nits

Author: compiler-errors

compiler/rustc_hir_typeck/src/fallback.rs

@@ -565,6 +565,8 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 Some(self.root_var(self.shallow_resolve(ty).ty_vid()?))
 }
 
+/// Given a set of diverging vids and coercions, walk the HIR to gather a
+/// set of suggestions which can be applied to preserve fallback to unit.
 fntry_to_suggest_annotations(
 &self,
 diverging_vids:&[ty::TyVid],
@@ -574,26 +576,34 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 self.tcx.hir().maybe_body_owned_by(self.body_id).expect("body id must have an owner");
 // For each diverging var, look through the HIR for a place to give it
 // a type annotation. We do this per var because we only really need one
-// per var.
+// suggestion to influence a var to be `()`.
 let suggestions = diverging_vids
 .iter()
 .copied()
 .filter_map(|vid| {
 let reachable_vids =
  graph::depth_first_search_as_undirected(coercions, vid).collect();
-VidVisitor{ reachable_vids,fcx:self}.visit_expr(body.value).break_value()
+AnnotateUnitFallbackVisitor{ reachable_vids,fcx:self}
+.visit_expr(body.value)
+.break_value()
 })
 .collect();
  errors::SuggestAnnotations{ suggestions }
 }
 }
 
-/// Try to collect a useful suggestion to preserve fallback to `()`.
-structVidVisitor<'a,'tcx>{
+/// Try to walk the HIR to find a place to insert a useful suggestion
+/// to preserve fallback to `()` in 2024.
+structAnnotateUnitFallbackVisitor<'a,'tcx>{
 reachable_vids:FxHashSet<ty::TyVid>,
 fcx:&'aFnCtxt<'a,'tcx>,
 }
-impl<'tcx>VidVisitor<'_,'tcx>{
+impl<'tcx>AnnotateUnitFallbackVisitor<'_,'tcx>{
+// For a given path segment, if it's missing a turbofish, try to suggest adding
+// one so we can constrain an argument to `()`. To keep the suggestion simple,
+// we want to simply suggest `_` for all the other args. This (for now) only
+// works when there are only type variables (and region variables, since we can
+// elide them)...
 fnsuggest_for_segment(
 &self,
 arg_segment:&'tcx hir::PathSegment<'tcx>,
@@ -627,7 +637,7 @@ impl<'tcx> VidVisitor<'_, 'tcx> {
 ControlFlow::Continue(())
 }
 }
-impl<'tcx>Visitor<'tcx>forVidVisitor<'_,'tcx>{
+impl<'tcx>Visitor<'tcx>forAnnotateUnitFallbackVisitor<'_,'tcx>{
 typeResult = ControlFlow<errors::SuggestAnnotation>;
 
 fnvisit_ty(&mutself,hir_ty:&'tcx hir::Ty<'tcx>) -> Self::Result{
@@ -657,9 +667,7 @@ impl<'tcx> Visitor<'tcx> for VidVisitor<'_, 'tcx> {
 return hir::intravisit::walk_qpath(self, qpath, id);
 }
 };
-// Alternatively, try to turbofish `::<_, (), _>` (ignoring lifetimes,
-// since we don't need to turbofish those; they'll be inferred).
-// FIXME: Same logic could work for types...
+// Alternatively, try to turbofish `::<_, (), _>`.
 ifletSome(def_id) = self.fcx.typeck_results.borrow().qpath_res(qpath, id).opt_def_id(){
 self.suggest_for_segment(arg_segment, def_id, id)?;
 }
@@ -668,6 +676,7 @@ impl<'tcx> Visitor<'tcx> for VidVisitor<'_, 'tcx> {
 
 fnvisit_expr(&mutself,expr:&'tcx hir::Expr<'tcx>) -> Self::Result{
 // Try to suggest adding an explicit qself `()` to a trait method path.
+// i.e. changing `Default::default()` to `<() as Default>::default()`.
 iflet hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
  && letRes::Def(DefKind::AssocFn, def_id) = path.res
  && self.fcx.tcx.trait_of_item(def_id).is_some()
@@ -679,7 +688,7 @@ impl<'tcx> Visitor<'tcx> for VidVisitor<'_, 'tcx> {
 let span = path.span.shrink_to_lo().to(trait_segment.ident.span);
 returnControlFlow::Break(errors::SuggestAnnotation::Path(span));
 }
-// Or elseturbofishing the method
+// Or else, try suggesting turbofishing the method args.
 iflet hir::ExprKind::MethodCall(segment, ..) = expr.kind
  && letSome(def_id) =
 self.fcx.typeck_results.borrow().type_dependent_def_id(expr.hir_id)
@@ -690,6 +699,7 @@ impl<'tcx> Visitor<'tcx> for VidVisitor<'_, 'tcx> {
 }
 
 fnvisit_local(&mutself,local:&'tcx hir::LetStmt<'tcx>) -> Self::Result{
+// For a local, try suggest annotating the type if it's missing.
 ifletNone = local.ty
  && let ty = self.fcx.typeck_results.borrow().node_type(local.hir_id)
  && letSome(vid) = self.fcx.root_vid(ty)