Reduce error duplication for invalid placeholder types in fn types

src/librustc_typeck/astconv.rs

@@ -2883,16 +2883,17 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let bare_fn_ty =
             ty::Binder::bind(tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi));
 
-        if !self.allow_ty_infer() {
+        if let (false, Some(ident_span)) = (self.allow_ty_infer(), ident_span) {
             // We always collect the spans for placeholder types when evaluating `fn`s, but we
             // only want to emit an error complaining about them if infer types (`_`) are not
-            // allowed. `allow_ty_infer` gates this behavior.
+            // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
+            // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
             crate::collect::placeholder_type_error(
                 tcx,
-                ident_span.map(|sp| sp.shrink_to_hi()).unwrap_or(DUMMY_SP),
+                ident_span.shrink_to_hi(),
                 &generics.params[..],
                 visitor.0,
-                ident_span.is_some(),
+                true,
             );
         }
 

src/librustc_typeck/collect.rs

@@ -1503,9 +1503,13 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
             AstConv::ty_of_fn(&icx, header.unsafety, header.abi, decl, &generics, Some(ident.span))
         }
 
-        ForeignItem(&hir::ForeignItem { kind: ForeignItemKind::Fn(ref fn_decl, _, _), .. }) => {
+        ForeignItem(&hir::ForeignItem {
+            kind: ForeignItemKind::Fn(ref fn_decl, _, _),
+            ident,
+            ..
+        }) => {
             let abi = tcx.hir().get_foreign_abi(hir_id);
-            compute_sig_of_foreign_fn_decl(tcx, def_id, fn_decl, abi)
+            compute_sig_of_foreign_fn_decl(tcx, def_id, fn_decl, abi, ident)
         }
 
         Ctor(data) | Variant(hir::Variant { data, .. }) if data.ctor_hir_id().is_some() => {
@@ -2118,6 +2122,7 @@ fn compute_sig_of_foreign_fn_decl<'tcx>(
     def_id: DefId,
     decl: &'tcx hir::FnDecl<'tcx>,
     abi: abi::Abi,
+    ident: Ident,
 ) -> ty::PolyFnSig<'tcx> {
     let unsafety = if abi == abi::Abi::RustIntrinsic {
         intrinsic_operation_unsafety(&tcx.item_name(def_id).as_str())
@@ -2130,7 +2135,7 @@ fn compute_sig_of_foreign_fn_decl<'tcx>(
         abi,
         decl,
         &hir::Generics::empty(),
-        None,
+        Some(ident.span),
     );
 
     // Feature gate SIMD types in FFI, since I am not sure that the

src/test/ui/typeck/typeck_type_placeholder_item.rs

@@ -32,7 +32,6 @@ fn test7(x: _) { let _x: usize = x; }
 
 fn test8(_f: fn() -> _) { }
 //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
-//~| ERROR the type placeholder `_` is not allowed within types on item signatures
 
 struct Test9;
 
@@ -99,7 +98,6 @@ pub fn main() {
 
     fn fn_test8(_f: fn() -> _) { }
     //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
-    //~| ERROR the type placeholder `_` is not allowed within types on item signatures
 
     struct FnTest9;