Permit and require stability attributes on default type parameters

Co-Authored-By: Avi Dessauer <avi.the.coder@gmail.com>

Author: varkor

src/librustc_passes/reachable.rs

@@ -309,6 +309,7 @@ impl<'a, 'tcx> ReachableContext<'a, 'tcx> {
             | Node::Ctor(..)
             | Node::Field(_)
             | Node::Ty(_)
+            | Node::GenericParam(_)
             | Node::MacroDef(_) => {}
             _ => {
                 bug!(

src/librustc_passes/stability.rs

@@ -278,6 +278,20 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
     fn visit_macro_def(&mut self, md: &'tcx hir::MacroDef<'tcx>) {
         self.annotate(md.hir_id, &md.attrs, md.span, AnnotationKind::Required, |_| {});
     }
+
+    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::Required
+            }
+            _ => AnnotationKind::Prohibited,
+        };
+
+        self.annotate(p.hir_id, &p.attrs, p.span, kind, |v| {
+            intravisit::walk_generic_param(v, p);
+        });
+    }
 }
 
 struct MissingStabilityAnnotations<'a, 'tcx> {
@@ -348,6 +362,17 @@ impl<'a, 'tcx> Visitor<'tcx> for MissingStabilityAnnotations<'a, 'tcx> {
     fn visit_macro_def(&mut self, md: &'tcx hir::MacroDef<'tcx>) {
         self.check_missing_stability(md.hir_id, md.span, "macro");
     }
+
+    fn visit_generic_param(&mut self, p: &'tcx hir::GenericParam<'tcx>) {
+        match &p.kind {
+            // FIXME(const_generics:defaults)
+            hir::GenericParamKind::Type { default, .. } if default.is_some() => {
+                self.check_missing_stability(p.hir_id, p.span, "default type parameter");
+            }
+            _ => {}
+        }
+        intravisit::walk_generic_param(self, p);
+    }
 }
 
 fn new_index(tcx: TyCtxt<'tcx>) -> Index<'tcx> {

src/librustc_privacy/lib.rs

@@ -946,6 +946,20 @@ impl Visitor<'tcx> for EmbargoVisitor<'tcx> {
             module_id = self.tcx.hir().get_parent_node(module_id);
         }
     }
+
+    fn visit_generic_param(&mut self, p: &'tcx hir::GenericParam<'tcx>) {
+        // Generic parameters cannot be directly accessed by users, so restricting privacy is
+        // unimportant here.
+        // However, parameters must be (at least) `Reachable` in order for stability requirements to
+        // be enforced (see `check_missing_stability` in `src/librustc_passes/stability.rs`).
+        match &p.kind {
+            // FIXME(const_generics:defaults)
+            hir::GenericParamKind::Type { default, .. } if default.is_some() => {
+                self.update(p.hir_id, Some(AccessLevel::Reachable));
+            }
+            _ => {}
+        }
+    }
 }
 
 impl ReachEverythingInTheInterfaceVisitor<'_, 'tcx> {