Rollup merge of rust-lang#69194 - Centril:assoc-extern-fuse, r=petrochenkov

parse: fuse associated and extern items up to defaultness

Language changes:

- The grammar of extern `type` aliases is unified with associated ones, and becomes:
  ```rust
  TypeItem = "type" ident generics {":" bounds}? where_clause {"=" type}? ";" ;
  ```

  Semantic restrictions (`ast_validation`) are added to forbid any parameters in `generics`, any bounds in `bounds`, and any predicates in `where_clause`, as well as the presence of a type expression (`= u8`).

  (Work still remains to fuse this with free `type` aliases, but this can be done later.)

- The grammar of constants and static items (free, associated, and extern) now permits the absence of an expression, and becomes:

  ```rust
  GlobalItem = {"const" {ident | "_"} | "static" "mut"? ident} {"=" expr}? ";" ;
  ```

  - A semantic restriction is added to enforce the presence of the expression (the body).
  - A semantic restriction is added to reject `const _` in associated contexts.

Together, these changes allow us to fuse the grammar of associated items and extern items up to `default`ness which is the main goal of the PR.

-----------------------

We are now very close to fully fusing the entirely of item parsing and their ASTs. To progress further, we must make a decision: should we parse e.g. `default use foo::bar;` and whatnot? Accepting that is likely easiest from a parsing perspective, as it does not require using look-ahead, but it is perhaps not too onerous to only accept it for `fn`s (and all their various qualifiers), `const`s, `static`s, and `type`s.

r? @petrochenkov

Author: Centril

src/librustc_ast_lowering/item.rs

@@ -269,26 +269,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
             }
             ItemKind::Static(ref t, m, ref e) => {
-                let ty = self.lower_ty(
-                    t,
-                    if self.sess.features_untracked().impl_trait_in_bindings {
-                        ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
-                    } else {
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
-                    },
-                );
-                hir::ItemKind::Static(ty, m, self.lower_const_body(span, Some(e)))
+                let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
+                hir::ItemKind::Static(ty, m, body_id)
             }
             ItemKind::Const(ref t, ref e) => {
-                let ty = self.lower_ty(
-                    t,
-                    if self.sess.features_untracked().impl_trait_in_bindings {
-                        ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
-                    } else {
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
-                    },
-                );
-                hir::ItemKind::Const(ty, self.lower_const_body(span, Some(e)))
+                let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
+                hir::ItemKind::Const(ty, body_id)
             }
             ItemKind::Fn(FnSig { ref decl, header }, ref generics, ref body) => {
                 let fn_def_id = self.resolver.definitions().local_def_id(id);
@@ -457,6 +443,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
         //     not cause an assertion failure inside the `lower_defaultness` function.
     }
 
+    fn lower_const_item(
+        &mut self,
+        ty: &Ty,
+        span: Span,
+        body: Option<&Expr>,
+    ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
+        let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
+            ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
+        } else {
+            ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
+        };
+        let ty = self.lower_ty(ty, itctx);
+        (ty, self.lower_const_body(span, body))
+    }
+
     fn lower_use_tree(
         &mut self,
         tree: &UseTree,
@@ -678,11 +679,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
                 }
-                ForeignItemKind::Static(ref t, m) => {
+                ForeignItemKind::Static(ref t, m, _) => {
                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
                     hir::ForeignItemKind::Static(ty, m)
                 }
-                ForeignItemKind::Ty => hir::ForeignItemKind::Type,
+                ForeignItemKind::Const(ref t, _) => {
+                    // For recovery purposes.
+                    let ty = self.lower_ty(t, ImplTraitContext::disallowed());
+                    hir::ForeignItemKind::Static(ty, Mutability::Not)
+                }
+                ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
             },
             vis: self.lower_visibility(&i.vis, None),
@@ -759,32 +765,27 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 
         let (generics, kind) = match i.kind {
-            AssocItemKind::Const(ref ty, ref default) => {
-                let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
+            AssocItemKind::Static(ref ty, _, ref default) // Let's pretend this is a `const`.
+            | AssocItemKind::Const(ref ty, ref default) => {
                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
-                (
-                    generics,
-                    hir::TraitItemKind::Const(
-                        ty,
-                        default.as_ref().map(|x| self.lower_const_body(i.span, Some(x))),
-                    ),
-                )
+                let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
+                (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
             }
-            AssocItemKind::Fn(ref sig, None) => {
+            AssocItemKind::Fn(ref sig, ref generics, None) => {
                 let names = self.lower_fn_params_to_names(&sig.decl);
                 let (generics, sig) =
-                    self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
+                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
             }
-            AssocItemKind::Fn(ref sig, Some(ref body)) => {
+            AssocItemKind::Fn(ref sig, ref generics, Some(ref body)) => {
                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
                 let (generics, sig) =
-                    self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
+                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
             }
-            AssocItemKind::TyAlias(ref bounds, ref default) => {
+            AssocItemKind::TyAlias(ref generics, ref bounds, ref default) => {
                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
-                let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
+                let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
                 let kind = hir::TraitItemKind::Type(
                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
                     ty,
@@ -806,10 +807,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
-        let (kind, has_default) = match i.kind {
-            AssocItemKind::Const(_, ref default) => (hir::AssocItemKind::Const, default.is_some()),
-            AssocItemKind::TyAlias(_, ref default) => (hir::AssocItemKind::Type, default.is_some()),
-            AssocItemKind::Fn(ref sig, ref default) => {
+        let (kind, has_default) = match &i.kind {
+            AssocItemKind::Static(_, _, default) // Let's pretend this is a `const` for recovery.
+            | AssocItemKind::Const(_, default) => {
+                (hir::AssocItemKind::Const, default.is_some())
+            }
+            AssocItemKind::TyAlias(_, _, default) => (hir::AssocItemKind::Type, default.is_some()),
+            AssocItemKind::Fn(sig, _, default) => {
                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
             }
             AssocItemKind::Macro(..) => unimplemented!(),
@@ -832,22 +836,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 
         let (generics, kind) = match i.kind {
-            AssocItemKind::Const(ref ty, ref expr) => {
-                let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
+            AssocItemKind::Static(ref ty, _, ref expr) | AssocItemKind::Const(ref ty, ref expr) => {
                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
                 (
-                    generics,
+                    hir::Generics::empty(),
                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
                 )
             }
-            AssocItemKind::Fn(ref sig, ref body) => {
+            AssocItemKind::Fn(ref sig, ref generics, ref body) => {
                 self.current_item = Some(i.span);
                 let asyncness = sig.header.asyncness;
                 let body_id =
                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
                 let impl_trait_return_allow = !self.is_in_trait_impl;
                 let (generics, sig) = self.lower_method_sig(
-                    &i.generics,
+                    generics,
                     sig,
                     impl_item_def_id,
                     impl_trait_return_allow,
@@ -856,8 +859,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                 (generics, hir::ImplItemKind::Method(sig, body_id))
             }
-            AssocItemKind::TyAlias(_, ref ty) => {
-                let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
+            AssocItemKind::TyAlias(ref generics, _, ref ty) => {
+                let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
                 let kind = match ty {
                     None => {
                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
@@ -901,14 +904,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
             vis: self.lower_visibility(&i.vis, Some(i.id)),
             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
             kind: match &i.kind {
-                AssocItemKind::Const(..) => hir::AssocItemKind::Const,
-                AssocItemKind::TyAlias(_, ty) => {
+                AssocItemKind::Static(..) // Let's pretend this is a `const` for recovery.
+                | AssocItemKind::Const(..) => hir::AssocItemKind::Const,
+                AssocItemKind::TyAlias(_, _, ty) => {
                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
                         None => hir::AssocItemKind::Type,
                         Some(_) => hir::AssocItemKind::OpaqueTy,
                     }
                 }
-                AssocItemKind::Fn(sig, _) => {
+                AssocItemKind::Fn(sig, _, _) => {
                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
                 }
                 AssocItemKind::Macro(..) => unimplemented!(),

src/librustc_ast_lowering/lib.rs

@@ -490,7 +490,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 self.lctx.allocate_hir_id_counter(item.id);
                 let owner = match (&item.kind, ctxt) {
                     // Ignore patterns in trait methods without bodies.
-                    (AssocItemKind::Fn(_, None), AssocCtxt::Trait) => None,
+                    (AssocItemKind::Fn(_, _, None), AssocCtxt::Trait) => None,
                     _ => Some(item.id),
                 };
                 self.with_hir_id_owner(owner, |this| visit::walk_assoc_item(this, item, ctxt));

src/librustc_ast_passes/ast_validation.rs

@@ -22,6 +22,9 @@ use syntax::expand::is_proc_macro_attr;
 use syntax::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
 use syntax::walk_list;
 
+const MORE_EXTERN: &str =
+    "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
+
 /// Is `self` allowed semantically as the first parameter in an `FnDecl`?
 enum SelfSemantic {
     Yes,
@@ -423,14 +426,62 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_impl_assoc_type_no_bounds(&self, bounds: &[GenericBound]) {
+    fn check_type_no_bounds(&self, bounds: &[GenericBound], ctx: &str) {
         let span = match bounds {
             [] => return,
             [b0] => b0.span(),
             [b0, .., bl] => b0.span().to(bl.span()),
         };
         self.err_handler()
-            .struct_span_err(span, "bounds on associated `type`s in `impl`s have no effect")
+            .struct_span_err(span, &format!("bounds on `type`s in {} have no effect", ctx))
+            .emit();
+    }
+
+    fn check_foreign_ty_genericless(&self, generics: &Generics) {
+        let cannot_have = |span, descr, remove_descr| {
+            self.err_handler()
+                .struct_span_err(
+                    span,
+                    &format!("`type`s inside `extern` blocks cannot have {}", descr),
+                )
+                .span_suggestion(
+                    span,
+                    &format!("remove the {}", remove_descr),
+                    String::new(),
+                    Applicability::MaybeIncorrect,
+                )
+                .span_label(self.current_extern_span(), "`extern` block begins here")
+                .note(MORE_EXTERN)
+                .emit();
+        };
+
+        if !generics.params.is_empty() {
+            cannot_have(generics.span, "generic parameters", "generic parameters");
+        }
+
+        if !generics.where_clause.predicates.is_empty() {
+            cannot_have(generics.where_clause.span, "`where` clauses", "`where` clause");
+        }
+    }
+
+    fn check_foreign_kind_bodyless(&self, ident: Ident, kind: &str, body: Option<Span>) {
+        let body = match body {
+            None => return,
+            Some(body) => body,
+        };
+        self.err_handler()
+            .struct_span_err(ident.span, &format!("incorrect `{}` inside `extern` block", kind))
+            .span_label(ident.span, "cannot have a body")
+            .span_label(body, "the invalid body")
+            .span_label(
+                self.current_extern_span(),
+                format!(
+                    "`extern` blocks define existing foreign {0}s and {0}s \
+                    inside of them cannot have a body",
+                    kind
+                ),
+            )
+            .note(MORE_EXTERN)
             .emit();
     }
 
@@ -458,7 +509,7 @@ impl<'a> AstValidator<'a> {
                 "`extern` blocks define existing foreign functions and functions \
                 inside of them cannot have a body",
             )
-            .note("for more information, visit https://doc.rust-lang.org/std/keyword.extern.html")
+            .note(MORE_EXTERN)
             .emit();
     }
 
@@ -531,6 +582,16 @@ impl<'a> AstValidator<'a> {
             }
         }
     }
+
+    fn check_item_named(&self, ident: Ident, kind: &str) {
+        if ident.name != kw::Underscore {
+            return;
+        }
+        self.err_handler()
+            .struct_span_err(ident.span, &format!("`{}` items in this context need a name", kind))
+            .span_label(ident.span, format!("`_` is not a valid name for this `{}` item", kind))
+            .emit();
+    }
 }
 
 enum GenericPosition {
@@ -900,6 +961,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.err_handler().span_err(item.span, "unions cannot have zero fields");
                 }
             }
+            ItemKind::Const(.., None) => {
+                let msg = "free constant item without body";
+                self.error_item_without_body(item.span, "constant", msg, " = <expr>;");
+            }
+            ItemKind::Static(.., None) => {
+                let msg = "free static item without body";
+                self.error_item_without_body(item.span, "static", msg, " = <expr>;");
+            }
             _ => {}
         }
 
@@ -912,7 +981,15 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self.check_foreign_fn_bodyless(fi.ident, body.as_deref());
                 self.check_foreign_fn_headerless(fi.ident, fi.span, sig.header);
             }
-            ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
+            ForeignItemKind::TyAlias(generics, bounds, body) => {
+                self.check_foreign_kind_bodyless(fi.ident, "type", body.as_ref().map(|b| b.span));
+                self.check_type_no_bounds(bounds, "`extern` blocks");
+                self.check_foreign_ty_genericless(generics);
+            }
+            ForeignItemKind::Static(_, _, body) => {
+                self.check_foreign_kind_bodyless(fi.ident, "static", body.as_ref().map(|b| b.span));
+            }
+            ForeignItemKind::Const(..) | ForeignItemKind::Macro(..) => {}
         }
 
         visit::walk_foreign_item(self, fi)
@@ -1154,25 +1231,29 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 AssocItemKind::Const(_, body) => {
                     self.check_impl_item_provided(item.span, body, "constant", " = <expr>;");
                 }
-                AssocItemKind::Fn(_, body) => {
+                AssocItemKind::Fn(_, _, body) => {
                     self.check_impl_item_provided(item.span, body, "function", " { <body> }");
                 }
-                AssocItemKind::TyAlias(bounds, body) => {
+                AssocItemKind::TyAlias(_, bounds, body) => {
                     self.check_impl_item_provided(item.span, body, "type", " = <type>;");
-                    self.check_impl_assoc_type_no_bounds(bounds);
+                    self.check_type_no_bounds(bounds, "`impl`s");
                 }
                 _ => {}
             }
         }
 
         if ctxt == AssocCtxt::Trait || self.in_trait_impl {
             self.invalid_visibility(&item.vis, None);
-            if let AssocItemKind::Fn(sig, _) = &item.kind {
+            if let AssocItemKind::Fn(sig, _, _) = &item.kind {
                 self.check_trait_fn_not_const(sig.header.constness);
                 self.check_trait_fn_not_async(item.span, sig.header.asyncness);
             }
         }
 
+        if let AssocItemKind::Const(..) = item.kind {
+            self.check_item_named(item.ident, "const");
+        }
+
         self.with_in_trait_impl(false, |this| visit::walk_assoc_item(this, item, ctxt));
     }
 }