error for impl trait alias

Author: durka

src/librustc_resolve/lib.rs

@@ -390,12 +390,18 @@ impl PatternSource {
  }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum AliasPossibility {
+ No,
+ Maybe,
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum PathSource<'a> {
  // Type paths `Path`.
  Type,
  // Trait paths in bounds or impls.
- Trait,
+ Trait(AliasPossibility),
  // Expression paths `path`, with optional parent context.
  Expr(Option<&'a Expr>),
  // Paths in path patterns `Path`.
@@ -415,7 +421,7 @@ enum PathSource<'a> {
 impl<'a> PathSource<'a> {
  fn namespace(self) -> Namespace {
  match self {
- PathSource::Type | PathSource::Trait | PathSource::Struct |
+ PathSource::Type | PathSource::Trait(_) | PathSource::Struct |
  PathSource::Visibility | PathSource::ImportPrefix => TypeNS,
  PathSource::Expr(..) | PathSource::Pat | PathSource::TupleStruct => ValueNS,
  PathSource::TraitItem(ns) => ns,
@@ -427,23 +433,23 @@ impl<'a> PathSource<'a> {
  PathSource::Visibility | PathSource::ImportPrefix => true,
  PathSource::Type | PathSource::Expr(..) | PathSource::Pat |
  PathSource::Struct | PathSource::TupleStruct |
- PathSource::Trait | PathSource::TraitItem(..) => false,
+ PathSource::Trait(_) | PathSource::TraitItem(..) => false,
  }
  }
 
  fn defer_to_typeck(self) -> bool {
  match self {
  PathSource::Type | PathSource::Expr(..) | PathSource::Pat |
  PathSource::Struct | PathSource::TupleStruct => true,
- PathSource::Trait | PathSource::TraitItem(..) |
+ PathSource::Trait(_) | PathSource::TraitItem(..) |
  PathSource::Visibility | PathSource::ImportPrefix => false,
  }
  }
 
  fn descr_expected(self) -> &'static str {
  match self {
  PathSource::Type => "type",
- PathSource::Trait => "trait",
+ PathSource::Trait(_) => "trait",
  PathSource::Pat => "unit struct/variant or constant",
  PathSource::Struct => "struct, variant or union type",
  PathSource::TupleStruct => "tuple struct/variant",
@@ -472,7 +478,11 @@ impl<'a> PathSource<'a> {
  Def::TyForeign(..) => true,
  _ => false,
  },
- PathSource::Trait => match def {
+ PathSource::Trait(AliasPossibility::No) => match def {
+ Def::Trait(..) => true,
+ _ => false,
+ },
+ PathSource::Trait(AliasPossibility::Maybe) => match def {
  Def::Trait(..) => true,
  Def::TraitAlias(..) => true,
  _ => false,
@@ -531,8 +541,8 @@ impl<'a> PathSource<'a> {
  __diagnostic_used!(E0577);
  __diagnostic_used!(E0578);
  match (self, has_unexpected_resolution) {
- (PathSource::Trait, true) => "E0404",
- (PathSource::Trait, false) => "E0405",
+ (PathSource::Trait(_), true) => "E0404",
+ (PathSource::Trait(_), false) => "E0405",
  (PathSource::Type, true) => "E0573",
  (PathSource::Type, false) => "E0412",
  (PathSource::Struct, true) => "E0574",
@@ -694,7 +704,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Resolver<'a> {
  tref: &'tcx ast::PolyTraitRef,
  m: &'tcx ast::TraitBoundModifier) {
  self.smart_resolve_path(tref.trait_ref.ref_id, None,
- &tref.trait_ref.path, PathSource::Trait);
+ &tref.trait_ref.path, PathSource::Trait(AliasPossibility::Maybe));
  visit::walk_poly_trait_ref(self, tref, m);
  }
  fn visit_variant(&mut self,
@@ -2095,7 +2105,7 @@ impl<'a> Resolver<'a> {
  &path,
  trait_ref.path.span,
  trait_ref.path.segments.last().unwrap().span,
- PathSource::Trait)
+ PathSource::Trait(AliasPossibility::No))
  .base_def();
  if def != Def::Err {
  new_id = Some(def.def_id());
@@ -2647,7 +2657,7 @@ impl<'a> Resolver<'a> {
  err.span_label(span, format!("did you mean `{}!(...)`?", path_str));
  return (err, candidates);
  }
- (Def::TyAlias(..), PathSource::Trait) => {
+ (Def::TyAlias(..), PathSource::Trait(_)) => {
  err.span_label(span, "type aliases cannot be used for traits");
  return (err, candidates);
  }

src/test/compile-fail/trait-alias.rs

@@ -18,7 +18,9 @@ trait Alias2<T: Clone = ()> = Default;
  //~^^^ERROR trait aliases are not yet fully implemented
 
 impl Alias1 { //~ERROR expected type, found trait alias
- fn foo() {}
+}
+
+impl Alias1 for () { //~ERROR expected trait, found trait alias
 }
 
 fn main() {}