Add has_default to GenericParamDefKind::Const

This currently creates a field which is always false on GenericParamDefKind for future use when
consts are permitted to have defaults

Update const_generics:default locations

Previously just ignored them, now actually do something about them.

Fix using type check instead of value

Add parsing

This adds all the necessary changes to lower const-generics defaults from parsing.

Change P<Expr> to AnonConst

This matches the arguments passed to instantiations of const generics, and makes it specific to
just anonymous constants.

Attempt to fix lowering bugs

Author: JulianKnodt

compiler/rustc_ast/src/ast.rs

@@ -368,6 +368,7 @@ pub enum GenericParamKind {
         ty: P<Ty>,
         /// Span of the `const` keyword.
         kw_span: Span,
+        default: Option<AnonConst>,
     },
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -790,7 +790,8 @@ pub fn noop_flat_map_generic_param<T: MutVisitor>(
         GenericParamKind::Type { default } => {
             visit_opt(default, |default| vis.visit_ty(default));
         }
-        GenericParamKind::Const { ty, kw_span: _ } => {
+        GenericParamKind::Const { ty, kw_span: _, default } => {
+            visit_opt(default, |default| vis.visit_anon_const(default));
             vis.visit_ty(ty);
         }
     }