Static rustc_lint::let_underscore::LET_UNDERSCORE_DROP
source · pub static LET_UNDERSCORE_DROP: &Lint
Expand description
The let_underscore_drop
lint checks for statements which don’t bind
an expression which has a non-trivial Drop implementation to anything,
causing the expression to be dropped immediately instead of at end of
scope.
Example
struct SomeStruct;
impl Drop for SomeStruct {
fn drop(&mut self) {
println!("Dropping SomeStruct");
}
}
fn main() {
#[warn(let_underscore_drop)]
// SomeStruct is dropped immediately instead of at end of scope,
// so "Dropping SomeStruct" is printed before "end of main".
// The order of prints would be reversed if SomeStruct was bound to
// a name (such as "_foo").
let _ = SomeStruct;
println!("end of main");
}
{{produces}}
Explanation
Statements which assign an expression to an underscore causes the
expression to immediately drop instead of extending the expression’s
lifetime to the end of the scope. This is usually unintended,
especially for types like MutexGuard
, which are typically used to
lock a mutex for the duration of an entire scope.
If you want to extend the expression’s lifetime to the end of the scope,
assign an underscore-prefixed name (such as _foo
) to the expression.
If you do actually want to drop the expression immediately, then
calling std::mem::drop
on the expression is clearer and helps convey
intent.