Fix a comment.

nnethercote · nnethercote · commit c3e5b3e8a8a8 · 2026-05-05T09:01:45.000+10:00
The explanation given for the non-derive use is incorrect.
diff --git a/compiler/rustc_type_ir/src/region_kind.rs b/compiler/rustc_type_ir/src/region_kind.rs
@@ -215,8 +215,9 @@ impl<I: Interner> fmt::Debug for RegionKind<I> {
     }
 }
 
+// This impl could be derived with `StableHash_NoContext`, but we instead write it by hand in order
+// to panic on `ReVar`.
 #[cfg(feature = "nightly")]
-// This is not a derived impl because a derive would require `I: StableHash`
 impl<I: Interner> StableHash for RegionKind<I>
 where
     I::EarlyParamRegion: StableHash,

Original file line number	Diff line number	Diff line change
`@@ -215,8 +215,9 @@ impl<I: Interner> fmt::Debug for RegionKind<I> {`
`215`	`215`	`}`
`216`	`216`	`}`
`217`	`217`
	`218`	+// This impl could be derived with `StableHash_NoContext`, but we instead write it by hand in order
	`219`	+// to panic on `ReVar`.
`218`	`220`	`#[cfg(feature = "nightly")]`
`219`		-// This is not a derived impl because a derive would require `I: StableHash`
`220`	`221`	`impl<I: Interner> StableHash for RegionKind<I>`
`221`	`222`	`where`
`222`	`223`	`I::EarlyParamRegion: StableHash,`