typeck: report placeholder type error w/out span

This commit fixes a regression introduced in rust-lang#70369 which
meant that an error was not being emitted for invalid placeholder types
when there wasn't a span available.

Signed-off-by: David Wood <david@davidtw.co>

Author: davidtwco

src/librustc_typeck/astconv.rs

@@ -3049,14 +3049,14 @@ 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 let (false, Some(ident_span)) = (self.allow_ty_infer(), ident_span) {
+        if !self.allow_ty_infer() {
             // 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. 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.shrink_to_hi(),
+                ident_span.map(|sp| sp.shrink_to_hi()),
                 &generics.params[..],
                 visitor.0,
                 true,

src/librustc_typeck/collect.rs

@@ -129,20 +129,23 @@ struct CollectItemTypesVisitor<'tcx> {
 /// all already existing generic type parameters to avoid suggesting a name that is already in use.
 crate fn placeholder_type_error(
     tcx: TyCtxt<'tcx>,
-    span: Span,
+    span: Option<Span>,
     generics: &[hir::GenericParam<'_>],
     placeholder_types: Vec<Span>,
     suggest: bool,
 ) {
     if placeholder_types.is_empty() {
         return;
     }
-    let type_name = generics.next_type_param_name(None);
 
+    let type_name = generics.next_type_param_name(None);
     let mut sugg: Vec<_> =
         placeholder_types.iter().map(|sp| (*sp, (*type_name).to_string())).collect();
+
     if generics.is_empty() {
-        sugg.push((span, format!("<{}>", type_name)));
+        if let Some(span) = span {
+            sugg.push((span, format!("<{}>", type_name)));
+        }
     } else if let Some(arg) = generics.iter().find(|arg| match arg.name {
         hir::ParamName::Plain(Ident { name: kw::Underscore, .. }) => true,
         _ => false,
@@ -158,6 +161,7 @@ crate fn placeholder_type_error(
             format!(", {}", type_name),
         ));
     }
+
     let mut err = bad_placeholder_type(tcx, placeholder_types);
     if suggest {
         err.multipart_suggestion(
@@ -186,7 +190,7 @@ fn reject_placeholder_type_signatures_in_item(tcx: TyCtxt<'tcx>, item: &'tcx hir
     let mut visitor = PlaceholderHirTyCollector::default();
     visitor.visit_item(item);
 
-    placeholder_type_error(tcx, generics.span, &generics.params[..], visitor.0, suggest);
+    placeholder_type_error(tcx, Some(generics.span), &generics.params[..], visitor.0, suggest);
 }
 
 impl Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
@@ -722,7 +726,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
             // Account for `const C: _;` and `type T = _;`.
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(tcx, DUMMY_SP, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false);
         }
 
         hir::TraitItemKind::Type(_, None) => {}
@@ -745,7 +749,7 @@ fn convert_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::HirId) {
             // Account for `type T = _;`
             let mut visitor = PlaceholderHirTyCollector::default();
             visitor.visit_impl_item(impl_item);
-            placeholder_type_error(tcx, DUMMY_SP, &[], visitor.0, false);
+            placeholder_type_error(tcx, None, &[], visitor.0, false);
         }
         hir::ImplItemKind::Const(..) => {}
     }

src/test/ui/issues/issue-74086.rs

@@ -0,0 +1,4 @@
+fn main() {
+    static BUG: fn(_) -> u8 = |_| 8;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures [E0121]
+}

src/test/ui/issues/issue-74086.stderr

@@ -0,0 +1,12 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-74086.rs:2:20
+   |
+LL |     static BUG: fn(_) -> u8 = |_| 8;
+   |                    ^
+   |                    |
+   |                    not allowed in type signatures
+   |                    help: use type parameters instead: `T`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.

src/test/ui/typeck/typeck_type_placeholder_item.rs

@@ -32,6 +32,7 @@ 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;
 
@@ -98,6 +99,7 @@ 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;