core: Document why Pointee::Metadata can't have 'static bound

Enselic · lukas-code · Enselic · commit d16da3b8b2be · 2025-02-15T17:22:56.000+01:00
Co-authored-by: Lukas &lt;26522220+lukas-code@users.noreply.github.com&gt;
diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs
@@ -61,6 +61,8 @@ pub trait Pointee {
     // NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
     // in `library/core/src/ptr/metadata.rs`
     // in sync with those here:
+    // NOTE: The metadata of `dyn Trait + 'a` is `DynMetadata<dyn Trait + 'a>`
+    // so a `'static` bound must not be added.
     type Metadata: fmt::Debug + Copy + Send + Sync + Ord + Hash + Unpin + Freeze;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -61,6 +61,8 @@ pub trait Pointee {`
`61`	`61`	// NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
`62`	`62`	// in `library/core/src/ptr/metadata.rs`
`63`	`63`	`// in sync with those here:`
	`64`	+ // NOTE: The metadata of `dyn Trait + 'a` is `DynMetadata<dyn Trait + 'a>`
	`65`	+ // so a `'static` bound must not be added.
`64`	`66`	`type Metadata: fmt::Debug + Copy + Send + Sync + Ord + Hash + Unpin + Freeze;`
`65`	`67`	`}`
`66`	`68`