints.
Example
#![allow(unused)] fn main() { let x = 123u8; match x { 0..=100 => { println!("small"); } 100..=255 => { println!("large"); } } }
This will produce:
warning: multiple patterns overlap on their endpoints
--> lint_example.rs:5:5
|
4 | 0..=100 => { println!("small"); }
| ------- this range overlaps on `100_u8`...
5 | 100..=255 => { println!("large"); }
| ^^^^^^^^^ ... with this range
|
= note: you likely meant to write mutually exclusive ranges
= note: `#[warn(overlapping_range_endpoints)]` on by default
Explanation
It is likely a mistake to have range patterns in a match expression that overlap in this
way. Check that the beginning and end values are what you expect, and keep in mind that
with ..=
the left and right bounds are inclusive.
path-statements
The path_statements
lint detects path statements with no effect.
Example
#![allow(unused)] fn main() { let x = 42; x; }
This will produce:
warning: path statement with no effect
--> lint_example.rs:4:1
|
4 | x;
| ^^
|
= note: `#[warn(path_statements)]` on by default
Explanation
It is usually a mistake to have a statement that has no effect.
private-in-public
The private_in_public
lint detects private items in public
interfaces not caught by the old implementation.
Example
#![allow(unused)] struct SemiPriv; mod m1 { struct Priv; impl super::SemiPriv { pub fn f(_: Priv) {} } } fn main() {}
This will produce:
warning: private type `Priv` in public interface (error E0446)
--> lint_example.rs:7:9
|
7 | pub fn f(_: Priv) {}
| ^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
= note: `#[warn(private_in_public)]` on by default
Explanation
The visibility rules are intended to prevent exposing private items in public interfaces. This is a future-incompatible lint to transition this to a hard error in the future. See issue #34537 for more details.
redundant-semicolons
The redundant_semicolons
lint detects unnecessary trailing
semicolons.
Example
#![allow(unused)] fn main() { let _ = 123;; }
This will produce:
warning: unnecessary trailing semicolon
--> lint_example.rs:2:13
|
2 | let _ = 123;;
| ^ help: remove this semicolon
|
= note: `#[warn(redundant_semicolons)]` on by default
Explanation
Extra semicolons are not needed, and may be removed to avoid confusion and visual clutter.
renamed-and-removed-lints
The renamed_and_removed_lints
lint detects lints that have been
renamed or removed.
Example
#![allow(unused)] #![deny(raw_pointer_derive)] fn main() { }
This will produce:
warning: lint `raw_pointer_derive` has been removed: using derive with raw pointers is ok
--> lint_example.rs:1:9
|
1 | #![deny(raw_pointer_derive)]
| ^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(renamed_and_removed_lints)]` on by default
Explanation
To fix this, either remove the lint or use the new name. This can help avoid confusion about lints that are no longer valid, and help maintain consistency for renamed lints.
repr-transparent-external-private-fields
The repr_transparent_external_private_fields
lint
detects types marked #[repr(transparent)]
that (transitively)
contain an external ZST type marked #[non_exhaustive]
or containing
private fields
Example
#![deny(repr_transparent_external_private_fields)]
use foo::NonExhaustiveZst;
#[repr(transparent)]
struct Bar(u32, ([u32; 0], NonExhaustiveZst));
This will produce:
error: zero-sized fields in repr(transparent) cannot contain external non-exhaustive types
--> src/main.rs:5:28
|
5 | struct Bar(u32, ([u32; 0], NonExhaustiveZst));
| ^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> src/main.rs:1:9
|
1 | #![deny(repr_transparent_external_private_fields)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
= note: this struct contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, and makes it not a breaking change to become non-zero-sized in the future.
Explanation
Previous, Rust accepted fields that contain external private zero-sized types, even though it should not be a breaking change to add a non-zero-sized field to that private type.
This is a future-incompatible lint to transition this to a hard error in the future. See issue #78586 for more details.
semicolon-in-expressions-from-macros
The semicolon_in_expressions_from_macros
lint detects trailing semicolons
in macro bodies when the macro is invoked in expression position.
This was previous accepted, but is being phased out.
Example
#![deny(semicolon_in_expressions_from_macros)] macro_rules! foo { () => { true; } } fn main() { let val = match true { true => false, _ => foo!() }; }
This will produce:
error: trailing semicolon in macro used in expression position
--> lint_example.rs:3:17
|
3 | () => { true; }
| ^
...
9 | _ => foo!()
| ------ in this macro invocation
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(semicolon_in_expressions_from_macros)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
Explanation
Previous, Rust ignored trailing semicolon in a macro body when a macro was invoked in expression position. However, this makes the treatment of semicolons in the language inconsistent, and could lead to unexpected runtime behavior in some circumstances (e.g. if the macro author expects a value to be dropped).
This is a future-incompatible lint to transition this to a hard error in the future. See issue #79813 for more details.
special-module-name
The special_module_name
lint detects module
declarations for files that have a special meaning.
Example
mod lib; fn main() { lib::run(); }
This will produce:
warning: found module declaration for lib.rs
--> lint_example.rs:1:1
|
1 | mod lib;
| ^^^^^^^^
|
= note: lib.rs is the root of this crate's library target
= help: to refer to it from other targets, use the library's name as the path
= note: `#[warn(special_module_name)]` on by default
Explanation
Cargo recognizes lib.rs
and main.rs
as the root of a
library or binary crate, so declaring them as modules
will lead to miscompilation of the crate unless configured
explicitly.
To access a library from a binary target within the same crate,
use your_crate_name::
as the path instead of lib::
:
// bar/src/lib.rs fn run() { // ... } // bar/src/main.rs fn main() { bar::run(); }
Binary targets cannot be used as libraries and so declaring one as a module is not allowed.
stable-features
The stable_features
lint detects a feature
attribute that
has since been made stable.
Example
#![feature(test_accepted_feature)] fn main() {}
This will produce:
warning: the feature `test_accepted_feature` has been stable since 1.0.0 and no longer requires an attribute to enable
--> lint_example.rs:1:12
|
1 | #![feature(test_accepted_feature)]
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(stable_features)]` on by default
Explanation
When a feature is stabilized, it is no longer necessary to include a
#![feature]
attribute for it. To fix, simply remove the
#![feature]
attribute.
suspicious-auto-trait-impls
The suspicious_auto_trait_impls
lint checks for potentially incorrect
implementations of auto traits.
Example
#![allow(unused)] fn main() { struct Foo<T>(T); unsafe impl<T> Send for Foo<*const T> {} }
This will produce:
warning: cross-crate traits with a default impl, like `Send`, should not be specialized
--> lint_example.rs:4:1
|
4 | unsafe impl<T> Send for Foo<*const T> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= warning: this will change its meaning in a future release!
= note: for more information, see issue #93367 <https://github.com/rust-lang/rust/issues/93367>
= note: `*const T` is not a generic parameter
note: try using the same sequence of generic parameters as the struct definition
--> lint_example.rs:2:1
|
2 | struct Foo<T>(T);
| ^^^^^^^^^^^^^
= note: `#[warn(suspicious_auto_trait_impls)]` on by default
Explanation
A type can implement auto traits, e.g. Send
, Sync
and Unpin
,
in two different ways: either by writing an explicit impl or if
all fields of the type implement that auto trait.
The compiler disables the automatic implementation if an explicit one exists for given type constructor. The exact rules governing this are currently unsound, quite subtle, and will be modified in the future. This change will cause the automatic implementation to be disabled in more cases, potentially breaking some code.
temporary-cstring-as-ptr
The temporary_cstring_as_ptr
lint detects getting the inner pointer of
a temporary CString
.
Example
#![allow(unused)] fn main() { #![allow(unused)] use std::ffi::CString; let c_str = CString::new("foo").unwrap().as_ptr(); }
This will produce:
warning: getting the inner pointer of a temporary `CString`
--> lint_example.rs:4:42
|
4 | let c_str = CString::new("foo").unwrap().as_ptr();
| ---------------------------- ^^^^^^ this pointer will be invalid
| |
| this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
|
= note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
= help: for more information, see https://doc.rust-lang.org/reference/destructors.html
= note: `#[warn(temporary_cstring_as_ptr)]` on by default
Explanation
The inner pointer of a CString
lives only as long as the CString
it
points to. Getting the inner pointer of a temporary CString
allows the CString
to be dropped at the end of the statement, as it is not being referenced as far as the typesystem
is concerned. This means outside of the statement the pointer will point to freed memory, which
causes undefined behavior if the pointer is later dereferenced.
trivial-bounds
The trivial_bounds
lint detects trait bounds that don't depend on
any type parameters.
Example
#![allow(unused)] #![feature(trivial_bounds)] fn main() { pub struct A where i32: Copy; }
This will produce:
warning: trait bound i32: Copy does not depend on any type or lifetime parameters
--> lint_example.rs:3:25
|
3 | pub struct A where i32: Copy;
| ^^^^
|
= note: `#[warn(trivial_bounds)]` on by default
Explanation
Usually you would not write a trait bound that you know is always
true, or never true. However, when using macros, the macro may not
know whether or not the constraint would hold or not at the time when
generating the code. Currently, the compiler does not alert you if the
constraint is always true, and generates an error if it is never true.
The trivial_bounds
feature changes this to be a warning in both
cases, giving macros more freedom and flexibility to generate code,
while still providing a signal when writing non-macro code that
something is amiss.
See RFC 2056 for more details. This feature is currently only available on the nightly channel, see tracking issue #48214.
type-alias-bounds
The type_alias_bounds
lint detects bounds in type aliases.
Example
#![allow(unused)] fn main() { type SendVec<T: Send> = Vec<T>; }
This will produce:
warning: bounds on generic parameters are not enforced in type aliases
--> lint_example.rs:2:17
|
2 | type SendVec<T: Send> = Vec<T>;
| ^^^^
|
= note: `#[warn(type_alias_bounds)]` on by default
help: the bound will not be checked when the type alias is used, and should be removed
|
2 - type SendVec<T: Send> = Vec<T>;
2 + type SendVec<T> = Vec<T>;
|
Explanation
The trait bounds in a type alias are currently ignored, and should not be included to avoid confusion. This was previously allowed unintentionally; this may become a hard error in the future.
tyvar-behind-raw-pointer
The tyvar_behind_raw_pointer
lint detects raw pointer to an
inference variable.
Example
#![allow(unused)] fn main() { // edition 2015 let data = std::ptr::null(); let _ = &data as *const *const (); if data.is_null() {} }
This will produce:
warning: type annotations needed
--> lint_example.rs:6:9
|
6 | if data.is_null() {}
| ^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
= note: `#[warn(tyvar_behind_raw_pointer)]` on by default
Explanation
This kind of inference was previously allowed, but with the future arrival of arbitrary self types, this can introduce ambiguity. To resolve this, use an explicit type instead of relying on type inference.
This is a future-incompatible lint to transition this to a hard error in the 2018 edition. See issue #46906 for more details. This is currently a hard-error on the 2018 edition, and is "warn" by default in the 2015 edition.
uncommon-codepoints
The uncommon_codepoints
lint detects uncommon Unicode codepoints in
identifiers.
Example
#![allow(unused)] fn main() { #![allow(unused)] const µ: f64 = 0.000001; }
This will produce:
warning: identifier contains uncommon Unicode codepoints
--> lint_example.rs:3:7
|
3 | const µ: f64 = 0.000001;
| ^
|
= note: `#[warn(uncommon_codepoints)]` on by default
Explanation
This lint warns about using characters which are not commonly used, and may cause visual confusion.
This lint is triggered by identifiers that contain a codepoint that is not part of the set of "Allowed" codepoints as described by Unicode® Technical Standard #39 Unicode Security Mechanisms Section 3.1 General Security Profile for Identifiers.
Note that the set of uncommon codepoints may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
unconditional-recursion
The unconditional_recursion
lint detects functions that cannot
return without calling themselves.
Example
#![allow(unused)] fn main() { fn foo() { foo(); } }
This will produce:
warning: function cannot return without recursing
--> lint_example.rs:2:1
|
2 | fn foo() {
| ^^^^^^^^ cannot return without recursing
3 | foo();
| ----- recursive call site
|
= help: a `loop` may express intention better if this is on purpose
= note: `#[warn(unconditional_recursion)]` on by default
Explanation
It is usually a mistake to have a recursive call that does not have
some condition to cause it to terminate. If you really intend to have
an infinite loop, using a loop
expression is recommended.
undefined-naked-function-abi
The undefined_naked_function_abi
lint detects naked function definitions that
either do not specify an ABI or specify the Rust ABI.
Example
#![allow(unused)] #![feature(asm_experimental_arch, naked_functions)] fn main() { use std::arch::asm; #[naked] pub fn default_abi() -> u32 { unsafe { asm!("", options(noreturn)); } } #[naked] pub extern "Rust" fn rust_abi() -> u32 { unsafe { asm!("", options(noreturn)); } } }
This will produce:
warning: Rust ABI is unsupported in naked functions
--> lint_example.rs:7:1
|
7 | pub fn default_abi() -> u32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(undefined_naked_function_abi)]` on by default
Explanation
The Rust ABI is currently undefined. Therefore, naked functions should specify a non-Rust ABI.
unexpected-cfgs
The unexpected_cfgs
lint detects unexpected conditional compilation conditions.
Example
rustc --check-cfg 'names()'
#[cfg(widnows)]
fn foo() {}
This will produce:
warning: unknown condition name used
--> lint_example.rs:1:7
|
1 | #[cfg(widnows)]
| ^^^^^^^
|
= note: `#[warn(unexpected_cfgs)]` on by default
Explanation
This lint is only active when a --check-cfg='names(...)'
option has been passed
to the compiler and triggers whenever an unknown condition name or value is used.
The known condition include names or values passed in --check-cfg
, --cfg
, and some
well-knows names and values built into the compiler.
unfulfilled-lint-expectations
The unfulfilled_lint_expectations
lint detects lint trigger expectations
that have not been fulfilled.
Example
#![allow(unused)] #![feature(lint_reasons)] fn main() { #[expect(unused_variables)] let x = 10; println!("{}", x); }
This will produce:
warning: this lint expectation is unfulfilled
--> lint_example.rs:4:10
|
4 | #[expect(unused_variables)]
| ^^^^^^^^^^^^^^^^
|
= note: `#[warn(unfulfilled_lint_expectations)]` on by default
Explanation
It was expected that the marked code would emit a lint. This expectation has not been fulfilled.
The expect
attribute can be removed if this is intended behavior otherwise
it should be investigated why the expected lint is no longer issued.
In rare cases, the expectation might be emitted at a different location than
shown in the shown code snippet. In most cases, the #[expect]
attribute
works when added to the outer scope. A few lints can only be expected
on a crate level.
Part of RFC 2383. The progress is being tracked in #54503
uninhabited-static
The uninhabited_static
lint detects uninhabited statics.
Example
#![allow(unused)] fn main() { enum Void {} extern { static EXTERN: Void; } }
This will produce:
warning: static of uninhabited type
--> lint_example.rs:4:5
|
4 | static EXTERN: Void;
| ^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error
= note: `#[warn(uninhabited_static)]` on by default
Explanation
Statics with an uninhabited type can never be initialized, so they are impossible to define.
However, this can be side-stepped with an extern static
, leading to problems later in the
compiler which assumes that there are no initialized uninhabited places (such as locals or
statics). This was accidentally allowed, but is being phased out.
unknown-lints
The unknown_lints
lint detects unrecognized lint attributes.
Example
#![allow(unused)] #![allow(not_a_real_lint)] fn main() { }
This will produce:
warning: unknown lint: `not_a_real_lint`
--> lint_example.rs:1:10
|
1 | #![allow(not_a_real_lint)]
| ^^^^^^^^^^^^^^^
|
= note: `#[warn(unknown_lints)]` on by default
Explanation
It is usually a mistake to specify a lint that does not exist. Check the spelling, and check the lint listing for the correct name. Also consider if you are using an old version of the compiler, and the lint is only available in a newer version.
unnameable-test-items
The unnameable_test_items
lint detects #[test]
functions
that are not able to be run by the test harness because they are in a
position where they are not nameable.
Example
fn main() { #[test] fn foo() { // This test will not fail because it does not run. assert_eq!(1, 2); } }
This will produce:
warning: cannot test inner items
--> lint_example.rs:2:5
|
2 | #[test]
| ^^^^^^^
|
= note: `#[warn(unnameable_test_items)]` on by default
= note: this warning originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
Explanation
In order for the test harness to run a test, the test function must be located in a position where it can be accessed from the crate root. This generally means it must be defined in a module, and not anywhere else such as inside another function. The compiler previously allowed this without an error, so a lint was added as an alert that a test is not being used. Whether or not this should be allowed has not yet been decided, see RFC 2471 and issue #36629.
unreachable-code
The unreachable_code
lint detects unreachable code paths.
Example
#![allow(unused)] fn main() { panic!("we never go past here!"); let x = 5; }
This will produce:
warning: unreachable statement
--> lint_example.rs:4:1
|
2 | panic!("we never go past here!");
| -------------------------------- any code following this expression is unreachable
3 |
4 | let x = 5;
| ^^^^^^^^^^ unreachable statement
|
= note: `#[warn(unreachable_code)]` on by default
Explanation
Unreachable code may signal a mistake or unfinished code. If the code is no longer in use, consider removing it.
unreachable-patterns
The unreachable_patterns
lint detects unreachable patterns.
Example
#![allow(unused)] fn main() { let x = 5; match x { y => (), 5 => (), } }
This will produce:
warning: unreachable pattern
--> lint_example.rs:5:5
|
4 | y => (),
| - matches any value
5 | 5 => (),
| ^ unreachable pattern
|
= note: `#[warn(unreachable_patterns)]` on by default
Explanation
This usually indicates a mistake in how the patterns are specified or
ordered. In this example, the y
pattern will always match, so the
five is impossible to reach. Remember, match arms match in order, you
probably wanted to put the 5
case above the y
case.
unstable-name-collisions
The unstable_name_collisions
lint detects that you have used a name
that the standard library plans to add in the future.
Example
#![allow(unused)] fn main() { trait MyIterator : Iterator { // is_sorted is an unstable method that already exists on the Iterator trait fn is_sorted(self) -> bool where Self: Sized {true} } impl<T: ?Sized> MyIterator for T where T: Iterator { } let x = vec![1, 2, 3]; let _ = x.iter().is_sorted(); }
This will produce:
warning: an associated function with this name may be added to the standard library in the future
--> lint_example.rs:10:18
|
10 | let _ = x.iter().is_sorted();
| ^^^^^^^^^
|
= warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
= note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
= help: call with fully qualified syntax `MyIterator::is_sorted(...)` to keep using the current method
= help: add `#![feature(is_sorted)]` to the crate attributes to enable `is_sorted`
= note: `#[warn(unstable_name_collisions)]` on by default
Explanation
When new methods are added to traits in the standard library, they are
usually added in an "unstable" form which is only available on the
nightly channel with a feature
attribute. If there is any
pre-existing code which extends a trait to have a method with the same
name, then the names will collide. In the future, when the method is
stabilized, this will cause an error due to the ambiguity. This lint
is an early-warning to let you know that there may be a collision in
the future. This can be avoided by adding type annotations to
disambiguate which trait method you intend to call, such as
MyIterator::is_sorted(my_iter)
or renaming or removing the method.
unstable-syntax-pre-expansion
The unstable_syntax_pre_expansion
lint detects the use of unstable
syntax that is discarded during attribute expansion.
Example
#![allow(unused)] fn main() { #[cfg(FALSE)] macro foo() {} }
This will produce:
warning: `macro` is experimental
--> lint_example.rs:3:1
|
3 | macro foo() {}
| ^^^^^^^^^^^^^^
|
= note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
= help: add `#![feature(decl_macro)]` to the crate attributes to enable
= warning: unstable syntax can change at any point in the future, causing a hard error!
= note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
Explanation
The input to active attributes such as #[cfg]
or procedural macro
attributes is required to be valid syntax. Previously, the compiler only
gated the use of unstable syntax features after resolving #[cfg]
gates
and expanding procedural macros.
To avoid relying on unstable syntax, move the use of unstable syntax into a position where the compiler does not parse the syntax, such as a functionlike macro.
#![allow(unused)] fn main() { #![deny(unstable_syntax_pre_expansion)] macro_rules! identity { ( $($tokens:tt)* ) => { $($tokens)* } } #[cfg(FALSE)] identity! { macro foo() {} } }
This is a future-incompatible lint to transition this to a hard error in the future. See issue #65860 for more details.
unsupported-calling-conventions
The unsupported_calling_conventions
lint is output whenever there is a use of the
stdcall
, fastcall
, thiscall
, vectorcall
calling conventions (or their unwind
variants) on targets that cannot meaningfully be supported for the requested target.
For example stdcall
does not make much sense for a x86_64 or, more apparently, powerpc
code, because this calling convention was never specified for those targets.
Historically MSVC toolchains have fallen back to the regular C calling convention for targets other than x86, but Rust doesn't really see a similar need to introduce a similar hack across many more targets.
Example
extern "stdcall" fn stdcall() {}
This will produce:
warning: use of calling convention not supported on this target
--> $DIR/unsupported.rs:39:1
|
LL | extern "stdcall" fn stdcall() {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unsupported_calling_conventions)]` on by default
= warning: this was previously accepted by the compiler but is being phased out;
it will become a hard error in a future release!
= note: for more information, see issue ...
Explanation
On most of the targets the behaviour of stdcall
and similar calling conventions is not
defined at all, but was previously accepted due to a bug in the implementation of the
compiler.
unused-allocation
The unused_allocation
lint detects unnecessary allocations that can
be eliminated.
Example
#![feature(box_syntax)] fn main() { let a = (box [1, 2, 3]).len(); }
This will produce:
warning: unnecessary allocation, use `&` instead
--> lint_example.rs:3:13
|
3 | let a = (box [1, 2, 3]).len();
| ^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_allocation)]` on by default
Explanation
When a box
expression is immediately coerced to a reference, then
the allocation is unnecessary, and a reference (using &
or &mut
)
should be used instead to avoid the allocation.
unused-assignments
The unused_assignments
lint detects assignments that will never be read.
Example
#![allow(unused)] fn main() { let mut x = 5; x = 6; }
This will produce:
warning: value assigned to `x` is never read
--> lint_example.rs:3:1
|
3 | x = 6;
| ^
|
= help: maybe it is overwritten before being read?
= note: `#[warn(unused_assignments)]` on by default
Explanation
Unused assignments may signal a mistake or unfinished code. If the
variable is never used after being assigned, then the assignment can
be removed. Variables with an underscore prefix such as _x
will not
trigger this lint.
unused-attributes
The unused_attributes
lint detects attributes that were not used by
the compiler.
Example
#![allow(unused)] #![ignore] fn main() { }
This will produce:
warning: `#[ignore]` only has an effect on functions
--> lint_example.rs:1:1
|
1 | #![ignore]
| ^^^^^^^^^^
|
= note: `#[warn(unused_attributes)]` on by default
Explanation
Unused attributes may indicate the attribute is placed in the wrong
position. Consider removing it, or placing it in the correct position.
Also consider if you intended to use an inner attribute (with a !
such as #![allow(unused)]
) which applies to the item the attribute
is within, or an outer attribute (without a !
such as
#[allow(unused)]
) which applies to the item following the
attribute.
unused-braces
The unused_braces
lint detects unnecessary braces around an
expression.
Example
#![allow(unused)] fn main() { if { true } { // ... } }
This will produce:
warning: unnecessary braces around `if` condition
--> lint_example.rs:2:4
|
2 | if { true } {
| ^^ ^^
|
= note: `#[warn(unused_braces)]` on by default
help: remove these braces
|
2 - if { true } {
2 + if true {
|
Explanation
The braces are not needed, and should be removed. This is the preferred style for writing these expressions.
unused-comparisons
The unused_comparisons
lint detects comparisons made useless by
limits of the types involved.
Example
#![allow(unused)] fn main() { fn foo(x: u8) { x >= 0; } }
This will produce:
warning: comparison is useless due to type limits
--> lint_example.rs:3:5
|
3 | x >= 0;
| ^^^^^^
|
= note: `#[warn(unused_comparisons)]` on by default
Explanation
A useless comparison may indicate a mistake, and should be fixed or removed.
unused-doc-comments
The unused_doc_comments
lint detects doc comments that aren't used
by rustdoc
.
Example
#![allow(unused)] fn main() { /// docs for x let x = 12; }
This will produce:
warning: unused doc comment
--> lint_example.rs:2:1
|
2 | /// docs for x
| ^^^^^^^^^^^^^^
3 | let x = 12;
| ----------- rustdoc does not generate documentation for statements
|
= help: use `//` for a plain comment
= note: `#[warn(unused_doc_comments)]` on by default
Explanation
rustdoc
does not use doc comments in all positions, and so the doc
comment will be ignored. Try changing it to a normal comment with //
to avoid the warning.
unused-features
The unused_features
lint detects unused or unknown features found in
crate-level feature
attributes.
Note: This lint is currently not functional, see issue #44232 for more details.
unused-imports
The unused_imports
lint detects imports that are never used.
Example
#![allow(unused)] fn main() { use std::collections::HashMap; }
This will produce:
warning: unused import: `std::collections::HashMap`
--> lint_example.rs:2:5
|
2 | use std::collections::HashMap;
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
Explanation
Unused imports may signal a mistake or unfinished code, and clutter
the code, and should be removed. If you intended to re-export the item
to make it available outside of the module, add a visibility modifier
like pub
.
unused-labels
The unused_labels
lint detects labels that are never used.
Example
#![allow(unused)] fn main() { 'unused_label: loop {} }
This will produce:
warning: unused label
--> lint_example.rs:2:1
|
2 | 'unused_label: loop {}
| ^^^^^^^^^^^^^
|
= note: `#[warn(unused_labels)]` on by default
Explanation
Unused labels may signal a mistake or unfinished code. To silence the
warning for the individual label, prefix it with an underscore such as
'_my_label:
.
unused-macros
The unused_macros
lint detects macros that were not used.
Note that this lint is distinct from the unused_macro_rules
lint,
which checks for single rules that never match of an otherwise used
macro, and thus never expand.
Example
macro_rules! unused { () => {}; } fn main() { }
This will produce:
warning: unused macro definition: `unused`
--> lint_example.rs:1:14
|
1 | macro_rules! unused {
| ^^^^^^
|
= note: `#[warn(unused_macros)]` on by default
Explanation
Unused macros may signal a mistake or unfinished code. To silence the
warning for the individual macro, prefix the name with an underscore
such as _my_macro
. If you intended to export the macro to make it
available outside of the crate, use the macro_export
attribute.
unused-must-use
The unused_must_use
lint detects unused result of a type flagged as
#[must_use]
.
Example
fn returns_result() -> Result<(), ()> { Ok(()) } fn main() { returns_result(); }
This will produce:
warning: unused `Result` that must be used
--> lint_example.rs:6:5
|
6 | returns_result();
| ^^^^^^^^^^^^^^^^^
|
= note: this `Result` may be an `Err` variant, which should be handled
= note: `#[warn(unused_must_use)]` on by default
Explanation
The #[must_use]
attribute is an indicator that it is a mistake to
ignore the value. See the reference for more details.
unused-mut
The unused_mut
lint detects mut variables which don't need to be
mutable.
Example
#![allow(unused)] fn main() { let mut x = 5; }
This will produce:
warning: variable does not need to be mutable
--> lint_example.rs:2:5
|
2 | let mut x = 5;
| ----^
| |
| help: remove this `mut`
|
= note: `#[warn(unused_mut)]` on by default
Explanation
The preferred style is to only mark variables as mut
if it is
required.
unused-parens
The unused_parens
lint detects if
, match
, while
and return
with parentheses; they do not need them.
Examples
#![allow(unused)] fn main() { if(true) {} }
This will produce:
warning: unnecessary parentheses around `if` condition
--> lint_example.rs:2:3
|
2 | if(true) {}
| ^ ^
|
= note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
|
2 - if(true) {}
2 + if true {}
|
Explanation
The parentheses are not needed, and should be removed. This is the preferred style for writing these expressions.
unused-unsafe
The unused_unsafe
lint detects unnecessary use of an unsafe
block.
Example
#![allow(unused)] fn main() { unsafe {} }
This will produce:
warning: unnecessary `unsafe` block
--> lint_example.rs:2:1
|
2 | unsafe {}
| ^^^^^^ unnecessary `unsafe` block
|
= note: `#[warn(unused_unsafe)]` on by default
Explanation
If nothing within the block requires unsafe
, then remove the
unsafe
marker because it is not required and may cause confusion.
unused-variables
The unused_variables
lint detects variables which are not used in
any way.
Example
#![allow(unused)] fn main() { let x = 5; }
This will produce:
warning: unused variable: `x`
--> lint_example.rs:2:5
|
2 | let x = 5;
| ^ help: if this is intentional, prefix it with an underscore: `_x`
|
= note: `#[warn(unused_variables)]` on by default
Explanation
Unused variables may signal a mistake or unfinished code. To silence
the warning for the individual variable, prefix it with an underscore
such as _x
.
warnings
The warnings
lint allows you to change the level of other
lints which produce warnings.
Example
#![allow(unused)] #![deny(warnings)] fn main() { fn foo() {} }
This will produce:
error: function `foo` is never used
--> lint_example.rs:3:4
|
3 | fn foo() {}
| ^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(warnings)]
| ^^^^^^^^
= note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`
Explanation
The warnings
lint is a bit special; by changing its level, you
change every other warning that would produce a warning to whatever
value you'd like. As such, you won't ever trigger this lint in your
code directly.
where-clauses-object-safety
The where_clauses_object_safety
lint detects for object safety of
where clauses.
Example
trait Trait {} trait X { fn foo(&self) where Self: Trait; } impl X for () { fn foo(&self) {} } impl Trait for dyn X {} // Segfault at opt-level 0, SIGILL otherwise. pub fn main() { <dyn X as X>::foo(&()); }
This will produce:
warning: the trait `X` cannot be made into an object
--> lint_example.rs:3:14
|
3 | trait X { fn foo(&self) where Self: Trait; }
| ^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> lint_example.rs:3:14
|
3 | trait X { fn foo(&self) where Self: Trait; }
| - ^^^ ...because method `foo` references the `Self` type in its `where` clause
| |
| this trait cannot be made into an object...
= help: consider moving `foo` to another trait
= note: `#[warn(where_clauses_object_safety)]` on by default
Explanation
The compiler previously allowed these object-unsafe bounds, which was incorrect. This is a future-incompatible lint to transition this to a hard error in the future. See issue #51443 for more details.
while-true
The while_true
lint detects while true { }
.
Example
#![allow(unused)] fn main() { while true { } }
This will produce:
warning: denote infinite loops with `loop { ... }`
--> lint_example.rs:2:1
|
2 | while true {
| ^^^^^^^^^^ help: use `loop`
|
= note: `#[warn(while_true)]` on by default
Explanation
while true
should be replaced with loop
. A loop
expression is
the preferred way to write an infinite loop because it more directly
expresses the intent of the loop.