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Auto merge of #140106 - dianne:deref-pat-usefulness, r=Nadrieril

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allow deref patterns to participate in exhaustiveness analysis

Per [this proposal](https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Exhaustiveness), this PR allows deref patterns to participate in exhaustiveness analysis. Currently all deref patterns enforce `DerefPure` bounds on their scrutinees, so this assumes all patterns it's analyzing are well-behaved. This also doesn't support [mixed exhaustiveness](https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Mixed-exhaustiveness), and instead emits an error if deref patterns are used together with normal constructors. I think mixed exhaustiveness would be nice to have (especially if we eventually want to support arbitrary `Deref` impls[^1]), but it'd require more work to get reasonable diagnostics[^2].

Tracking issue for deref patterns: #87121

r? `@Nadrieril`

[^1]: Regardless of whether we support limited exhaustiveness checking for untrusted `Deref` or always require other arms to be exhaustive, I think it'd be useful to allow mixed matching for user-defined smart pointers. And it'd be strange if it worked there but not for `Cow`.

[^2]: I think listing out witnesses of non-exhaustiveness can be confusing when they're not necessarily disjoint, and when you only need to cover some of them, so we'd probably want special formatting and/or explanatory subdiagnostics. And if it's implemented similarly to unions, we'd probably also want some way of merging witnesses; the way witnesses for unions can appear duplicated is pretty unfortunate. I'm not sure yet how the diagnostics should look, especially for deeply nested patterns.
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bors 2025-05-08 02:16:45 +00:00
commit 4c2b4ca4f2
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crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs
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@ -301,6 +301,7 @@ impl<'db> MatchCheckCtx<'db> {
// ignore this issue.
Ref => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
Slice(_) => unimplemented!(),
DerefPattern(_) => unimplemented!(),
&Str(void) => match void {},
Wildcard | NonExhaustive | Hidden | PrivateUninhabited => PatKind::Wild,
Never => PatKind::Never,
@ -351,6 +352,7 @@ impl PatCx for MatchCheckCtx<'_> {
},
Ref => 1,
Slice(..) => unimplemented!(),
DerefPattern(..) => unimplemented!(),
Never | Bool(..) | IntRange(..) | F16Range(..) | F32Range(..) | F64Range(..)
| F128Range(..) | Str(..) | Opaque(..) | NonExhaustive | PrivateUninhabited
| Hidden | Missing | Wildcard => 0,
@ -411,6 +413,7 @@ impl PatCx for MatchCheckCtx<'_> {
}
},
Slice(_) => unreachable!("Found a `Slice` constructor in match checking"),
DerefPattern(_) => unreachable!("Found a `DerefPattern` constructor in match checking"),
Never | Bool(..) | IntRange(..) | F16Range(..) | F32Range(..) | F64Range(..)
| F128Range(..) | Str(..) | Opaque(..) | NonExhaustive | PrivateUninhabited
| Hidden | Missing | Wildcard => {