Auto merge of #112319 - oli-obk:assoc_ty_sized_bound_for_object_safety2, r=compiler-errors

Don't require associated types with Self: Sized bounds in `dyn Trait` objects

Trait objects require *all* associated types to be specified, even if the associated type has an explicit `where Self: Sized` bound. The following snippet does not compile on master, but does with this PR.

```rust
fn _assert_is_object_safe(_: &dyn Foo) {}

pub trait Foo {
    type Bar where Self: Sized;
}
```

In contrast, if a `Self: Sized` bound is added to a method, the methodjust isn't callable on trait objects, but the trait can be made object safe just fine.

```rust
fn _assert_is_object_safe(_: &dyn Foo) {}

pub trait Foo {
    fn foo() where Self: Sized;
}
```

This PR closes this inconsistency (though it still exists for associated constants).

Additionally this PR adds a new lint that informs users they can remove associated type bounds from their trait objects if those associated type bounds have a `where Self: Sized` bound, and are thus useless.

r? `@compiler-errors`

1 files changed, 4 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 6f942e0bc86..0e1c6d19e31 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -2197,6 +2197,10 @@ rustc_queries! {
         desc { "getting cfg-ed out item names" }
         separate_provide_extern
     }
+
+    query generics_require_sized_self(def_id: DefId) -> bool {
+        desc { "check whether the item has a `where Self: Sized` bound" }
+    }
 }
 
 rustc_query_append! { define_callbacks! }