renumber: handle ReturnTy better

src/librustc/mir/visit.rs

@@ -292,11 +292,10 @@ macro_rules! basic_blocks {
                     self.visit_visibility_scope_data(scope);
                 }
 
-                let lookup = TyContext::SourceInfo(SourceInfo {
+                self.visit_ty(&$($mutability)* mir.return_ty, TyContext::ReturnTy(SourceInfo {
                     span: mir.span,
                     scope: ARGUMENT_VISIBILITY_SCOPE,
-                });
-                self.visit_ty(&$($mutability)* mir.return_ty, lookup);
+                }));
 
                 for local in mir.local_decls.indices() {
                     self.visit_local_decl(local, & $($mutability)* mir.local_decls[local]);
@@ -821,9 +820,11 @@ pub enum TyContext {
         source_info: SourceInfo,
     },
 
-    Location(Location),
+    /// The return type of the function.
+    ReturnTy(SourceInfo),
 
-    SourceInfo(SourceInfo),
+    /// A type found at some location.
+    Location(Location),
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

src/librustc_mir/transform/nll/renumber.rs

@@ -92,8 +92,15 @@ impl<'a, 'gcx, 'tcx> MutVisitor<'tcx> for NLLVisitor<'a, 'gcx, 'tcx> {
     fn visit_ty(&mut self, ty: &mut Ty<'tcx>, ty_context: TyContext) {
         let is_arg = match ty_context {
             TyContext::LocalDecl { local, .. } => self.is_argument_or_return_slot(local),
-            _ => false,
+            TyContext::ReturnTy(..) => true,
+            TyContext::Location(..) => false,
         };
+        debug!(
+            "visit_ty(ty={:?}, is_arg={:?}, ty_context={:?})",
+            ty,
+            is_arg,
+            ty_context
+        );
 
         let old_ty = *ty;
         *ty = if is_arg {
@@ -101,6 +108,7 @@ fn visit_ty(&mut self, ty: &mut Ty<'tcx>, ty_context: TyContext) {
         } else {
             self.renumber_regions(ty_context, &old_ty)
         };
+        debug!("visit_ty: ty={:?}", ty);
     }
 
     fn visit_substs(&mut self, substs: &mut &'tcx Substs<'tcx>, location: Location) {