SemVer Compatibility

This chapter provides details on what is conventionally considered a compatible or breaking SemVer change for new releases of a package. See the SemVer compatibility section for details on what SemVer is, and how Cargo uses it to ensure compatibility of libraries.

These are only guidelines, and not necessarily hard-and-fast rules that all projects will obey. The Change categories section details how this guide classifies the level and severity of a change. Most of this guide focuses on changes that will cause cargo and rustc to fail to build something that previously worked. Almost every change carries some risk that it will negatively affect the runtime behavior, and for those cases it is usually a judgment call by the project maintainers whether or not it is a SemVer-incompatible change.

See also rust-semverver, which is an experimental tool that attempts to programmatically check compatibility rules.

Change categories

All of the policies listed below are categorized by the level of change:

Major change: a change that requires a major SemVer bump.
Minor change: a change that requires only a minor SemVer bump.
Possibly-breaking change: a change that some projects may consider major and others consider minor.

The "Possibly-breaking" category covers changes that have the potential to break during an update, but may not necessarily cause a breakage. The impact of these changes should be considered carefully. The exact nature will depend on the change and the principles of the project maintainers.

Some projects may choose to only bump the patch number on a minor change. It is encouraged to follow the SemVer spec, and only apply bug fixes in patch releases. However, a bug fix may require an API change that is marked as a "minor change", and shouldn't affect compatibility. This guide does not take a stance on how each individual "minor change" should be treated, as the difference between minor and patch changes are conventions that depend on the nature of the change.

Some changes are marked as "minor", even though they carry the potential risk of breaking a build. This is for situations where the potential is extremely low, and the potentially breaking code is unlikely to be written in idiomatic Rust, or is specifically discouraged from use.

This guide uses the terms "major" and "minor" assuming this relates to a "1.0.0" release or later. Initial development releases starting with "0.y.z" can treat changes in "y" as a major release, and "z" as a minor release. "0.0.z" releases are always major changes. This is because Cargo uses the convention that only changes in the left-most non-zero component are considered incompatible.

API compatibility
Tooling and environment compatibility
Application compatibility

API compatibility

All of the examples below contain three parts: the original code, the code after it has been modified, and an example usage of the code that could appear in another project. In a minor change, the example usage should successfully build with both the before and after versions.

Major: renaming/moving/removing any public items

The absence of a publicly exposed item will cause any uses of that item to fail to compile.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo() {}

///////////////////////////////////////////////////////////
// After
// ... item has been removed

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    updated_crate::foo(); // Error: cannot find function `foo`
}

This includes adding any sort of cfg attribute which can change which items or behavior is available based on conditional compilation.

Mitigating strategies:

Mark items to be removed as deprecated, and then remove them at a later date in a SemVer-breaking release.
Mark renamed items as deprecated, and use a pub use item to re-export to the old name.

Minor: adding new public items

Adding new, public items is a minor change.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
// ... absence of item

///////////////////////////////////////////////////////////
// After
pub fn foo() {}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
// `foo` is not used since it didn't previously exist.

Note that in some rare cases this can be a breaking change due to glob imports. For example, if you add a new trait, and a project has used a glob import that brings that trait into scope, and the new trait introduces an associated item that conflicts with any types it is implemented on, this can cause a compile-time error due to the ambiguity. Example:

// Breaking change example

///////////////////////////////////////////////////////////
// Before
// ... absence of trait

///////////////////////////////////////////////////////////
// After
pub trait NewTrait {
    fn foo(&self) {}
}

impl NewTrait for i32 {}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::*;

pub trait LocalTrait {
    fn foo(&self) {}
}

impl LocalTrait for i32 {}

fn main() {
    123i32.foo(); // Error:  multiple applicable items in scope
}

This is not considered a major change because conventionally glob imports are a known forwards-compatibility hazard. Glob imports of items from external crates should be avoided.

Major: adding a private struct field when all current fields are public

When a private field is added to a struct that previously had all public fields, this will break any code that attempts to construct it with a struct literal.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo {
    pub f1: i32,
}

///////////////////////////////////////////////////////////
// After
pub struct Foo {
    pub f1: i32,
    f2: i32,
}

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    let x = updated_crate::Foo { f1: 123 }; // Error: cannot construct `Foo`
}

Mitigation strategies:

Do not add new fields to all-public field structs.
Mark structs as #[non_exhaustive] when first introducing a struct to prevent users from using struct literal syntax, and instead provide a constructor method and/or Default implementation.

Major: adding a public field when no private field exists

When a public field is added to a struct that has all public fields, this will break any code that attempts to construct it with a struct literal.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo {
    pub f1: i32,
}

///////////////////////////////////////////////////////////
// After
pub struct Foo {
    pub f1: i32,
    pub f2: i32,
}

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    let x = updated_crate::Foo { f1: 123 }; // Error: missing field `f2`
}

Mitigation strategies:

Do not add new new fields to all-public field structs.
Mark structs as #[non_exhaustive] when first introducing a struct to prevent users from using struct literal syntax, and instead provide a constructor method and/or Default implementation.

Minor: adding or removing private fields when at least one already exists

It is safe to add or remove private fields from a struct when the struct already has at least one private field.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
#[derive(Default)]
pub struct Foo {
    f1: i32,
}

///////////////////////////////////////////////////////////
// After
#[derive(Default)]
pub struct Foo {
    f2: f64,
}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
fn main() {
    // Cannot access private fields.
    let x = updated_crate::Foo::default();
}

This is safe because existing code cannot use a struct literal to construct it, nor exhaustively match its contents.

Note that for tuple structs, this is a major change if the tuple contains public fields, and the addition or removal of a private field changes the index of any public field.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
#[derive(Default)]
pub struct Foo(pub i32, i32);

///////////////////////////////////////////////////////////
// After
#[derive(Default)]
pub struct Foo(f64, pub i32, i32);

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    let x = updated_crate::Foo::default();
    let y = x.0; // Error: is private
}

Minor: going from a tuple struct with all private fields (with at least one field) to a normal struct, or vice versa

Changing a tuple struct to a normal struct (or vice-versa) is safe if all fields are private.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
#[derive(Default)]
pub struct Foo(i32);

///////////////////////////////////////////////////////////
// After
#[derive(Default)]
pub struct Foo {
    f1: i32,
}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
fn main() {
    // Cannot access private fields.
    let x = updated_crate::Foo::default();
}

This is safe because existing code cannot use a struct literal to construct it, nor match its contents.

Major: adding new enum variants (without `non_exhaustive`)

It is a breaking change to add a new enum variant if the enum does not use the #[non_exhaustive] attribute.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub enum E {
    Variant1,
}

///////////////////////////////////////////////////////////
// After
pub enum E {
    Variant1,
    Variant2,
}

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    use updated_crate::E;
    let x = E::Variant1;
    match x { // Error: `Variant2` not covered
        E::Variant1 => {}
    }
}

Mitigation strategies:

When introducing the enum, mark it as #[non_exhaustive] to force users to use wildcard patterns to catch new variants.

Major: adding new fields to an enum variant

It is a breaking change to add new fields to an enum variant because all fields are public, and constructors and matching will fail to compile.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub enum E {
    Variant1 { f1: i32 },
}

///////////////////////////////////////////////////////////
// After
pub enum E {
    Variant1 { f1: i32, f2: i32 },
}

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    use updated_crate::E;
    let x = E::Variant1 { f1: 1 }; // Error: missing f2
    match x {
        E::Variant1 { f1 } => {} // Error: missing f2
    }
}

Mitigation strategies:

When introducing the enum, mark the variant as non_exhaustive so that it cannot be constructed or matched without wildcards.
```
pub enum E {
    #[non_exhaustive]
    Variant1{f1: i32}
}
```
When introducing the enum, use an explicit struct as a value, where you can have control over the field visibility.
```
pub struct Foo {
   f1: i32,
   f2: i32,
}
pub enum E {
    Variant1(Foo)
}
```

Major: adding a non-defaulted trait item

It is a breaking change to add a non-defaulted item to a trait. This will break any implementors of the trait.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}

///////////////////////////////////////////////////////////
// After
pub trait Trait {
    fn foo(&self);
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Trait;
struct Foo;

impl Trait for Foo {}  // Error: not all trait items implemented

Mitigation strategies:

Always provide a default implementation or value for new associated trait items.
When introducing the trait, use the sealed trait technique to prevent users outside of the crate from implementing the trait.

Major: any change to trait item signatures

It is a breaking change to make any change to a trait item signature. This can break external implementors of the trait.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {
    fn f(&self, x: i32) {}
}

///////////////////////////////////////////////////////////
// After
pub trait Trait {
    // For sealed traits or normal functions, this would be a minor change
    // because generalizing with generics strictly expands the possible uses.
    // But in this case, trait implementations must use the same signature.
    fn f<V>(&self, x: V) {}
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Trait;
struct Foo;

impl Trait for Foo {
    fn f(&self, x: i32) {}  // Error: trait declaration has 1 type parameter
}

Mitigation strategies:

Introduce new items with default implementations to cover the new functionality instead of modifying existing items.
When introducing the trait, use the sealed trait technique to prevent users outside of the crate from implementing the trait.

Possibly-breaking: adding a defaulted trait item

It is usually safe to add a defaulted trait item. However, this can sometimes cause a compile error. For example, this can introduce an ambiguity if a method of the same name exists in another trait.

// Breaking change example

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}

///////////////////////////////////////////////////////////
// After
pub trait Trait {
    fn foo(&self) {}
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Trait;
struct Foo;

trait LocalTrait {
    fn foo(&self) {}
}

impl Trait for Foo {}
impl LocalTrait for Foo {}

fn main() {
    let x = Foo;
    x.foo(); // Error: multiple applicable items in scope
}

Note that this ambiguity does not exist for name collisions on inherent implementations, as they take priority over trait items.

See trait-object-safety for a special case to consider when adding trait items.

Mitigation strategies:

Some projects may deem this acceptable breakage, particularly if the new item name is unlikely to collide with any existing code. Choose names carefully to help avoid these collisions. Additionally, it may be acceptable to require downstream users to add disambiguation syntax to select the correct function when updating the dependency.

Major: adding a trait item that makes the trait non-object safe

It is a breaking change to add a trait item that changes the trait to not be object safe.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}

///////////////////////////////////////////////////////////
// After
pub trait Trait {
    // An associated const makes the trait not object-safe.
    const CONST: i32 = 123;
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Trait;
struct Foo;

impl Trait for Foo {}

fn main() {
    let obj: Box<dyn Trait> = Box::new(Foo); // Error: cannot be made into an object
}

It is safe to do the converse (making a non-object safe trait into a safe one).

Major: adding a type parameter without a default

It is a breaking change to add a type parameter without a default to a trait.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}

///////////////////////////////////////////////////////////
// After
pub trait Trait<T> {}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Trait;
struct Foo;

impl Trait for Foo {}  // Error: missing generics

Mitigating strategies:

See adding a defaulted trait type parameter.

Minor: adding a defaulted trait type parameter

It is safe to add a type parameter to a trait as long as it has a default. External implementors will use the default without needing to specify the parameter.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}

///////////////////////////////////////////////////////////
// After
pub trait Trait<T = i32> {}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::Trait;
struct Foo;

impl Trait for Foo {}

Possibly-breaking change: adding any inherent items

Usually adding inherent items to an implementation should be safe because inherent items take priority over trait items. However, in some cases the collision can cause problems if the name is the same as an implemented trait item with a different signature.

// Breaking change example

///////////////////////////////////////////////////////////
// Before
pub struct Foo;

///////////////////////////////////////////////////////////
// After
pub struct Foo;

impl Foo {
    pub fn foo(&self) {}
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Foo;

trait Trait {
    fn foo(&self, x: i32) {}
}

impl Trait for Foo {}

fn main() {
    let x = Foo;
    x.foo(1); // Error: this function takes 0 arguments
}

Note that if the signatures match, there would not be a compile-time error, but possibly a silent change in runtime behavior (because it is now executing a different function).

Mitigation strategies:

Some projects may deem this acceptable breakage, particularly if the new item name is unlikely to collide with any existing code. Choose names carefully to help avoid these collisions. Additionally, it may be acceptable to require downstream users to add disambiguation syntax to select the correct function when updating the dependency.

Major: tightening generic bounds

It is a breaking change to tighten generic bounds on a type since this can break users expecting the looser bounds.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo<A> {
    pub f1: A,
}

///////////////////////////////////////////////////////////
// After
pub struct Foo<A: Eq> {
    pub f1: A,
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Foo;

fn main() {
    let s = Foo { f1: 1.23 }; // Error: the trait bound `{float}: Eq` is not satisfied
}

Minor: loosening generic bounds

It is safe to loosen the generic bounds on a type, as it only expands what is allowed.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo<A: Clone> {
    pub f1: A,
}

///////////////////////////////////////////////////////////
// After
pub struct Foo<A> {
    pub f1: A,
}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::Foo;

fn main() {
    let s = Foo { f1: 123 };
}

Minor: adding defaulted type parameters

It is safe to add a type parameter to a type as long as it has a default. All existing references will use the default without needing to specify the parameter.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
#[derive(Default)]
pub struct Foo {}

///////////////////////////////////////////////////////////
// After
#[derive(Default)]
pub struct Foo<A = i32> {
    f1: A,
}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::Foo;

fn main() {
    let s: Foo = Default::default();
}

Minor: generalizing a type to use generics (with identical types)

A struct or enum field can change from a concrete type to a generic type parameter, provided that the change results in an identical type for all existing use cases. For example, the following change is permitted:

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo(pub u8);

///////////////////////////////////////////////////////////
// After
pub struct Foo<T = u8>(pub T);

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::Foo;

fn main() {
    let s: Foo = Foo(123);
}

because existing uses of Foo are shorthand for Foo<u8> which yields the identical field type.

Major: generalizing a type to use generics (with possibly different types)

Changing a struct or enum field from a concrete type to a generic type parameter can break if the type can change.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo<T = u8>(pub T, pub u8);

///////////////////////////////////////////////////////////
// After
pub struct Foo<T = u8>(pub T, pub T);

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::Foo;

fn main() {
    let s: Foo<f32> = Foo(3.14, 123); // Error: mismatched types
}

Minor: changing a generic type to a more generic type

It is safe to change a generic type to a more generic one. For example, the following adds a generic parameter that defaults to the original type, which is safe because all existing users will be using the same type for both fields, the the defaulted parameter does not need to be specified.

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub struct Foo<T>(pub T, pub T);

///////////////////////////////////////////////////////////
// After
pub struct Foo<T, U = T>(pub T, pub U);

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::Foo;

fn main() {
    let s: Foo<f32> = Foo(1.0, 2.0);
}

Major: adding/removing function parameters

Changing the arity of a function is a breaking change.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo() {}

///////////////////////////////////////////////////////////
// After
pub fn foo(x: i32) {}

///////////////////////////////////////////////////////////
// Example usage that will break.
fn main() {
    updated_crate::foo(); // Error: this function takes 1 argument
}

Mitigating strategies:

Introduce a new function with the new signature and possibly deprecate the old one.
Introduce functions that take a struct argument, where the struct is built with the builder pattern. This allows new fields to be added to the struct in the future.

Possibly-breaking: introducing a new function type parameter

Usually, adding a non-defaulted type parameter is safe, but in some cases it can be a breaking change:

// Breaking change example

///////////////////////////////////////////////////////////
// Before
pub fn foo<T>() {}

///////////////////////////////////////////////////////////
// After
pub fn foo<T, U>() {}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::foo;

fn main() {
    foo::<u8>(); // Error: this function takes 2 generic arguments but 1 generic argument was supplied
}

However, such explicit calls are rare enough (and can usually be written in other ways) that this breakage is usually acceptable. One should take into account how likely it is that the function in question is being called with explicit type arguments.

Minor: generalizing a function to use generics (supporting original type)

The type of a parameter to a function, or its return value, can be generalized to use generics, including by introducing a new type parameter, as long as it can be instantiated to the original type. For example, the following changes are allowed:

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo(x: u8) -> u8 {
    x
}
pub fn bar<T: Iterator<Item = u8>>(t: T) {}

///////////////////////////////////////////////////////////
// After
use std::ops::Add;
pub fn foo<T: Add>(x: T) -> T {
    x
}
pub fn bar<T: IntoIterator<Item = u8>>(t: T) {}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::{bar, foo};

fn main() {
    foo(1);
    bar(vec![1, 2, 3].into_iter());
}

because all existing uses are instantiations of the new signature.

Perhaps somewhat surprisingly, generalization applies to trait objects as well, given that every trait implements itself:

// MINOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub trait Trait {}
pub fn foo(t: &dyn Trait) {}

///////////////////////////////////////////////////////////
// After
pub trait Trait {}
pub fn foo<T: Trait + ?Sized>(t: &T) {}

///////////////////////////////////////////////////////////
// Example use of the library that will safely work.
use updated_crate::{foo, Trait};

struct Foo;
impl Trait for Foo {}

fn main() {
    let obj = Foo;
    foo(&obj);
}

(The use of ?Sized is essential; otherwise you couldn't recover the original signature.)

Introducing generics in this way can potentially create type inference failures. These are usually rare, and may be acceptable breakage for some projects, as this can be fixed with additional type annotations.

// Breaking change example

///////////////////////////////////////////////////////////
// Before
pub fn foo() -> i32 {
    0
}

///////////////////////////////////////////////////////////
// After
pub fn foo<T: Default>() -> T {
    Default::default()
}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::foo;

fn main() {
    let x = foo(); // Error: type annotations needed
}

Major: generalizing a function to use generics with type mismatch

It is a breaking change to change a function parameter or return type if the generic type constrains or changes the types previously allowed. For example, the following adds a generic constraint that may not be satisfied by existing code:

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
pub fn foo(x: Vec<u8>) {}

///////////////////////////////////////////////////////////
// After
pub fn foo<T: Copy + IntoIterator<Item = u8>>(x: T) {}

///////////////////////////////////////////////////////////
// Example usage that will break.
use updated_crate::foo;

fn main() {
    foo(vec![1, 2, 3]); // Error: `Copy` is not implemented for `Vec<u8>`
}

Major: switching from `no_std` support to requiring `std`

If your library specifically supports a no_std environment, it is a breaking change to make a new release that requires std.

// MAJOR CHANGE

///////////////////////////////////////////////////////////
// Before
#![no_std]
pub fn foo() {}

///////////////////////////////////////////////////////////
// After
pub fn foo() {
    std::time::SystemTime::now();
}

///////////////////////////////////////////////////////////
// Example usage that will break.
// This will fail to link for no_std targets because they don't have a `std` crate.
#![no_std]
use updated_crate::foo;

fn example() {
    foo();
}

Mitigation strategies:

A common idiom to avoid this is to include a std Cargo feature that optionally enables std support, and when the feature is off, the library can be used in a no_std environment.

Tooling and environment compatibility

Possibly-breaking: changing the minimum version of Rust required

Introducing the use of new features in a new release of Rust can break projects that are using older versions of Rust. This also includes using new features in a new release of Cargo, and requiring the use of a nightly-only feature in a crate that previously worked on stable.

Some projects choose to allow this in a minor release for various reasons. It is usually relatively easy to update to a newer version of Rust. Rust also has a rapid 6-week release cycle, and some projects will provide compatibility within a window of releases (such as the current stable release plus N previous releases). Just keep in mind that some large projects may not be able to update their Rust toolchain rapidly.

Mitigation strategies:

Use Cargo features to make the new features opt-in.
Provide a large window of support for older releases.
Copy the source of new standard library items if possible so that you can continue to use an older version but take advantage of the new feature.
Provide a separate branch of older minor releases that can receive backports of important bugfixes.
Keep an eye out for the [cfg(version(..))] and #[cfg(accessible(..))] features which provide an opt-in mechanism for new features. These are currently unstable and only available in the nightly channel.

Possibly-breaking: changing the platform and environment requirements

There is a very wide range of assumptions a library makes about the environment that it runs in, such as the host platform, operating system version, available services, filesystem support, etc. It can be a breaking change if you make a new release that restricts what was previously supported, for example requiring a newer version of an operating system. These changes can be difficult to track, since you may not always know if a change breaks in an environment that is not automatically tested.

Some projects may deem this acceptable breakage, particularly if the breakage is unlikely for most users, or the project doesn't have the resources to support all environments. Another notable situation is when a vendor discontinues support for some hardware or OS, the project may deem it reasonable to also discontinue support.

Mitigation strategies:

Document the platforms and environments you specifically support.
Test your code on a wide range of environments in CI.

Cargo

Minor: adding a new Cargo feature

It is usually safe to add new Cargo features. If the feature introduces new changes that cause a breaking change, this can cause difficulties for projects that have stricter backwards-compatibility needs. In that scenario, avoid adding the feature to the "default" list, and possibly document the consequences of enabling the feature.

# MINOR CHANGE

###########################################################
# Before
[features]
# ..empty

###########################################################
# After
[features]
std = []

Major: removing a Cargo feature

It is usually a breaking change to remove Cargo features. This will cause an error for any project that enabled the feature.

# MAJOR CHANGE

###########################################################
# Before
[features]
logging = []

###########################################################
# After
[dependencies]
# ..logging removed

Mitigation strategies:

Clearly document your features. If there is an internal or experimental feature, mark it as such, so that users know the status of the feature.
Leave the old feature in Cargo.toml, but otherwise remove its functionality. Document that the feature is deprecated, and remove it in a future major SemVer release.

Major: removing a feature from a feature list if that changes functionality or public items

If removing a feature from another feature, this can break existing users if they are expecting that functionality to be available through that feature.

# Breaking change example

###########################################################
# Before
[features]
default = ["std"]
std = []

###########################################################
# After
[features]
default = []  # This may cause packages to fail if they are expecting std to be enabled.
std = []

Possibly-breaking: removing an optional dependency

Removing an optional dependency can break a project using your library because another project may be enabling that dependency via Cargo features.

# Breaking change example

###########################################################
# Before
[dependencies]
curl = { version = "0.4.31", optional = true }

###########################################################
# After
[dependencies]
# ..curl removed

Mitigation strategies:

Clearly document your features. If the optional dependency is not included in the documented list of features, then you may decide to consider it safe to change undocumented entries.
Leave the optional dependency, and just don't use it within your library.
Replace the optional dependency with a Cargo feature that does nothing, and document that it is deprecated.
Use high-level features which enable optional dependencies, and document those as the preferred way to enable the extended functionality. For example, if your library has optional support for something like "networking", create a generic feature name "networking" that enables the optional dependencies necessary to implement "networking". Then document the "networking" feature.

Minor: changing dependency features

It is usually safe to change the features on a dependency, as long as the feature does not introduce a breaking change.

# MINOR CHANGE

###########################################################
# Before
[dependencies]
rand = { version = "0.7.3", features = ["small_rng"] }


###########################################################
# After
[dependencies]
rand = "0.7.3"

Minor: adding dependencies

It is usually safe to add new dependencies, as long as the new dependency does not introduce new requirements that result in a breaking change. For example, adding a new dependency that requires nightly in a project that previously worked on stable is a major change.

# MINOR CHANGE

###########################################################
# Before
[dependencies]
# ..empty

###########################################################
# After
[dependencies]
log = "0.4.11"

Application compatibility

Cargo projects may also include executable binaries which have their own interfaces (such as a CLI interface, OS-level interaction, etc.). Since these are part of the Cargo package, they often use and share the same version as the package. You will need to decide if and how you want to employ a SemVer contract with your users in the changes you make to your application. The potential breaking and compatible changes to an application are too numerous to list, so you are encouraged to use the spirit of the SemVer spec to guide your decisions on how to apply versioning to your application, or at least document what your commitments are.

The Cargo Book