Auto merge of #47416 - petrochenkov:remove-impl-for-dot-dot, r=petrochenkov

Remove `impl Foo for .. {}` in favor `auto trait Foo {}`

Rebase of https://github.com/rust-lang/rust/pull/46480 with restored parsing support.

6 files changed, 39 insertions, 80 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index e08a2cbfd08..9c0622e7bef 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -2022,10 +2022,6 @@ pub enum ItemKind {
     ///
     /// E.g. `trait Foo = Bar + Quux;`
     TraitAlias(Generics, TyParamBounds),
-    /// Auto trait implementation.
-    ///
-    /// E.g. `impl Trait for .. {}` or `impl<T> Trait<T> for .. {}`
-    AutoImpl(Unsafety, TraitRef),
     /// An implementation.
     ///
     /// E.g. `impl<A> Foo<A> { .. }` or `impl<A> Trait for Foo<A> { .. }`
@@ -2064,8 +2060,7 @@ impl ItemKind {
             ItemKind::TraitAlias(..) => "trait alias",
             ItemKind::Mac(..) |
             ItemKind::MacroDef(..) |
-            ItemKind::Impl(..) |
-            ItemKind::AutoImpl(..) => "item"
+            ItemKind::Impl(..) => "item"
         }
     }
 }
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index b474a299058..c3bf5dbff5c 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -1532,13 +1532,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                    "trait aliases are not yet fully implemented");
             }
 
-            ast::ItemKind::AutoImpl(..) => {
-                gate_feature_post!(&self, optin_builtin_traits,
-                                   i.span,
-                                   "auto trait implementations are experimental \
-                                    and possibly buggy");
-            }
-
             ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, ref impl_items) => {
                 if polarity == ast::ImplPolarity::Negative {
                     gate_feature_post!(&self, optin_builtin_traits,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 6682a3439f1..c304e3a9f50 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -911,9 +911,6 @@ pub fn noop_fold_item_kind<T: Folder>(i: ItemKind, folder: &mut T) -> ItemKind {
             let generics = folder.fold_generics(generics);
             ItemKind::Union(folder.fold_variant_data(struct_def), generics)
         }
-        ItemKind::AutoImpl(unsafety, ref trait_ref) => {
-            ItemKind::AutoImpl(unsafety, folder.fold_trait_ref((*trait_ref).clone()))
-        }
         ItemKind::Impl(unsafety,
                        polarity,
                        defaultness,
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 489e5e78cce..ad9c802ac85 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -4084,14 +4084,14 @@ impl<'a> Parser<'a> {
         self.token.is_keyword(keywords::Extern) && self.look_ahead(1, |t| t != &token::ModSep)
     }
 
-    fn eat_auto_trait(&mut self) -> bool {
-        if self.token.is_keyword(keywords::Auto)
-            && self.look_ahead(1, |t| t.is_keyword(keywords::Trait))
-        {
-            self.eat_keyword(keywords::Auto) && self.eat_keyword(keywords::Trait)
-        } else {
-            false
-        }
+    fn is_auto_trait_item(&mut self) -> bool {
+        // auto trait
+        (self.token.is_keyword(keywords::Auto)
+            && self.look_ahead(1, |t| t.is_keyword(keywords::Trait)))
+        || // unsafe auto trait
+        (self.token.is_keyword(keywords::Unsafe) &&
+         self.look_ahead(1, |t| t.is_keyword(keywords::Auto)) &&
+         self.look_ahead(2, |t| t.is_keyword(keywords::Trait)))
     }
 
     fn is_defaultness(&self) -> bool {
@@ -4194,7 +4194,8 @@ impl<'a> Parser<'a> {
                 node: StmtKind::Item(macro_def),
                 span: lo.to(self.prev_span),
             }
-        // Starts like a simple path, but not a union item or item with `crate` visibility.
+        // Starts like a simple path, being careful to avoid contextual keywords
+        // such as a union items, item with `crate` visibility or auto trait items.
         // Our goal here is to parse an arbitrary path `a::b::c` but not something that starts
         // like a path (1 token), but it fact not a path.
         // `union::b::c` - path, `union U { ... }` - not a path.
@@ -4204,7 +4205,8 @@ impl<'a> Parser<'a> {
                   !self.token.is_qpath_start() &&
                   !self.is_union_item() &&
                   !self.is_crate_vis() &&
-                  !self.is_extern_non_path() {
+                  !self.is_extern_non_path() &&
+                  !self.is_auto_trait_item() {
             let pth = self.parse_path(PathStyle::Expr)?;
 
             if !self.eat(&token::Not) {
@@ -5374,11 +5376,9 @@ impl<'a> Parser<'a> {
     /// Parses items implementations variants
     ///    impl<T> Foo { ... }
     ///    impl<T> ToString for &'static T { ... }
-    ///    impl Send for .. {}
     fn parse_item_impl(&mut self,
                        unsafety: ast::Unsafety,
                        defaultness: Defaultness) -> PResult<'a, ItemInfo> {
-        let impl_span = self.span;
 
         // First, parse type parameters if necessary.
         let mut generics = self.parse_generics()?;
@@ -5421,48 +5421,35 @@ impl<'a> Parser<'a> {
             None
         };
 
-        if opt_trait.is_some() && self.eat(&token::DotDot) {
-            if generics.is_parameterized() {
-                self.span_err(impl_span, "auto trait implementations are not \
-                                          allowed to have generics");
-            }
-
-            if let ast::Defaultness::Default = defaultness {
-                self.span_err(impl_span, "`default impl` is not allowed for \
-                                         auto trait implementations");
-            }
-
-            self.expect(&token::OpenDelim(token::Brace))?;
-            self.expect(&token::CloseDelim(token::Brace))?;
-            Ok((keywords::Invalid.ident(),
-             ItemKind::AutoImpl(unsafety, opt_trait.unwrap()), None))
-        } else {
-            if opt_trait.is_some() {
-                ty = self.parse_ty()?;
+        if opt_trait.is_some() {
+            ty = if self.eat(&token::DotDot) {
+                P(Ty { node: TyKind::Err, span: self.prev_span, id: ast::DUMMY_NODE_ID })
+            } else {
+                self.parse_ty()?
             }
-            generics.where_clause = self.parse_where_clause()?;
+        }
+        generics.where_clause = self.parse_where_clause()?;
 
-            self.expect(&token::OpenDelim(token::Brace))?;
-            let attrs = self.parse_inner_attributes()?;
+        self.expect(&token::OpenDelim(token::Brace))?;
+        let attrs = self.parse_inner_attributes()?;
 
-            let mut impl_items = vec![];
-            while !self.eat(&token::CloseDelim(token::Brace)) {
-                let mut at_end = false;
-                match self.parse_impl_item(&mut at_end) {
-                    Ok(item) => impl_items.push(item),
-                    Err(mut e) => {
-                        e.emit();
-                        if !at_end {
-                            self.recover_stmt_(SemiColonMode::Break, BlockMode::Break);
-                        }
+        let mut impl_items = vec![];
+        while !self.eat(&token::CloseDelim(token::Brace)) {
+            let mut at_end = false;
+            match self.parse_impl_item(&mut at_end) {
+                Ok(item) => impl_items.push(item),
+                Err(mut e) => {
+                    e.emit();
+                    if !at_end {
+                        self.recover_stmt_(SemiColonMode::Break, BlockMode::Break);
                     }
                 }
             }
-
-            Ok((keywords::Invalid.ident(),
-             ItemKind::Impl(unsafety, polarity, defaultness, generics, opt_trait, ty, impl_items),
-             Some(attrs)))
         }
+
+        Ok((keywords::Invalid.ident(),
+            ItemKind::Impl(unsafety, polarity, defaultness, generics, opt_trait, ty, impl_items),
+            Some(attrs)))
     }
 
     fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
@@ -6387,7 +6374,8 @@ impl<'a> Parser<'a> {
             let is_auto = if self.eat_keyword(keywords::Trait) {
                 IsAuto::No
             } else {
-                self.eat_auto_trait();
+                self.expect_keyword(keywords::Auto)?;
+                self.expect_keyword(keywords::Trait)?;
                 IsAuto::Yes
             };
             let (ident, item_, extra_attrs) =
@@ -6501,7 +6489,8 @@ impl<'a> Parser<'a> {
             let is_auto = if self.eat_keyword(keywords::Trait) {
                 IsAuto::No
             } else {
-                self.eat_auto_trait();
+                self.expect_keyword(keywords::Auto)?;
+                self.expect_keyword(keywords::Trait)?;
                 IsAuto::Yes
             };
             // TRAIT ITEM
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index dd343a2384b..5374bf180f4 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1294,18 +1294,6 @@ impl<'a> State<'a> {
                 self.head(&visibility_qualified(&item.vis, "union"))?;
                 self.print_struct(struct_def, generics, item.ident, item.span, true)?;
             }
-            ast::ItemKind::AutoImpl(unsafety, ref trait_ref) => {
-                self.head("")?;
-                self.print_visibility(&item.vis)?;
-                self.print_unsafety(unsafety)?;
-                self.word_nbsp("impl")?;
-                self.print_trait_ref(trait_ref)?;
-                self.s.space()?;
-                self.word_space("for")?;
-                self.word_space("..")?;
-                self.bopen()?;
-                self.bclose(item.span)?;
-            }
             ast::ItemKind::Impl(unsafety,
                           polarity,
                           defaultness,
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index bd68d7cf8f5..b5fc9236ad3 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -259,9 +259,6 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
             visitor.visit_generics(type_parameters);
             visitor.visit_enum_def(enum_definition, type_parameters, item.id, item.span)
         }
-        ItemKind::AutoImpl(_, ref trait_ref) => {
-            visitor.visit_trait_ref(trait_ref)
-        }
         ItemKind::Impl(_, _, _,
                  ref type_parameters,
                  ref opt_trait_reference,