• Feature Name: not applicable
  • Start Date: 2015-05-04
  • RFC PR: rust-lang/rfcs#1105
  • Rust Issue: N/A


This RFC proposes a comprehensive set of guidelines for which changes to stable APIs are considered breaking from a semver perspective, and which are not. These guidelines are intended for both the standard library and for the crates.io ecosystem.

This does not mean that the standard library should be completely free to make non-semver-breaking changes; there are sometimes still risks of ecosystem pain that need to be taken into account. Rather, this RFC makes explicit an initial set of changes that absolutely cannot be made without a semver bump.

Along the way, it also discusses some interactions with potential language features that can help mitigate pain for non-breaking changes.

The RFC covers only API issues; other issues related to language features, lints, type inference, command line arguments, Cargo, and so on are considered out of scope.

The stability promise specifically does not apply to unstable features, even if they are accidentally usable on the Stable release channel under certain conditions such as because of bugs in the compiler.


Both Rust and its library ecosystem have adopted semver, a technique for versioning platforms/libraries partly in terms of the effect on the code that uses them. In a nutshell, the versioning scheme has three components::

  1. Major: must be incremented for changes that break client code.
  2. Minor: incremented for backwards-compatible feature additions.
  3. Patch: incremented for backwards-compatible bug fixes.

Rust 1.0.0 will mark the beginning of our commitment to stability, and from that point onward it will be important to be clear about what constitutes a breaking change, in order for semver to play a meaningful role. As we will see, this question is more subtle than one might think at first – and the simplest approach would make it effectively impossible to grow the standard library.

The goal of this RFC is to lay out a comprehensive policy for what must be considered a breaking API change from the perspective of semver, along with some guidance about non-semver-breaking changes.

Detailed design

For clarity, in the rest of the RFC, we will use the following terms:

  • Major change: a change that requires a major semver bump.
  • Minor change: a change that requires only a minor semver bump.
  • Breaking change: a change that, strictly speaking, can cause downstream code to fail to compile.

What we will see is that in Rust today, almost any change is technically a breaking change. For example, given the way that globs currently work, adding any public item to a library can break its clients (more on that later). But not all breaking changes are equal.

So, this RFC proposes that all major changes are breaking, but not all breaking changes are major.


Principles of the policy

The basic design of the policy is that the same code should be able to run against different minor revisions. Furthermore, minor changes should require at most a few local annotations to the code you are developing, and in principle no changes to your dependencies.

In more detail:

  • Minor changes should require at most minor amounts of work upon upgrade. For example, changes that may require occasional type annotations or use of UFCS to disambiguate are not automatically “major” changes. (But in such cases, one must evaluate how widespread these “minor” changes are).

  • In principle, it should be possible to produce a version of dependency code that will not break when upgrading other dependencies, or Rust itself, to a new minor revision. This goes hand-in-hand with the above bullet; as we will see, it’s possible to save a fully “elaborated” version of upstream code that does not require any disambiguation. The “in principle” refers to the fact that getting there may require some additional tooling or language support, which this RFC outlines.

That means that any breakage in a minor release must be very “shallow”: it must always be possible to locally fix the problem through some kind of disambiguation that could have been done in advance (by using more explicit forms) or other annotation (like disabling a lint). It means that minor changes can never leave you in a state that requires breaking changes to your own code.

Although this general policy allows some (very limited) breakage in minor releases, it is not a license to make these changes blindly. The breakage that this RFC permits, aside from being very simple to fix, is also unlikely to occur often in practice. The RFC will discuss measures that should be employed in the standard library to ensure that even these minor forms of breakage do not cause widespread pain in the ecosystem.

Scope of the policy

The policy laid out by this RFC applies to stable, public APIs in the standard library. Eventually, stability attributes will be usable in external libraries as well (this will require some design work), but for now public APIs in external crates should be understood as de facto stable after the library reaches 1.0.0 (per semver).

Policy by language feature

Most of the policy is simplest to lay out with reference to specific language features and the way that APIs using them can, and cannot, evolve in a minor release.

Breaking changes are assumed to be major changes unless otherwise stated. The RFC covers many, but not all breaking changes that are major; it covers all breaking changes that are considered minor.


Major change: going from stable to nightly

Changing a crate from working on stable Rust to requiring a nightly is considered a breaking change. That includes using #[feature] directly, or using a dependency that does so. Crate authors should consider using Cargo “features” for their crate to make such use opt-in.

Minor change: altering the use of Cargo features

Cargo packages can provide opt-in features, which enable #[cfg] options. When a common dependency is compiled, it is done so with the union of all features opted into by any packages using the dependency. That means that adding or removing a feature could technically break other, unrelated code.

However, such breakage always represents a bug: packages are supposed to support any combination of features, and if another client of the package depends on a given feature, that client should specify the opt-in themselves.


Major change: renaming/moving/removing any public items.

Although renaming an item might seem like a minor change, according to the general policy design this is not a permitted form of breakage: it’s not possible to annotate code in advance to avoid the breakage, nor is it possible to prevent the breakage from affecting dependencies.

Of course, much of the effect of renaming/moving/removing can be achieved by instead using deprecation and pub use, and the standard library should not be afraid to do so! In the long run, we should consider hiding at least some old deprecated items from the docs, and could even consider putting out a major version solely as a kind of “garbage collection” for long-deprecated APIs.

Minor change: adding new public items.

Note that adding new public items is currently a breaking change, due to glob imports. For example, the following snippet of code will break if the foo module introduces a public item called bar:

use foo::*;
fn bar() { ... }

The problem here is that glob imports currently do not allow any of their imports to be shadowed by an explicitly-defined item.

This is considered a minor change because under the principles of this RFC: the glob imports could have been written as more explicit (expanded) use statements. It is also plausible to do this expansion automatically for a crate’s dependencies, to prevent breakage in the first place.

(This RFC also suggests permitting shadowing of a glob import by any explicit item. This has been the intended semantics of globs, but has not been implemented. The details are left to a future RFC, however.)


See “Signatures in type definitions” for some general remarks about changes to the actual types in a struct definition.

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

This change has the effect of making external struct literals impossible to write, which can break code irreparably.

Major change: adding a public field when no private field exists.

This change retains the ability to use struct literals, but it breaks existing uses of such literals; it likewise breaks exhaustive matches against the struct.

Minor change: adding or removing private fields when at least one already exists (before and after the change).

No existing code could be relying on struct literals for the struct, nor on exhaustively matching its contents, and client code will likewise be oblivious to the addition of further private fields.

For tuple structs, this is only a minor change if furthermore all fields are currently private. (Tuple structs with mixtures of public and private fields are bad practice in any case.)

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

This is technically a breaking change:

// in some other module:
pub struct Foo(SomeType);

// in downstream code
let Foo(_) = foo;

Changing Foo to a normal struct can break code that matches on it – but there is never any real reason to match on it in that circumstance, since you cannot extract any fields or learn anything of interest about the struct.


See “Signatures in type definitions” for some general remarks about changes to the actual types in an enum definition.

Major change: adding new variants.

Exhaustiveness checking means that a match that explicitly checks all the variants for an enum will break if a new variant is added. It is not currently possible to defend against this breakage in advance.

A postponed RFC discusses a language feature that allows an enum to be marked as “extensible”, which modifies the way that exhaustiveness checking is done and would make it possible to extend the enum without breakage.

Major change: adding new fields to a variant.

If the enum is public, so is the full contents of all of its variants. As per the rules for structs, this means it is not allowed to add any new fields (which will automatically be public).

If you wish to allow for this kind of extensibility, consider introducing a new, explicit struct for the variant up front.


Major change: adding a non-defaulted item.

Adding any item without a default will immediately break all trait implementations.

It’s possible that in the future we will allow some kind of “sealing” to say that a trait can only be used as a bound, not to provide new implementations; such a trait would allow arbitrary items to be added.

Major change: any non-trivial change to item signatures.

Because traits have both implementors and consumers, any change to the signature of e.g. a method will affect at least one of the two parties. So, for example, abstracting a concrete method to use generics instead might work fine for clients of the trait, but would break existing implementors. (Note, as above, the potential for “sealed” traits to alter this dynamic.)

Minor change: adding a defaulted item.

Adding a defaulted item is technically a breaking change:

trait Trait1 {}
trait Trait2 {
    fn foo(&self);

fn use_both<T: Trait1 + Trait2>(t: &T) {

If a foo method is added to Trait1, even with a default, it would cause a dispatch ambiguity in use_both, since the call to foo could be referring to either trait.

(Note, however, that existing implementations of the trait are fine.)

According to the basic principles of this RFC, such a change is minor: it is always possible to annotate the call t.foo() to be more explicit in advance using UFCS: Trait2::foo(t). This kind of annotation could be done automatically for code in dependencies (see Elaborated source). And it would also be possible to mitigate this problem by allowing method renaming on trait import.

While the scenario of adding a defaulted method to a trait may seem somewhat obscure, the exact same hazards arise with implementing existing traits (see below), which is clearly vital to allow; we apply a similar policy to both.

All that said, it is incumbent on library authors to ensure that such “minor” changes are in fact minor in practice: if a conflict like t.foo() is likely to arise at all often in downstream code, it would be advisable to explore a different choice of names. More guidelines for the standard library are given later on.

There are two circumstances when adding a defaulted item is still a major change:

  • The new item would change the trait from object safe to non-object safe.
  • The trait has a defaulted associated type and the item being added is a defaulted function/method. In this case, existing impls that override the associated type will break, since the function/method default will not apply. (See the associated item RFC).
  • Adding a default to an existing associated type is likewise a major change if the trait has defaulted methods, since it will invalidate use of those defaults for the methods in existing trait impls.

Minor change: adding a defaulted type parameter.

As with “Signatures in type definitions”, traits are permitted to add new type parameters as long as defaults are provided (which is backwards compatible).

Trait implementations

Major change: implementing any “fundamental” trait.

A recent RFC introduced the idea of “fundamental” traits which are so basic that not implementing such a trait right off the bat is considered a promise that you will never implement the trait. The Sized and Fn traits are examples.

The coherence rules take advantage of fundamental traits in such a way that adding a new implementation of a fundamental trait to an existing type can cause downstream breakage. Thus, such impls are considered major changes.

Minor change: implementing any non-fundamental trait.

Unfortunately, implementing any existing trait can cause breakage:

// Crate A
    pub trait Trait1 {
        fn foo(&self);

    pub struct Foo; // does not implement Trait1

// Crate B
    use crateA::Trait1;

    trait Trait2 {
        fn foo(&self);

    impl Trait2 for crateA::Foo { .. }

    fn use_foo(f: &crateA::Foo) {

If crate A adds an implementation of Trait1 for Foo, the call to f.foo() in crate B will yield a dispatch ambiguity (much like the one we saw for defaulted items). Thus technically implementing any existing trait is a breaking change! Completely prohibiting such a change is clearly a non-starter.

However, as before, this kind of breakage is considered “minor” by the principles of this RFC (see “Adding a defaulted item” above).

Inherent implementations

Minor change: adding any inherent items.

Adding an inherent item cannot lead to dispatch ambiguity, because inherent items trump any trait items with the same name.

However, introducing an inherent item can lead to breakage if the signature of the item does not match that of an in scope, implemented trait:

// Crate A
    pub struct Foo;

// Crate B
    trait Trait {
        fn foo(&self);

    impl Trait for crateA::Foo { .. }

    fn use_foo(f: &crateA::Foo) {

If crate A adds a method:

impl Foo {
    fn foo(&self, x: u8) { ... }

then crate B would no longer compile, since dispatch would prefer the inherent impl, which has the wrong type.

Once more, this is considered a minor change, since UFCS can disambiguate (see “Adding a defaulted item” above).

It’s worth noting, however, that if the signatures did happen to match then the change would no longer cause a compilation error, but might silently change runtime behavior. The case where the same method for the same type has meaningfully different behavior is considered unlikely enough that the RFC is willing to permit it to be labeled as a minor change – and otherwise, inherent methods could never be added after the fact.

Other items

Most remaining items do not have any particularly unique items:

Cross-cutting concerns

Behavioral changes

This RFC is largely focused on API changes which may, in particular, cause downstream code to stop compiling. But in some sense it is even more pernicious to make a change that allows downstream code to continue compiling, but causes its runtime behavior to break.

This RFC does not attempt to provide a comprehensive policy on behavioral changes, which would be extremely difficult. In general, APIs are expected to provide explicit contracts for their behavior via documentation, and behavior that is not part of this contract is permitted to change in minor revisions. (Remember: this RFC is about setting a minimum bar for when major version bumps are required.)

This policy will likely require some revision over time, to become more explicit and perhaps lay out some best practices.

Signatures in type definitions

Major change: tightening bounds.

Adding new constraints on existing type parameters is a breaking change, since existing uses of the type definition can break. So the following is a major change:


// Before
struct Foo<A> { .. }

// After
struct Foo<A: Clone> { .. }

Minor change: loosening bounds.

Loosening bounds, on the other hand, cannot break code because when you reference Foo<A>, you do not learn anything about the bounds on A. (This is why you have to repeat any relevant bounds in impl blocks for Foo, for example.) So the following is a minor change:


// Before
struct Foo<A: Clone> { .. }

// After
struct Foo<A> { .. }

Minor change: adding defaulted type parameters.

All existing references to a type/trait definition continue to compile and work correctly after a new defaulted type parameter is added. So the following is a minor change:


// Before
struct Foo { .. }

// After
struct Foo<A = u8> { .. }

Minor change: generalizing to generics.

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:


// Before
struct Foo(pub u8);

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

because existing uses of Foo are shorthand for Foo<u8> which yields the identical field type. (Note: this is not actually true today, since default type parameters are not fully implemented. But this is the intended semantics.)

On the other hand, the following is not permitted:


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

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

since there may be existing uses of Foo with a non-default type parameter which would break as a result of the change.

It’s also permitted to change from a generic type to a more-generic one in a minor revision:


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

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

since, again, all existing uses of the type Foo<T> will yield the same field types as before.

Signatures in functions

All of the changes mentioned below are considered major changes in the context of trait methods, since they can break implementors.

Major change: adding/removing arguments.

At the moment, Rust does not provide defaulted arguments, so any change in arity is a breaking change.

Minor change: introducing a new type parameter.

Technically, adding a (non-defaulted) type parameter can break code:

// MINOR CHANGE (but causes breakage)

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

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

will break any calls like foo::<u8>. However, such explicit calls are rare enough (and can usually be written in other ways) that this breakage is considered minor. (However, one should take into account how likely it is that the function in question is being called with explicit type arguments). This RFC also suggests adding a ... notation to explicit parameter lists to keep them open-ended (see suggested language changes).

Such changes are an important ingredient of abstracting to use generics, as described next.

Minor change: generalizing to generics.

The type of an argument 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 change is allowed:


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

// After
fn foo<T: Add>(x: T) -> T;
fn bar<T: IntoIterator<Item = u8>>(t: T);

because all existing uses are instantiations of the new signature. On the other hand, the following isn’t allowed in a minor revision:


// Before
fn foo(x: Vec<u8>);

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

because the generics include a constraint not satisfied by the original type.

Introducing generics in this way can potentially create type inference failures, but these are considered acceptable per the principles of the RFC: they only require local annotations that could have been inserted in advance.

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


// Before
fn foo(t: &Trait);

// After
fn foo<T: Trait + ?Sized>(t: &T);

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


Minor change: introducing new lint warnings/errors

Lints are considered advisory, and changes that cause downstream code to receive additional lint warnings/errors are still considered “minor” changes.

Making this work well in practice will likely require some infrastructure work along the lines of this RFC issue

Mitigation for minor changes

The Crater tool

@brson has been hard at work on a tool called “Crater” which can be used to exercise changes on the entire crates.io ecosystem, looking for regressions. This tool will be indispensable when weighing the costs of a minor change that might cause some breakage – we can actually gauge what the breakage would look like in practice.

While this would, of course, miss code not available publicly, the hope is that code on crates.io is a broadly representative sample, good enough to turn up problems.

Any breaking, but minor change to the standard library must be evaluated through Crater before being committed.


One line of defense against a “minor” change causing significant breakage is the nightly release channel: we can get feedback about breakage long before it makes even into a beta release. And of course the beta cycle itself provides another line of defense.

Elaborated source

When compiling upstream dependencies, it is possible to generate an “elaborated” version of the source code where all dispatch is resolved to explicit UFCS form, all types are annotated, and all glob imports are replaced by explicit imports.

This fully-elaborated form is almost entirely immune to breakage due to any of the “minor changes” listed above.

You could imagine Cargo storing this elaborated form for dependencies upon compilation. That would in turn make it easy to update Rust, or some subset of dependencies, without breaking any upstream code (even in minor ways). You would be left only with very small, local changes to make to the code you own.

While this RFC does not propose any such tooling change right now, the point is mainly that there are a lot of options if minor changes turn out to cause breakage more often than anticipated.

Trait item renaming

One very useful mechanism would be the ability to import a trait while renaming some of its items, e.g. use some_mod::SomeTrait with {foo_method as bar}. In particular, when methods happen to conflict across traits defined in separate crates, a user of the two traits could rename one of the methods out of the way.

Thoughts on possible language changes (unofficial)

The following is just a quick sketch of some focused language changes that would help our API evolution story.

Glob semantics

As already mentioned, the fact that glob imports currently allow no shadowing is deeply problematic: in a technical sense, it means that the addition of any public item can break downstream code arbitrarily.

It would be much better for API evolution (and for ergonomics and intuition) if explicitly-defined items trump glob imports. But this is left to a future RFC.

Globs with fine-grained control

Another useful tool for working with globs would be the ability to exclude certain items from a glob import, e.g. something like:

use some_module::{* without Foo};

This is especially useful for the case where multiple modules being glob imported happen to export items with the same name.

Another possibility would be to not make it an error for two glob imports to bring the same name into scope, but to generate the error only at the point that the imported name was actually used. Then collisions could be resolved simply by adding a single explicit, shadowing import.

Default type parameters

Some of the minor changes for moving to more generic code depends on an interplay between defaulted type parameters and type inference, which has been accepted as an RFC but not yet implemented.

“Extensible” enums

There is already an RFC for an enum annotation that would make it possible to add variants without ever breaking downstream code.

Sealed traits

The ability to annotate a trait with some “sealed” marker, saying that no external implementations are allowed, would be useful in certain cases where a crate wishes to define a closed set of types that implements a particular interface. Such an attribute would make it possible to evolve the interface without a major version bump (since no downstream implementors can exist).

Defaulted parameters

Also known as “optional arguments” – an oft-requested feature. Allowing arguments to a function to be optional makes it possible to add new arguments after the fact without a major version bump.

Open-ended explicit type parameters

One hazard is that with today’s explicit type parameter syntax, you must always specify all type parameters: foo::<T, U>(x, y). That means that adding a new type parameter to foo can break code, even if a default is provided.

This could be easily addressed by adding a notation like ... to leave additional parameters unspecified: foo::<T, ...>(x, y).

[Amendment] Misuse of accessible(..)

RFC 2523 introduces #[cfg(accessible($path)]. Based on the accessibility of a to-the-current-crate external $path, the flag allows conditional compilation. When combined with #[cfg(feature = "unstable")], this has certain breakage risks. Such breakage due to misuse, as outlined in the RFC, is considered acceptable and not covered by our stability promises. Please see the RFC for more details.

Drawbacks and Alternatives

The main drawback to the approach laid out here is that it makes the stability and semver guarantees a bit fuzzier: the promise is not that code will never break, full stop, but rather that minor release breakage is of an extremely limited form, for which there are a variety of mitigation strategies. This approach tries to strike a middle ground between a very hard line for stability (which, for Rust, would rule out many forms of extension) and willy-nilly breakage: it’s an explicit, but pragmatic policy.

An alternative would be to take a harder line and find some other way to allow API evolution. Supposing that we resolved the issues around glob imports, the main problems with breakage have to do with adding new inherent methods or trait implementations – both of which are vital forms of evolution. It might be possible, in the standard library case, to provide some kind of version-based opt in to this evolution: a crate could opt in to breaking changes for a particular version of Rust, which might in turn be provided only through some cfg-like mechanism.

Note that these strategies are not mutually exclusive. Rust’s development processes involved a very steady, strong stream of breakage, and while we need to be very serious about stabilization, it is possible to take an iterative approach. The changes considered “major” by this RFC already move the bar very significantly from what was permitted pre-1.0. It may turn out that even the minor forms of breakage permitted here are, in the long run, too much to tolerate; at that point we could revise the policies here and explore some opt-in scheme, for example.

Unresolved questions

Behavioral issues

  • Is it permitted to change a contract from “abort” to “panic”? What about from “panic” to “return an Err”?

  • Should we try to lay out more specific guidance for behavioral changes at this point?