const-eval: allow constants to refer to mutable/external memory, but reject such constants as patterns by RalfJung · Pull Request #140942 · rust-lang/rust (original) (raw)

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@RalfJung

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@RalfJung

…reject such constants as patterns

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…rors

Rollup of 18 pull requests

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rust-timer added a commit that referenced this pull request

Jun 27, 2025

@rust-timer

Rollup merge of #140942 - RalfJung:const-ref-to-mut, r=oli-obk

const-eval: allow constants to refer to mutable/external memory, but reject such constants as patterns

This fixes #140653 by accepting code such as this:

static FOO: AtomicU32 = AtomicU32::new(0);
const C: &'static AtomicU32 = &FOO;

This can be written entirely in safe code, so there can't really be anything wrong with it.

We also accept the much more questionable following code, since it looks very similar to the interpreter:

static mut FOO2: u32 = 0;
const C2: &'static u32 = unsafe { &mut FOO2 };

Using this without causing UB is at least very hard (the details are unclear since it is related to how the aliasing model deals with the staging of const-eval vs runtime code).

If a constant like C2 is used as a pattern, we emit an error:

error: constant BAD_PATTERN cannot be used as pattern
  --> $DIR/const_refs_to_static_fail.rs:30:9
   |
LL |         BAD_PATTERN => {},
   |         ^^^^^^^^^^^
   |
   = note: constants that reference mutable or external memory cannot be used as pattern

(If you somehow manage to build a pattern with constant C, you'd get the same error, but that should be impossible: we don't have a type that can be used in patterns and that has interior mutability.)

The same treatment is afforded for shared references to extern static, for the same reason: the const evaluation is entirely fine with it, we just can't build a pattern for it -- and when using interior mutability, this can be totally sound.

We do still not accept anything where there is an &mut in the final value of the const, as that should always require unsafe code and it's hard to imagine a sound use-case that would require this.

compiler-errors added a commit to compiler-errors/rust that referenced this pull request

Jun 27, 2025

@compiler-errors

… r=oli-obk

const checks for lifetime-extended temporaries: avoid 'top-level scope' terminology

This error recently got changed in rust-lang#140942 to use the terminology of "top-level scope", but after further discussion in rust-lang/reference#1865 it seems the reference will not be using that terminology after all. So let's also remove it from the compiler again, and let's focus on what actually happens with these temporaries: their lifetime is extended until the end of the program.

r? @oli-obk @traviscross

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request

Jun 27, 2025

@matthiaskrgr

… r=oli-obk

const checks for lifetime-extended temporaries: avoid 'top-level scope' terminology

This error recently got changed in rust-lang#140942 to use the terminology of "top-level scope", but after further discussion in rust-lang/reference#1865 it seems the reference will not be using that terminology after all. So let's also remove it from the compiler again, and let's focus on what actually happens with these temporaries: their lifetime is extended until the end of the program.

r? @oli-obk @traviscross

rust-timer added a commit that referenced this pull request

Jun 28, 2025

@rust-timer

Rollup merge of #143092 - RalfJung:const-check-lifetime-ext, r=oli-obk

const checks for lifetime-extended temporaries: avoid 'top-level scope' terminology

This error recently got changed in #140942 to use the terminology of "top-level scope", but after further discussion in rust-lang/reference#1865 it seems the reference will not be using that terminology after all. So let's also remove it from the compiler again, and let's focus on what actually happens with these temporaries: their lifetime is extended until the end of the program.

r? @oli-obk @traviscross

github-actions bot pushed a commit to rust-lang/miri that referenced this pull request

Jun 28, 2025

@matthiaskrgr

const checks for lifetime-extended temporaries: avoid 'top-level scope' terminology

This error recently got changed in rust-lang/rust#140942 to use the terminology of "top-level scope", but after further discussion in rust-lang/reference#1865 it seems the reference will not be using that terminology after all. So let's also remove it from the compiler again, and let's focus on what actually happens with these temporaries: their lifetime is extended until the end of the program.

r? @oli-obk @traviscross

github-actions bot pushed a commit to model-checking/verify-rust-std that referenced this pull request

Jul 4, 2025

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@traviscross traviscross added the relnotes

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wip-sync pushed a commit to NetBSD/pkgsrc-wip that referenced this pull request

Sep 20, 2025

@he32

Pkgsrc changes:

Upstream changes relative to 1.89.0:

Version 1.90 (2025-09-18)

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tmeijn pushed a commit to tmeijn/dotfiles that referenced this pull request

Sep 24, 2025

@tmeijn

This MR contains the following updates:

Package Update Change
rust minor 1.89.0 -> 1.90.0

MR created with the help of el-capitano/tools/renovate-bot.

Proposed changes to behavior should be submitted there as MRs.

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rust-lang/rust (rust)

v1.90.0

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netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request

Oct 18, 2025

@he32

Pkgsrc changes:

Upstream changes:

Version 1.90 (2025-09-18)

Language

Compiler

Platform Support

Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

Stabilized APIs

These previously stable APIs are now stable in const contexts:

Cargo

Rustdoc

Compatibility Notes

Version 1.89.0 (2025-08-07)

Language

Compiler

Platform Support

Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

Stabilized APIs

These previously stable APIs are now stable in const contexts:

Cargo

Rustdoc

Compatibility Notes

Internal Changes

These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools.

netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request

Oct 23, 2025

@he32

Pkgsrc changes:

Upstream changes:

Version 1.90 (2025-09-18)

Language

Compiler

Platform Support

Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

Stabilized APIs

These previously stable APIs are now stable in const contexts:

Cargo

Rustdoc

Compatibility Notes

Version 1.89.0 (2025-08-07)

Language

Compiler

Platform Support

Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

Stabilized APIs

These previously stable APIs are now stable in const contexts:

Cargo

Rustdoc

Compatibility Notes

Internal Changes

These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools.