SemVer Compatibility - The Cargo Book

# 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.

## 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
  - Items
    - Major: renaming/moving/removing any public items
    - Minor: adding new public items
  - Types
    - Major: Changing the alignment, layout, or size of a well-defined type
  - Structs
    - Major: adding a private struct field when all current fields are public
    - Major: adding a public field when no private field exists
    - Minor: adding or removing private fields when at least one already exists
    - Minor: going from a tuple struct with all private fields (with at least one field) to a normal struct, or vice versa
  - Enums
    - Major: adding new enum variants (without `non_exhaustive`)
    - Major: adding new fields to an enum variant
  - Traits
    - Major: adding a non-defaulted trait item
    - Major: any change to trait item signatures
    - Possibly-breaking: adding a defaulted trait item
    - Major: adding a trait item that makes the trait non-object safe
    - Major: adding a type parameter without a default
    - Minor: adding a defaulted trait type parameter
  - Implementations
    - Possibly-breaking change: adding any inherent items
  - Generics
    - Major: tightening generic bounds
    - Minor: loosening generic bounds
    - Minor: adding defaulted type parameters
    - Minor: generalizing a type to use generics (with identical types)
    - Major: generalizing a type to use generics (with possibly different types)
    - Minor: changing a generic type to a more generic type
    - Major: capturing more generic parameters in RPIT
  - Functions
    - Major: adding/removing function parameters
    - Possibly-breaking: introducing a new function type parameter
    - Minor: generalizing a function to use generics (supporting original type)
    - Major: generalizing a function to use generics with type mismatch
    - Minor: making an `unsafe` function safe
  - Attributes
    - Major: switching from `no_std` support to requiring `std`
    - Major: adding `non_exhaustive` to an existing enum, variant, or struct with no private fields
- Tooling and environment compatibility
  - Possibly-breaking: changing the minimum version of Rust required
  - Possibly-breaking: changing the platform and environment requirements
  - Minor: introducing new lints
  - Cargo
    - Minor: adding a new Cargo feature
    - Major: removing a Cargo feature
    - Major: removing a feature from a feature list if that changes functionality or public items
    - Possibly-breaking: removing an optional dependency
    - Minor: changing dependency features
    - Minor: adding dependencies
- 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: Changing the alignment, layout, or size of a well-defined type

It is a breaking change to change the alignment, layout, or size of a type that was previously well-defined.

In general, types that use the the default representation do not have a well-defined alignment, layout, or size. The compiler is free to alter the alignment, layout, or size, so code should not make any assumptions about it.

> Note: It may be possible for external crates to break if they make assumptions about the alignment, layout, or size of a type even if it is not well-defined. This is not considered a SemVer breaking change since those assumptions should not be made.

Some examples of changes that are not a breaking change are (assuming no other rules in this guide are violated):

- Adding, removing, reordering, or changing fields of a default representation struct, union, or enum in such a way that the change follows the other rules in this guide (for example, using `non_exhaustive` to allow those changes, or changes to private fields that are already private). See struct-add-private-field-when-public, struct-add-public-field-when-no-private, struct-private-fields-with-private, enum-fields-new.
- Adding variants to a default representation enum, if the enum uses `non_exhaustive`. This may change the alignment or size of the enumeration, but those are not well-defined. See enum-variant-new.
- Adding, removing, reordering, or changing private fields of a `repr(C)` struct, union, or enum, following the other rules in this guide (for example, using `non_exhaustive`, or adding private fields when other private fields already exist). See repr-c-private-change.
- Adding variants to a `repr(C)` enum, if the enum uses `non_exhaustive`. See repr-c-enum-variant-new.
- Adding `repr(C)` to a default representation struct, union, or enum. See repr-c-add.
- Adding `repr(<int>)` primitive representation to an enum. See repr-int-enum-add.
- Adding `repr(transparent)` to a default representation struct or enum. See repr-transparent-add.

Types that use the `repr` attribute can be said to have an alignment and layout that is defined in some way that code may make some assumptions about that may break as a result of changing that type.

In some cases, types with a `repr` attribute may not have an alignment, layout, or size that is well-defined. In these cases, it may be safe to make changes to the types, though care should be exercised. For example, types with private fields that do not otherwise document their alignment, layout, or size guarantees cannot be relied upon by external crates since the public API does not fully define the alignment, layout, or size of the type.

A common example where a type with private fields is well-defined is a type with a single private field with a generic type, using `repr(transparent)`, and the prose of the documentation discusses that it is transparent to the generic type. For example, see `UnsafeCell`.

Some examples of breaking changes are:

- Adding `repr(packed)` to a struct or union. See repr-packed-add.
- Adding `repr(align)` to a struct, union, or enum. See repr-align-add.
- Removing `repr(packed)` from a struct or union. See repr-packed-remove.
- Changing the value N of `repr(packed(N))` if that changes the alignment or layout. See repr-packed-n-change.
- Changing the value N of `repr(align(N))` if that changes the alignment. See repr-align-n-change.
- Removing `repr(align)` from a struct, union, or enum. See repr-align-remove.
- Changing the order of public fields of a `repr(C)` type. See repr-c-shuffle.
- Removing `repr(C)` from a struct, union, or enum. See repr-c-remove.
- Removing `repr(<int>)` from an enum. See repr-int-enum-remove.
- Changing the primitive representation of a `repr(<int>)` enum. See repr-int-enum-change.
- Removing `repr(transparent)` from a struct or enum. See repr-transparent-remove.

#### Minor: `repr(C)` add, remove, or change a private field

It is usually safe to add, remove, or change a private field of a `repr(C)` struct, union, or enum, assuming it follows the other guidelines in this guide (see struct-add-private-field-when-public, struct-add-public-field-when-no-private, struct-private-fields-with-private, enum-fields-new).

For example, adding private fields can only be done if there are already other private fields, or it is `non_exhaustive`. Public fields may be added if there are private fields, or it is `non_exhaustive`, and the addition does not alter the layout of the other fields.

However, this may change the size and alignment of the type. Care should be taken if the size or alignment changes. Code should not make assumptions about the size or alignment of types with private fields or `non_exhaustive` unless it has a documented size or alignment.

    // MINOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[derive(Default)]
    #[repr(C)]
    pub struct Example {
        pub f1: i32,
        f2: i32, // a private field
    }

    ///////////////////////////////////////////////////////////
    // After
    #[derive(Default)]
    #[repr(C)]
    pub struct Example {
        pub f1: i32,
        f2: i32,
        f3: i32, // a new field
    }

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.
    fn main() {
        // NOTE: Users should not make assumptions about the size or alignment
        // since they are not documented.
        let f = updated_crate::Example::default();
    }

#### Minor: `repr(C)` add enum variant

It is usually safe to add variants to a `repr(C)` enum, if the enum uses `non_exhaustive`. See enum-variant-new for more discussion.

Note that this may be a breaking change since it changes the size and alignment of the type. See repr-c-private-change for similar concerns.

    // MINOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C)]
    #[non_exhaustive]
    pub enum Example {
        Variant1 { f1: i16 },
        Variant2 { f1: i32 },
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)]
    #[non_exhaustive]
    pub enum Example {
        Variant1 { f1: i16 },
        Variant2 { f1: i32 },
        Variant3 { f1: i64 }, // added
    }

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.
    fn main() {
        // NOTE: Users should not make assumptions about the size or alignment
        // since they are not specified. For example, this raised the size from 8
        // to 16 bytes.
        let f = updated_crate::Example::Variant2 { f1: 123 };
    }

#### Minor: Adding `repr(C)` to a default representation

It is safe to add `repr(C)` to a struct, union, or enum with the default representation. This is safe because users should not make assumptions about the alignment, layout, or size of types with the default representation.

    // MINOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub struct Example {
        pub f1: i32,
        pub f2: i16,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)] // added
    pub struct Example {
        pub f1: i32,
        pub f2: i16,
    }

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.
    fn main() {
        let f = updated_crate::Example { f1: 123, f2: 456 };
    }

#### Minor: Adding `repr(<int>)` to an enum

It is safe to add `repr(<int>)` primitive representation to an enum with the default representation. This is safe because users should not make assumptions about the alignment, layout, or size of an enum with the default representation.

    // MINOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub enum E {
        Variant1,
        Variant2(i32),
        Variant3 { f1: f64 },
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(i32)] // added
    pub enum E {
        Variant1,
        Variant2(i32),
        Variant3 { f1: f64 },
    }

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.
    fn main() {
        let x = updated_crate::E::Variant3 { f1: 1.23 };
    }

#### Minor: Adding `repr(transparent)` to a default representation struct or enum

It is safe to add `repr(transparent)` to a struct or enum with the default representation. This is safe because users should not make assumptions about the alignment, layout, or size of a struct or enum with the default representation.

    // MINOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[derive(Default)]
    pub struct Example<T>(T);

    ///////////////////////////////////////////////////////////
    // After
    #[derive(Default)]
    #[repr(transparent)] // added
    pub struct Example<T>(T);

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.
    fn main() {
        let x = updated_crate::Example::<i32>::default();
    }

#### Major: Adding `repr(packed)` to a struct or union

It is a breaking change to add `repr(packed)` to a struct or union. Making a type `repr(packed)` makes changes that can break code, such as being invalid to take a reference to a field, or causing truncation of disjoint closure captures.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub struct Example {
        pub f1: u8,
        pub f2: u16,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(packed)] // added
    pub struct Example {
        pub f1: u8,
        pub f2: u16,
    }

    ///////////////////////////////////////////////////////////
    // Example usage that will break.
    fn main() {
        let f = updated_crate::Example { f1: 1, f2: 2 };
        let x = &f.f2; // Error: reference to packed field is unaligned
    }

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub struct Example(pub i32, pub i32);

    ///////////////////////////////////////////////////////////
    // After
    #[repr(packed)]
    pub struct Example(pub i32, pub i32);

    ///////////////////////////////////////////////////////////
    // Example usage that will break.
    fn main() {
        let mut f = updated_crate::Example(123, 456);
        let c = || {
            // Without repr(packed), the closure precisely captures `&f.0`.
            // With repr(packed), the closure captures `&f` to avoid undefined behavior.
            let a = f.0;
        };
        f.1 = 789; // Error: cannot assign to `f.1` because it is borrowed
        c();
    }

#### Major: Adding `repr(align)` to a struct, union, or enum

It is a breaking change to add `repr(align)` to a struct, union, or enum. Making a type `repr(align)` would break any use of that type in a `repr(packed)` type because that combination is not allowed.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub struct Aligned {
        pub a: i32,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(align(8))] // added
    pub struct Aligned {
        pub a: i32,
    }

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

    #[repr(packed)]
    pub struct Packed { // Error: packed type cannot transitively contain a `#[repr(align)]` type
        f1: Aligned,
    }

    fn main() {
        let p = Packed {
            f1: Aligned { a: 123 },
        };
    }

#### Major: Removing `repr(packed)` from a struct or union

It is a breaking change to remove `repr(packed)` from a struct or union. This may change the alignment or layout that extern crates are relying on.

If any fields are public, then removing `repr(packed)` may change the way disjoint closure captures work. In some cases, this can cause code to break, similar to those outlined in the edition guide.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C, packed)]
    pub struct Packed {
        pub a: u8,
        pub b: u16,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)] // removed packed
    pub struct Packed {
        pub a: u8,
        pub b: u16,
    }

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

    fn main() {
        let p = Packed { a: 1, b: 2 };
        // Some assumption about the size of the type.
        // Without `packed`, this fails since the size is 4.
        const _: () = assert!(std::mem::size_of::<Packed>() == 3); // Error: evaluation of constant value failed
    }

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C, packed)]
    pub struct Packed {
        pub a: *mut i32,
        pub b: i32,
    }
    unsafe impl Send for Packed {}

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)] // removed packed
    pub struct Packed {
        pub a: *mut i32,
        pub b: i32,
    }
    unsafe impl Send for Packed {}

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

    fn main() {
        let mut x = 123;

        let p = Packed {
            a: &mut x as *mut i32,
            b: 456,
        };

        // When the structure was packed, the closure captures `p` which is Send.
        // When `packed` is removed, this ends up capturing `p.a` which is not Send.
        std::thread::spawn(move || unsafe {
            *(p.a) += 1; // Error: cannot be sent between threads safely
        });
    }

#### Major: Changing the value N of `repr(packed(N))` if that changes the alignment or layout

It is a breaking change to change the value of N of `repr(packed(N))` if that changes the alignment or layout. This may change the alignment or layout that external crates are relying on.

If the value `N` is lowered below the alignment of a public field, then that would break any code that attempts to take a reference of that field.

Note that some changes to `N` may not change the alignment or layout, for example increasing it when the current value is already equal to the natural alignment of the type.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(packed(4))]
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(packed(2))] // changed to 2
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

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

    fn main() {
        let p = Packed { a: 1, b: 2 };
        let x = &p.b; // Error: reference to packed field is unaligned
    }

#### Major: Changing the value N of `repr(align(N))` if that changes the alignment

It is a breaking change to change the value `N` of `repr(align(N))` if that changes the alignment. This may change the alignment that external crates are relying on.

This change should be safe to make if the type is not well-defined as discussed in type layout (such as having any private fields and having an undocumented alignment or layout).

Note that some changes to `N` may not change the alignment or layout, for example decreasing it when the current value is already equal to or less than the natural alignment of the type.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(align(8))]
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(align(4))] // changed to 4
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

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

    fn main() {
        let p = Packed { a: 1, b: 2 };
        // Some assumption about the size of the type.
        // The alignment has changed from 8 to 4.
        const _: () = assert!(std::mem::align_of::<Packed>() == 8); // Error: evaluation of constant value failed
    }

#### Major: Removing `repr(align)` from a struct, union, or enum

It is a breaking change to remove `repr(align)` from a struct, union, or enum, if their layout was well-defined. This may change the alignment or layout that external crates are relying on.

This change should be safe to make if the type is not well-defined as discussed in type layout (such as having any private fields and having an undocumented alignment).

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C, align(8))]
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)] // removed align
    pub struct Packed {
        pub a: u8,
        pub b: u32,
    }

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

    fn main() {
        let p = Packed { a: 1, b: 2 };
        // Some assumption about the size of the type.
        // The alignment has changed from 8 to 4.
        const _: () = assert!(std::mem::align_of::<Packed>() == 8); // Error: evaluation of constant value failed
    }

#### Major: Changing the order of public fields of a `repr(C)` type

It is a breaking change to change the order of public fields of a `repr(C)` type. External crates may be relying on the specific ordering of the fields.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C)]
    pub struct SpecificLayout {
        pub a: u8,
        pub b: u32,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(C)]
    pub struct SpecificLayout {
        pub b: u32, // changed order
        pub a: u8,
    }

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

    extern "C" {
        // This C function is assuming a specific layout defined in a C header.
        fn c_fn_get_b(x: &SpecificLayout) -> u32;
    }

    fn main() {
        let p = SpecificLayout { a: 1, b: 2 };
        unsafe { assert_eq!(c_fn_get_b(&p), 2) } // Error: value not equal to 2
    }

    mod cdep {
        // This simulates what would normally be something included from a build script.
        // This definition would be in a C header.
        #[repr(C)]
        pub struct SpecificLayout {
            pub a: u8,
            pub b: u32,
        }

        #[no_mangle]
        pub fn c_fn_get_b(x: &SpecificLayout) -> u32 {
            x.b
        }
    }

#### Major: Removing `repr(C)` from a struct, union, or enum

It is a breaking change to remove `repr(C)` from a struct, union, or enum. External crates may be relying on the specific layout of the type.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(C)]
    pub struct SpecificLayout {
        pub a: u8,
        pub b: u32,
    }

    ///////////////////////////////////////////////////////////
    // After
    // removed repr(C)
    pub struct SpecificLayout {
        pub a: u8,
        pub b: u32,
    }

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

    extern "C" {
        // This C function is assuming a specific layout defined in a C header.
        fn c_fn_get_b(x: &SpecificLayout) -> u32; // Error: is not FFI-safe
    }

    fn main() {
        let p = SpecificLayout { a: 1, b: 2 };
        unsafe { assert_eq!(c_fn_get_b(&p), 2) }
    }

    mod cdep {
        // This simulates what would normally be something included from a build script.
        // This definition would be in a C header.
        #[repr(C)]
        pub struct SpecificLayout {
            pub a: u8,
            pub b: u32,
        }

        #[no_mangle]
        pub fn c_fn_get_b(x: &SpecificLayout) -> u32 {
            x.b
        }
    }

#### Major: Removing `repr(<int>)` from an enum

It is a breaking change to remove `repr(<int>)` from an enum. External crates may be assuming that the discriminant is a specific size. For example, `std::mem::transmute` of an enum may fail.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(u16)]
    pub enum Example {
        Variant1,
        Variant2,
        Variant3,
    }

    ///////////////////////////////////////////////////////////
    // After
    // removed repr(u16)
    pub enum Example {
        Variant1,
        Variant2,
        Variant3,
    }

    ///////////////////////////////////////////////////////////
    // Example usage that will break.

    fn main() {
        let e = updated_crate::Example::Variant2;
        let i: u16 = unsafe { std::mem::transmute(e) }; // Error: cannot transmute between types of different sizes
    }

#### Major: Changing the primitive representation of a `repr(<int>)` enum

It is a breaking change to change the primitive representation of a `repr(<int>)` enum. External crates may be assuming that the discriminant is a specific size. For example, `std::mem::transmute` of an enum may fail.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(u16)]
    pub enum Example {
        Variant1,
        Variant2,
        Variant3,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[repr(u8)] // changed repr size
    pub enum Example {
        Variant1,
        Variant2,
        Variant3,
    }

    ///////////////////////////////////////////////////////////
    // Example usage that will break.

    fn main() {
        let e = updated_crate::Example::Variant2;
        let i: u16 = unsafe { std::mem::transmute(e) }; // Error: cannot transmute between types of different sizes
    }

#### Major: Removing `repr(transparent)` from a struct or enum

It is a breaking change to remove `repr(transparent)` from a struct or enum. External crates may be relying on the type having the alignment, layout, or size of the transparent field.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    #[repr(transparent)]
    pub struct Transparent<T>(T);

    ///////////////////////////////////////////////////////////
    // After
    // removed repr
    pub struct Transparent<T>(T);

    ///////////////////////////////////////////////////////////
    // Example usage that will break.
    #![deny(improper_ctypes)]
    use updated_crate::Transparent;

    extern "C" {
        fn c_fn() -> Transparent<f64>; // Error: is not FFI-safe
    }

    fn main() {}

### 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 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: `E::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 method takes 0 arguments but 1 argument was supplied
    }

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 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: capturing more generic parameters in RPIT

It is a breaking change to capture additional generic parameters in an RPIT (return-position impl trait).

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub fn f<'a, 'b>(x: &'a str, y: &'b str) -> impl Iterator<Item = char> + use<'a> {
        x.chars()
    }

    ///////////////////////////////////////////////////////////
    // After
    pub fn f<'a, 'b>(x: &'a str, y: &'b str) -> impl Iterator<Item = char> + use<'a, 'b> {
        x.chars().chain(y.chars())
    }

    ///////////////////////////////////////////////////////////
    // Example usage that will break.
    fn main() {
        let a = String::new();
        let b = String::new();
        let iter = updated_crate::f(&a, &b);
        drop(b); // Error: cannot move out of `b` because it is borrowed
    }

Adding generic parameters to an RPIT places additional constraints on how the resulting type may be used.

Note that there are implicit captures when the `use<>` syntax is not specified. In Rust 2021 and earlier editions, the lifetime parameters are only captured if they appear syntactically within a bound in the RPIT type signature. Starting in Rust 2024, all lifetime parameters are unconditionally captured. This means that starting in Rust 2024, the default is maximally compatible, requiring you to be explicit when you want to capture less, which is a SemVer commitment.

See the edition guide and the reference for more information on RPIT capturing.

It is a minor change to capture fewer generic parameters in an RPIT.

> Note: All in-scope type and const generic parameters must be either implicitly captured (no `+ use<…>` specified) or explicitly captured (must be listed in `+ use<…>`), and thus currently it is not allowed to change what is captured of those kinds of generics.

### 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: 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>`
    }

### Minor: making an `unsafe` function safe

A previously `unsafe` function can be made safe without breaking code.

Note however that it may cause the `unused_unsafe` lint to trigger as in the example below, which will cause local crates that have specified `#![deny(warnings)]` to stop compiling. Per introducing new lints, it is allowed for updates to introduce new warnings.

Going the other way (making a safe function `unsafe`) is a breaking change.

    // MINOR CHANGE

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

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

    ///////////////////////////////////////////////////////////
    // Example use of the library that will trigger a lint.
    use updated_crate::foo;

    unsafe fn bar(f: unsafe fn()) {
        f()
    }

    fn main() {
        unsafe { foo() }; // The `unused_unsafe` lint will trigger here
        unsafe { bar(foo) };
    }

Making a previously `unsafe` associated function or method on structs / enums safe is also a minor change, while the same is not true for associated function on traits (see any change to trait item signatures).

### 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.

### Major: adding `non_exhaustive` to an existing enum, variant, or struct with no private fields

Making items `#[non_exhaustive]` changes how they may be used outside the crate where they are defined:

- Non-exhaustive structs and enum variants cannot be constructed using struct literal syntax, including functional update syntax.
- Pattern matching on non-exhaustive structs requires `..` and matching on enums does not count towards exhaustiveness.
- Casting enum variants to their discriminant with `as` is not allowed.

Structs with private fields cannot be constructed using struct literal syntax regardless of whether `#[non_exhaustive]` is used. Adding `#[non_exhaustive]` to such a struct is not a breaking change.

    // MAJOR CHANGE

    ///////////////////////////////////////////////////////////
    // Before
    pub struct Foo {
        pub bar: usize,
    }

    pub enum Bar {
        X,
        Y(usize),
        Z { a: usize },
    }

    pub enum Quux {
        Var,
    }

    ///////////////////////////////////////////////////////////
    // After
    #[non_exhaustive]
    pub struct Foo {
        pub bar: usize,
    }

    pub enum Bar {
        #[non_exhaustive]
        X,

        #[non_exhaustive]
        Y(usize),

        #[non_exhaustive]
        Z { a: usize },
    }

    #[non_exhaustive]
    pub enum Quux {
        Var,
    }

    ///////////////////////////////////////////////////////////
    // Example usage that will break.
    use updated_crate::{Bar, Foo, Quux};

    fn main() {
        let foo = Foo { bar: 0 }; // Error: cannot create non-exhaustive struct using struct expression

        let bar_x = Bar::X; // Error: unit variant `X` is private
        let bar_y = Bar::Y(0); // Error: tuple variant `Y` is private
        let bar_z = Bar::Z { a: 0 }; // Error: cannot create non-exhaustive variant using struct expression

        let q = Quux::Var;
        match q {
            Quux::Var => 0,
            // Error: non-exhaustive patterns: `_` not covered
        };
    }

Mitigation strategies:

- Mark structs, enums, and enum variants as `#[non_exhaustive]` when first introducing them, rather than adding `#[non_exhaustive]` later on.

## 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.

It is generally recommended to treat this as a minor change, rather than as a major change, 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.

### Minor: introducing new lints

Some changes to a library may cause new lints to be triggered in users of that library. This should generally be considered a compatible change.

    // MINOR CHANGE

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

    ///////////////////////////////////////////////////////////
    // After
    #[deprecated]
    pub fn foo() {}

    ///////////////////////////////////////////////////////////
    // Example use of the library that will safely work.

    fn main() {
        updated_crate::foo(); // Warning: use of deprecated function
    }

Beware that it may be possible for this to technically cause a project to fail if they have explicitly denied the warning, and the updated crate is a direct dependency. Denying warnings should be done with care and the understanding that new lints may be introduced over time. However, library authors should be cautious about introducing new warnings and may want to consider the potential impact on their users.

The following lints are examples of those that may be introduced when updating a dependency:

- `deprecated` — Introduced when a dependency adds the `#[deprecated]` attribute to an item you are using.
- `unused_must_use` — Introduced when a dependency adds the `#[must_use]` attribute to an item where you are not consuming the result.
- `unused_unsafe` — Introduced when a dependency removes the `unsafe` qualifier from a function, and that is the only unsafe function called in an unsafe block.

Additionally, updating `rustc` to a new version may introduce new lints.

Transitive dependencies which introduce new lints should not usually cause a failure because Cargo uses `--cap-lints` to suppress all lints in dependencies.

Mitigating strategies:

- If you build with warnings denied, understand you may need to deal with resolving new warnings whenever you update your dependencies. If using RUSTFLAGS to pass `-Dwarnings`, also add the `-A` flag to allow lints that are likely to cause issues, such as `-Adeprecated`.
- Introduce deprecations behind a feature. For example `#[cfg_attr(feature = "deprecated", deprecated="use bar instead")]`. Then, when you plan to remove an item in a future SemVer breaking change, you can communicate with your users that they should enable the `deprecated` feature before updating to remove the use of the deprecated items. This allows users to choose when to respond to deprecations without needing to immediately respond to them. A downside is that it can be difficult to communicate to users that they need to take these manual steps to prepare for a major update.

### 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.

When there is an optional dependency, cargo implicitly defines a feature of the same name to provide a mechanism to enable the dependency and to check when it is enabled. This problem can be avoided by using the `dep:` syntax in the `[features]` table, which disables this implicit feature. Using `dep:` makes it possible to hide the existence of optional dependencies under more semantically-relevant names which can be more safely modified.

    # Breaking change example

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

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

    # MINOR CHANGE
    #
    # This example shows how to avoid breaking changes with optional dependencies.

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

    [features]
    networking = ["dep:curl"]

    ###########################################################
    # After
    [dependencies]
    # Here, one optional dependency was replaced with another.
    hyper = { version = "0.14.27", optional = true }

    [features]
    networking = ["dep:hyper"]

Mitigation strategies:

- Use the `dep:` syntax in the `[features]` table to avoid exposing optional dependencies in the first place. See optional dependencies for more information.
- 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.