first pass at default values for const generics

- Adds optional default values to const generic parameters in the AST
  and HIR
- Parses these optional default values
- Adds a `const_generics_defaults` feature gate

compiler/rustc_ast/src/ast.rs

@@ -368,6 +368,8 @@ pub enum GenericParamKind {
         ty: P<Ty>,
         /// Span of the `const` keyword.
         kw_span: Span,
+        /// Optional default value for the const generic param
+        default: Option<AnonConst>,
     },
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -790,8 +790,9 @@ 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 } => {
             vis.visit_ty(ty);
+            visit_opt(default, |default| vis.visit_anon_const(default));
         }
     }
     smallvec![param]

compiler/rustc_ast/src/visit.rs

@@ -578,7 +578,12 @@ pub fn walk_generic_param<'a, V: Visitor<'a>>(visitor: &mut V, param: &'a Generi
     match param.kind {
         GenericParamKind::Lifetime => (),
         GenericParamKind::Type { ref default } => walk_list!(visitor, visit_ty, default),
-        GenericParamKind::Const { ref ty, .. } => visitor.visit_ty(ty),
+        GenericParamKind::Const { ref ty, ref default, .. } => {
+            visitor.visit_ty(ty);
+            if let Some(default) = default {
+                visitor.visit_anon_const(default);
+            }
+        }
     }
 }