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This RFC has the goal of defining what sorts of breaking changes we will permit for the Rust language itself, and giving guidelines for how to go about making such changes.


With the release of 1.0, we need to establish clear policy on what precisely constitutes a "minor" vs "major" change to the Rust language itself (as opposed to libraries, which are covered by RFC 1105). This RFC proposes that minor releases may only contain breaking changes that fix compiler bugs or other type-system issues. Primarily, this means soundness issues where "innocent" code can cause undefined behavior (in the technical sense), but it also covers cases like compiler bugs and tightening up the semantics of "underspecified" parts of the language (more details below).

However, simply landing all breaking changes immediately could be very disruptive to the ecosystem. Therefore, the RFC also proposes specific measures to mitigate the impact of breaking changes, and some criteria when those measures might be appropriate.

In rare cases, it may be deemed a good idea to make a breaking change that is not a soundness problem or compiler bug, but rather correcting a defect in design. Such cases should be rare. But if such a change is deemed worthwhile, then the guidelines given here can still be used to mitigate its impact.

Detailed design

The detailed design is broken into two major sections: how to address soundness changes, and how to address other, opt-in style changes. We do not discuss non-breaking changes here, since obviously those are safe.

Soundness changes

When compiler or type-system bugs are encountered in the language itself (as opposed to in a library), clearly they ought to be fixed. However, it is important to fix them in such a way as to minimize the impact on the ecosystem.

The first step then is to evaluate the impact of the fix on the crates found in the website (using e.g. the crater tool). If impact is found to be "small" (which this RFC does not attempt to precisely define), then the fix can simply be landed. As today, the commit message of any breaking change should include the term [breaking-change] along with a description of how to resolve the problem, which helps those people who are affected to migrate their code. A description of the problem should also appear in the relevant subteam report.

In cases where the impact seems larger, any effort to ease the transition is sure to be welcome. The following are suggestions for possible steps we could take (not all of which will be applicable to all scenarios):

  1. Identify important crates (such as those with many dependants) and work with the crate author to correct the code as quickly as possible, ideally before the fix even lands.
  2. Work hard to ensure that the error message identifies the problem clearly and suggests the appropriate solution.
    • If we develop a rustfix tool, in some cases we may be able to extend that tool to perform the fix automatically.
  3. Provide an annotation that allows for a scoped "opt out" of the newer rules, as described below. While the change is still breaking, this at least makes it easy for crates to update and get back to compiling status quickly.
  4. Begin with a deprecation or other warning before issuing a hard error. In extreme cases, it might be nice to begin by issuing a deprecation warning for the unsound behavior, and only make the behavior a hard error after the deprecation has had time to circulate. This gives people more time to update their crates. However, this option may frequently not be available, because the source of a compilation error is often hard to pin down with precision.

Some of the factors that should be taken into consideration when deciding whether and how to minimize the impact of a fix:

  • How important is the change?
    • Soundness holes that can be easily exploited or which impact running code are obviously much more concerning than minor corner cases. There is somewhat in tension with the other factors: if there is, for example, a widely deployed vulnerability, fixing that vulnerability is important, but it will also cause a larger disruption.
  • How many crates on are affected?
    • This is a general proxy for the overall impact (since of course there will always be private crates that are not part of
  • Were particularly vital or widely used crates affected?
    • This could indicate that the impact will be wider than the raw number would suggest.
  • Does the change silently change the result of running the program, or simply cause additional compilation failures?
    • The latter, while frustrating, are easier to diagnose.
  • What changes are needed to get code compiling again? Are those changes obvious from the error message?
    • The more cryptic the error, the more frustrating it is when compilation fails.

What is a "compiler bug" or "soundness change"?

In the absence of a formal spec, it is hard to define precisely what constitutes a "compiler bug" or "soundness change" (see also the section below on underspecified parts of the language). The obvious cases are soundness violations in a rather strict sense:

  • Cases where the user is able to produce Undefined Behavior (UB) purely from safe code.
  • Cases where the user is able to produce UB using standard library APIs or other unsafe code that "should work".

However, there are other kinds of type-system inconsistencies that might be worth fixing, even if they cannot lead directly to UB. Bugs in the coherence system that permit uncontrolled overlap between impls are one example. Another example might be inference failures that cause code to compile which should not (because ambiguities exist). Finally, there is a list below of areas of the language which are generally considered underspecified.

We expect that there will be cases that fall on a grey line between bug and expected behavior, and discussion will be needed to determine where it falls. The recent conflict between Rc and scoped threads is an example of such a discusison: it was clear that both APIs could not be legal, but not clear which one was at fault. The results of these discussions will feed into the Rust spec as it is developed.

Opting out

In some cases, it may be useful to permit users to opt out of new type rules. The intention is that this "opt out" is used as a temporary crutch to make it easy to get the code up and running. Typically this opt out will thus be removed in a later release. But in some cases, particularly those cases where the severity of the problem is relatively small, it could be an option to leave the "opt out" mechanism in place permanently. In either case, use of the "opt out" API would trigger the deprecation lint.

Note that we should make every effort to ensure that crates which employ this opt out can be used compatibly with crates that do not.

Changes that alter dynamic semantics versus typing rules

In some cases, fixing a bug may not cause crates to stop compiling, but rather will cause them to silently start doing something different than they were doing before. In cases like these, the same principle of using mitigation measures to lessen the impact (and ease the transition) applies, but the precise strategy to be used will have to be worked out on a more case-by-case basis. This is particularly relevant to the underspecified areas of the language described in the next section.

Our approach to handling dynamic drop is a good example. Because we expect that moving to the complete non-zeroing dynamic drop semantics will break code, we've made an intermediate change that altered the compiler to fill with use a non-zero value, which helps to expose code that was implicitly relying on the current behavior (much of which has since been restructured in a more future-proof way).

Underspecified language semantics

There are a number of areas where the precise language semantics are currently somewhat underspecified. Over time, we expect to be fully defining the semantics of all of these areas. This may cause some existing code -- and in particular existing unsafe code -- to break or become invalid. Changes of this nature should be treated as soundness changes, meaning that we should attempt to mitigate the impact and ease the transition wherever possible.

Known areas where change is expected include the following:

  • Destructors semantics:
    • We plan to stop zeroing data and instead use marker flags on the stack, as specified in RFC 320. This may affect destructors that rely on ovewriting memory or using the unsafe_no_drop_flag attribute.
    • Currently, panicing in a destructor can cause unintentional memory leaks and other poor behavior (see #14875, #16135). We are likely to make panic in a destructor simply abort, but the precise mechanism is not yet decided.
    • Order of dtor execution within a data structure is somewhat inconsistent (see #744).
  • The legal aliasing rules between unsafe pointers is not fully settled (see #19733).
  • The interplay of assoc types and lifetimes is not fully settled and can lead to unsoundness in some cases (see #23442).
  • The trait selection algorithm is expected to be improved and made more complete over time. It is possible that this will affect existing code.
  • Overflow semantics: in particular, we may have missed some cases.
  • Memory allocation in unsafe code is currently unstable. We expect to be defining safe interfaces as part of the work on supporting tracing garbage collectors (see #415).
  • The treatment of hygiene in macros is uneven (see #22462, #24278). In some cases, changes here may be backwards compatible, or may be more appropriate only with explicit opt-in (or perhaps an alternate macro system altogether, such as this proposal).
  • Lints will evolve over time (both the lints that are enabled and the precise cases that lints catch). We expect to introduce a means to limit the effect of these changes on dependencies.
  • Stack overflow is currently detected via a segmented stack check prologue and results in an abort. We expect to experiment with a system based on guard pages in the future.
  • We currently abort the process on OOM conditions (exceeding the heap space, overflowing the stack). We may attempt to panic in such cases instead if possible.
  • Some details of type inference may change. For example, we expect to implement the fallback mechanism described in RFC 213, and we may wish to make minor changes to accommodate overloaded integer literals. In some cases, type inferences changes may be better handled via explicit opt-in.

There are other kinds of changes that can be made in a minor version that may break unsafe code but which are not considered breaking changes, because the unsafe code is relying on things known to be intentionally unspecified. One obvious example is the layout of data structures, which is considered undefined unless they have a #[repr(C)] attribute.

Although it is not directly covered by this RFC, it's worth noting in passing that some of the CLI flags to the compiler may change in the future as well. The -Z flags are of course explicitly unstable, but some of the -C, rustdoc, and linker-specific flags are expected to evolve over time (see e.g. #24451).


The primary drawback is that making breaking changes are disruptive, even when done with the best of intentions. The alternatives list some ways that we could avoid breaking changes altogether, and the downsides of each.

Notes on phasing


Rather than simply fixing soundness bugs, we could issue new major releases, or use some sort of opt-in mechanism to fix them conditionally. This was initially considered as an option, but eventually rejected for the following reasons:

  • Opting in to type system changes would cause deep splits between minor versions; it would also create a high maintenance burden in the compiler, since both older and newer versions would have to be supported.
  • It seems likely that all users of Rust will want to know that their code is sound and would not want to be working with unsafe constructs or bugs.
  • We already have several mitigation measures, such as opt-out or temporary deprecation, that can be used to ease the transition around a soundness fix. Moreover, separating out new type rules so that they can be "opted into" can be very difficult and would complicate the compiler internally; it would also make it harder to reason about the type system as a whole.

Unresolved questions

What precisely constitutes "small" impact? This RFC does not attempt to define when the impact of a patch is "small" or "not small". We will have to develop guidelines over time based on precedent. One of the big unknowns is how indicative the breakage we observe on will be of the total breakage that will occur: it is certainly possible that all crates on work fine, but the change still breaks a large body of code we do not have access to.

What attribute should we use to "opt out" of soundness changes? The section on breaking changes indicated that it may sometimes be appropriate to includ an "opt out" that people can use to temporarily revert to older, unsound type rules, but did not specify precisely what that opt-out should look like. Ideally, we would identify a specific attribute in advance that will be used for such purposes. In the past, we have simply created ad-hoc attributes (e.g., #[old_orphan_check]), but because custom attributes are forbidden by stable Rust, this has the unfortunate side-effect of meaning that code which opts out of the newer rules cannot be compiled on older compilers (even though it's using the older type system rules). If we introduce an attribute in advance we will not have this problem.

Are there any other circumstances in which we might perform a breaking change? In particular, it may happen from time to time that we wish to alter some detail of a stable component. If we believe that this change will not affect anyone, such a change may be worth doing, but we'll have to work out more precise guidelines. RFC 1156 is an example.