Provide local extern function arg names

compiler/rustc_middle/src/hir/mod.rs

@@ -160,9 +160,13 @@ pub fn provide(providers: &mut Providers) {
         } else if let Node::TraitItem(&TraitItem {
             kind: TraitItemKind::Fn(_, TraitFn::Required(idents)),
             ..
+        })
+        | Node::ForeignItem(&ForeignItem {
+            kind: ForeignItemKind::Fn(_, idents, _),
+            ..
         }) = hir.get(hir_id)
         {
-            tcx.arena.alloc_slice(idents)
+            idents
         } else {
             span_bug!(hir.span(hir_id), "fn_arg_names: unexpected item {:?}", id);
         }

src/test/ui/argument-suggestions/extern-fn-arg-names.rs

@@ -0,0 +1,9 @@
+extern "Rust" {
+    fn dstfn(src: i32, dst: err);
+    //~^ ERROR cannot find type `err` in this scope
+}
+
+fn main() {
+    dstfn(1);
+    //~^ ERROR this function takes 2 arguments but 1 argument was supplied
+}

src/test/ui/argument-suggestions/extern-fn-arg-names.stderr

@@ -0,0 +1,26 @@
+error[E0412]: cannot find type `err` in this scope
+  --> $DIR/extern-fn-arg-names.rs:2:29
+   |
+LL |     fn dstfn(src: i32, dst: err);
+   |                             ^^^ not found in this scope
+
+error[E0061]: this function takes 2 arguments but 1 argument was supplied
+  --> $DIR/extern-fn-arg-names.rs:7:5
+   |
+LL |     dstfn(1);
+   |     ^^^^^--- an argument is missing
+   |
+note: function defined here
+  --> $DIR/extern-fn-arg-names.rs:2:8
+   |
+LL |     fn dstfn(src: i32, dst: err);
+   |        ^^^^^
+help: provide the argument
+   |
+LL |     dstfn(1, /* dst */);
+   |          ~~~~~~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0061, E0412.
+For more information about an error, try `rustc --explain E0061`.