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promote_consts experiment: do not promote !Freeze shared references#142287

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promote_consts experiment: do not promote !Freeze shared references#142287
RalfJung wants to merge 1 commit intorust-lang:masterfrom
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@RalfJung RalfJung commented Jun 10, 2025

This is a crater experiment to measure the fallout from not promoting shared references to !Freeze types. IOW, this switches promotion from value-based reasoning to type-based reasoning about interior mutability.

Landing this PR would fix rust-lang/unsafe-code-guidelines#493 but I doubt we can get away with that.

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jun 10, 2025
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RalfJung commented Jun 10, 2025

@bors2 try

rust-bors bot added a commit that referenced this pull request Jun 10, 2025
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Auto merge of #142287 - RalfJung:no-promote-im, r=<try>
51087fb 
promote_consts: do not promote !Freeze shared references

This is a crater experiment to measure the fallout from not promoting shared references to `!Freeze` types. IOW, this switches promotion from value-based reasoning to type-based reasoning about interior mutability.

Landing this PR would fix rust-lang/unsafe-code-guidelines#493 but I doubt we can get away with that.
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rust-bors bot commented Jun 10, 2025

⌛ Trying commit 19a8ea3 with merge 51087fb…

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rust-bors bot commented Jun 10, 2025

💔 Test failed

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RalfJung commented Jun 10, 2025

@bors2 try

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rust-bors bot commented Jun 10, 2025

⌛ Trying commit deba557 with merge 901e6d5…

To cancel the try build, run the command @bors2 try cancel.

rust-bors bot added a commit that referenced this pull request Jun 10, 2025
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Auto merge of #142287 - RalfJung:no-promote-im, r=<try>
901e6d5 
promote_consts: do not promote !Freeze shared references

This is a crater experiment to measure the fallout from not promoting shared references to `!Freeze` types. IOW, this switches promotion from value-based reasoning to type-based reasoning about interior mutability.

Landing this PR would fix rust-lang/unsafe-code-guidelines#493 but I doubt we can get away with that.
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rust-log-analyzer commented Jun 10, 2025

The job mingw-check-1 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
error[E0515]: cannot return value referencing temporary value
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ra-ap-rustc_pattern_analysis-0.113.0/src/pat.rs:194:9
    |
194 | /         match self {
195 | |             PatOrWild::Wild => &Wildcard,
    | |                                 -------- temporary value created here
196 | |             PatOrWild::Pat(pat) => pat.ctor(),
197 | |         }
    | |_________^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.
[RUSTC-TIMING] ra_ap_rustc_pattern_analysis test:false 1.011
error: could not compile `ra-ap-rustc_pattern_analysis` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

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rust-bors bot commented Jun 10, 2025

☀️ Try build successful (CI)
Build commit: 901e6d5 (901e6d56697c79b2e80df6a487b2318ec6f57ee3)

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RalfJung commented Jun 10, 2025

@craterbot check

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craterbot commented Jun 10, 2025

🚧 Experiment pr-142287 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

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craterbot commented Jun 10, 2025

👌 Experiment pr-142287 created and queued.
🤖 Automatically detected try build 901e6d5
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot craterbot added the S-waiting-on-crater Status: Waiting on a crater run to be completed. label Jun 10, 2025
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RalfJung commented Jun 10, 2025
View reviewed changes
compiler/rustc_pattern_analysis/src/pat.rs
pub(crate) fn ctor(self) -> &'p Constructor<Cx> {
match self {
PatOrWild::Wild => &Wildcard,
PatOrWild::Wild => const { &Wildcard },
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@RalfJung RalfJung Jun 10, 2025 •
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Not even the compiler builds without some changes.^^

What happens here is that by making the const expression &Wildcard, the lifetime extension rules kick in, and those still do value-based reasoning and therefore allow this constant without ever involving promotion.

@RalfJung RalfJung changed the title promote_consts: do not promote !Freeze shared references promote_consts experiment: do not promote !Freeze shared references Jun 10, 2025
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craterbot commented Jun 11, 2025

🎉 Experiment pr-142287 is completed!
📊 6478 regressed and 6 fixed (645631 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Jun 11, 2025
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RalfJung commented Jun 11, 2025

Oh wow, this is even more than the 4k regressions we got last time... and last time we technically broke more code. I guess that's just a sign of how much the ecosystem has been growing...

@RalfJung RalfJung closed this Jun 11, 2025
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Promotion introduces UB into otherwise well-defined code

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