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

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.

Author: bors

crates/hir-ty/src/diagnostics/match_check/pat_analysis.rs

@@ -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 => {