Stop skewing inference in ?'s desugaring #122412
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r? @nnethercote rustbot has assigned @nnethercote. Use r? to explicitly pick a reviewer |
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| /// This is possible because `!` is uninhabited (has no values), so this function can't actually | ||
| /// be ever called at runtime. | ||
| /// | ||
| /// Even though `!` can be coerced to any type implicitly anyway (and indeed this function |
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"function is implemented"
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I have skimmed this PR and gave minor comments, but this kind of subtle lang/libs/type system change is the kind of thing I am perfectly not suited to reviewing. The "needs-fcp" and "I-lang-nominated" labels have been added, so this clearly will get some scrutiny via other channels, but I will take a guess at a suitable reviewer and reassign anyway... r? @cuviper |
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Is |
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@cuviper I think it could plausibly become stable, at least if/when |
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OK, well the actual implementation of r? libs-api |
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Do not use `?`-induced skewing of type inference in the compiler This prevents breakage from rust-lang#122412 and is generally a good idea. r? `@estebank`
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Rollup merge of rust-lang#122540 - WaffleLapkin:ununexpected, r=estebank Do not use `?`-induced skewing of type inference in the compiler This prevents breakage from rust-lang#122412 and is generally a good idea. r? `@estebank`
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API-wise this seems fine to add as unstable. I presume the next step is a crater run to evaluate the impact? |
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FTR, I've commonly used |
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We were unsure about how to think about this PR in the triage meeting today. Looking at this in particular,
it made me wonder whether it would be feasible to change |
@scottmcm I'm not sure. I don't think it's unfeasible, but it sure is harder than this. The issues are:
I'm not sure if this is a good idea or not. It's kinda weird. |
@Amanieu yes. |
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@chorman0773 can you explain what you mean by "if this will cause issues in This PR makes all |
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…-operator-to-not-screw-with-inference-will-it-obey, r=<try>
Stop skewing inference in ?'s desugaring
**NB**: this is a breaking change (although arguably a bug fix) and as such shouldn't be taken lightly.
This changes `expr?`'s desugaring like so (simplified, see code for more info):
```rust
// old
match expr {
Ok(val) => val,
Err(err) => return Err(err),
}
// new
match expr {
Ok(val) => val,
Err(err) => core::convert::absurd(return Err(err)),
}
// core::convert
pub const fn absurd<T>(x: !) -> T { x }
```
This prevents `!` from the `return` from skewing inference:
```rust
// previously: ok (never type spontaneous decay skews inference, `T = ()`)
// with this pr: can't infer the type for `T`
Err(())?;
```
Fixes rust-lang#51125
Closes rust-lang#39216
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Don't depend on `?` affecting type inference in weird ways This is likely to stop working in the future versions of Rust, see rust-lang/rust#122412 (comment).
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The job Click to see the possible cause of the failure (guessed by this bot) |
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☀️ Try build successful - checks-actions |
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@craterbot run start=master#385fa9d845dd326c6bbfd58c22244215e431948a end=try#645bb72776a6a56a1a8f52631a44bd082b2ba509 mode=check-only name=absurd-question-mark-desugar crates=https://crater-reports.s3.amazonaws.com/no-never-type-fallback/retry-regressed-list.txt (using #122955 regressions as the crate list, since this can only break things that #122955 broke) |
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👌 Experiment ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more |
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🚧 Experiment ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more |
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🎉 Experiment
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@rustbot labels -I-lang-nominated We discussed this in the triage meeting today. Some people liked this change, and most liked the spirit of it, but there were some concerns. In particular, the many cases like this: fn result() -> Result<bool, ()> {
- Err(())?;
+ Err::<(), _>(())?;
Ok(true)
}...resulted in much discussion regarding, e.g., the churn and whether the motivation of this proposal makes it worth it. We did not reach consensus here other than that probably no consensus is possible at this time without a revised proposal, so we can unnominate until and unless such a revised proposal is available. |
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NB: this is a breaking change (although arguably a bug fix) and as such shouldn't be taken lightly.
This changes
expr?'s desugaring like so (simplified, see code for more info):This prevents
!from thereturnfrom skewing inference:Fixes #51125
Closes #39216