Rollup merge of #137913 - compiler-errors:struct-field-default-generic, r=BoxyUwU

Allow struct field default values to reference struct's generics

Right now, the default field value feature (https://github.com/rust-lang/rust/issues/132162) lowers anon consts whose types may reference ADT params that the const doesn't inherit.

This PR fixes this, so that these defaults can reference ADTs' generics, and sets the `generics_of` parenting up correctly.

There doesn't seem to be a good reason not to support this, since the anon const has a well-defined type from the field, and the anon const doesn't interact with the type system like generic parameter defaults do.

r? `````@boxyuwu````` or reassign

I could also make this into an error if this seems problematic (https://github.com/rust-lang/rust/compare/master...compiler-errors:rust:default-field-value-implicit-param?expand=1)...... but I'd rather make this work and register an open question on the tracking issue about validating that this is well-vetted.

Fixes #137896

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -187,6 +187,8 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                         Some(parent_did)
                     }
                     Node::TyPat(_) => Some(parent_did),
+                    // Field default values inherit the ADT's generics.
+                    Node::Field(_) => Some(parent_did),
                     _ => None,
                 }
             }