Lint on extern in item declaration position

Author: Mark-Simulacrum

compiler/rustc_ast_lowering/src/item.rs

@@ -310,7 +310,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     );
                     let sig = hir::FnSig {
                         decl,
-                        header: this.lower_fn_header(header),
+                        header: this.lower_fn_header(header, fn_sig_span, id),
                         span: fn_sig_span,
                     };
                     hir::ItemKind::Fn(sig, generics, body_id)
@@ -806,13 +806,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
             AssocItemKind::Fn(_, ref sig, ref generics, None) => {
                 let names = self.lower_fn_params_to_names(&sig.decl);
                 let (generics, sig) =
-                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
+                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
             }
             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(generics, sig, trait_item_def_id, false, None);
+                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
             }
             AssocItemKind::TyAlias(_, ref generics, ref bounds, ref default) => {
@@ -882,6 +882,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     impl_item_def_id,
                     impl_trait_return_allow,
                     asyncness.opt_return_id(),
+                    i.id,
                 );
 
                 (generics, hir::ImplItemKind::Fn(sig, body_id))
@@ -1275,8 +1276,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
         fn_def_id: LocalDefId,
         impl_trait_return_allow: bool,
         is_async: Option<NodeId>,
+        id: NodeId,
     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
-        let header = self.lower_fn_header(sig.header);
+        let header = self.lower_fn_header(sig.header, sig.span, id);
         let (generics, decl) = self.add_in_band_defs(
             generics,
             fn_def_id,
@@ -1293,12 +1295,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         (generics, hir::FnSig { header, decl, span: sig.span })
     }
 
-    fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
+    fn lower_fn_header(&mut self, h: FnHeader, span: Span, id: NodeId) -> hir::FnHeader {
         hir::FnHeader {
             unsafety: self.lower_unsafety(h.unsafety),
             asyncness: self.lower_asyncness(h.asyncness),
             constness: self.lower_constness(h.constness),
-            abi: self.lower_extern(h.ext),
+            abi: self.lower_extern(h.ext, span, id),
         }
     }
 
@@ -1309,10 +1311,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         })
     }
 
-    pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
+    pub(super) fn lower_extern(&mut self, ext: Extern, span: Span, id: NodeId) -> abi::Abi {
         match ext {
             Extern::None => abi::Abi::Rust,
-            Extern::Implicit => abi::Abi::C,
+            Extern::Implicit => {
+                self.maybe_lint_missing_abi(span, id, abi::Abi::C);
+                abi::Abi::C
+            }
             Extern::Explicit(abi) => self.lower_abi(abi),
         }
     }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1290,14 +1290,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             TyKind::BareFn(ref f) => self.with_in_scope_lifetime_defs(&f.generic_params, |this| {
                 this.with_anonymous_lifetime_mode(AnonymousLifetimeMode::PassThrough, |this| {
+                    let span = this.sess.source_map().next_point(t.span.shrink_to_lo());
                     hir::TyKind::BareFn(this.arena.alloc(hir::BareFnTy {
                         generic_params: this.lower_generic_params(
                             &f.generic_params,
                             &NodeMap::default(),
                             ImplTraitContext::disallowed(),
                         ),
                         unsafety: this.lower_unsafety(f.unsafety),
-                        abi: this.lower_extern(f.ext),
+                        abi: this.lower_extern(f.ext, span, t.id),
                         decl: this.lower_fn_decl(&f.decl, None, false, None),
                         param_names: this.lower_fn_params_to_names(&f.decl),
                     }))