Match ergonomics 2024: Implement eat-one-layer #123512
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| if let Some(&mut Some(&x)) = &Some(&mut Some(0)) { | ||
| let _: u32 = x; | ||
| } | ||
| if let Some(&Some(&mut x)) = &mut Some(& Some(0)) { | ||
| let _: u32 = x; | ||
| } |
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These two are weird, shouldn't we be eating references in the same order as in the type?
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Does this typecheck?
if let Some(&Some(x)) = &mut Some(&Some(0)) {
let _: &mut u32 = x;
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Does this typecheck?
Not any longer, pushed a fix so we now properly downgrade the mutability of the inherited reference.
Shouldn't we be eating references in the same order as in the type?
No, that would be unnecessarily restrictive, as demonstrated by this newly added test:
if let Some(Some(&mut x)) = &Some(Some(&mut 0)) {
let _: &u32 = x;
}(let &mut x = &&mut 0 does not work currently, but I think it should, will try to fix edit: also works now)
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Hm, I would not expect these tests to work personally.
I would expect we want a rule like: let &mut x = <expr>; x: T iff let x = <expr>; x: &mut T, no?
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I don't see any good reason we should restrict it to that, when the currently implemented rule (if the pattern can match against the inherited reference, do so; otherwise match against a reference in the scrutinee type) is strictly more useful. let Some(&mut x) = &Some(&mut 0) is more convenient than let &Some(&mut (ref x)) = &Some(&mut 0), why should we force people to use the latter?
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Wouldn't let Some(&x) = &Some(&mut 0) work just as well?
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No, that would make x an &mut {integer} instead of an &{integer}, and then the compiler yells at you that you can't move out of a shared reference
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Oh I see. Well, I'm happy to merge your design, this is an implementation review not a design review. I'll just need some time to wrap my head around the new commits
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Some changes occurred in match checking cc @Nadrieril |
compiler/rustc_hir_typeck/src/pat.rs
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| AdjustMode::ResetAndConsumeRef(ref_pat_mutbl) => { | ||
| let mutbls_match = def_bm.0 == ByRef::Yes(ref_pat_mutbl); | ||
| if pat.span.at_least_rust_2024() && self.tcx.features().ref_pat_eat_one_layer_2024 { | ||
| let max_ref_mutbl = cmp::min(max_ref_mutbl, ref_pat_mutbl); |
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Intuitively I'd expect we can just cap def_bm by max_ref_mutbl just before returning from the function, and cap it by ref_pat_mutbl at the end of this if. Does that work? The peel_off_references change I just suggested could help.
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I made a slight tweak to this function, but didn't go as far as that. The capping by max_ref_mutbl has to be gated by edition/feature gate, so needs to stay inside the if.
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☔ The latest upstream changes (presumably #123991) made this pull request unmergeable. Please resolve the merge conflicts. |
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Looks good, ty! @bors r+ |
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…llaumeGomez Rollup of 7 pull requests Successful merges: - rust-lang#122811 (Move `SourceMap` initialization) - rust-lang#123512 (Match ergonomics 2024: Implement eat-one-layer) - rust-lang#123811 (Use queue-based `RwLock` on more platforms) - rust-lang#123859 (Remove uneeded clones now that TrustedStep implies Copy) - rust-lang#123979 (Subtype predicates only exist on inference types, so we can allow them to register opaque types within them.) - rust-lang#124016 (Outline default query and hook provider function implementations) - rust-lang#124023 (Allow workproducts without object files.) r? `@ghost` `@rustbot` modify labels: rollup
Rollup merge of rust-lang#123512 - Jules-Bertholet:ref-pat-eat-one-layer-2024, r=Nadrieril Match ergonomics 2024: Implement eat-one-layer r? `@Nadrieril` cc rust-lang#123076 `@rustbot` label A-edition-2024 A-patterns
r? @Nadrieril
cc #123076
@rustbot label A-edition-2024 A-patterns