Auto merge of #75384 - JulianKnodt:cg_def, r=varkor,lcnr

implement `feature(const_generics_defaults)`

Implements const generics defaults `struct Example<const N: usize=3>`, as well as a query for getting the default of a given const-parameter's def id. There are some remaining FIXME's but they were specified as not blocking for merging this PR. This also puts the defaults behind the unstable feature gate `#![feature(const_generics_defaults)]`.

~~This currently creates a field which is always false on `GenericParamDefKind` for future use when
consts are permitted to have defaults. I'm not sure if this is exactly what is best for adding default parameters, but I mimicked the style of type defaults, so hopefully this is ok.~~

r? `@lcnr`

compiler/rustc_passes/src/stability.rs

@@ -507,10 +507,9 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
 
     fn visit_generic_param(&mut self, p: &'tcx hir::GenericParam<'tcx>) {
         let kind = match &p.kind {
-            // FIXME(const_generics_defaults)
-            hir::GenericParamKind::Type { default, .. } if default.is_some() => {
-                AnnotationKind::Container
-            }
+            // Allow stability attributes on default generic arguments.
+            hir::GenericParamKind::Type { default: Some(_), .. }
+            | hir::GenericParamKind::Const { default: Some(_), .. } => AnnotationKind::Container,
             _ => AnnotationKind::Prohibited,
         };