Don't infer fn return type to return itself

compiler/rustc_hir_analysis/src/collect.rs

@@ -1146,24 +1146,22 @@ fn infer_return_ty_for_fn_sig<'tcx>(
 
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_ty(ty);
+
             let mut diag = bad_placeholder(tcx, visitor.0, "return type");
             let ret_ty = fn_sig.output();
+            // Don't leak types into signatures unless they're nameable!
+            // For example, if a function returns itself, we don't want that
+            // recursive function definition to leak out into the fn sig.
+            let mut should_recover = false;
+
             if let Some(ret_ty) = ret_ty.make_suggestable(tcx, false) {
                 diag.span_suggestion(
                     ty.span,
                     "replace with the correct return type",
                     ret_ty,
                     Applicability::MachineApplicable,
                 );
-            } else if matches!(ret_ty.kind(), ty::FnDef(..))
-                && let Some(fn_sig) = ret_ty.fn_sig(tcx).make_suggestable(tcx, false)
-            {
-                diag.span_suggestion(
-                    ty.span,
-                    "replace with the correct return type",
-                    fn_sig,
-                    Applicability::MachineApplicable,
-                );
+                should_recover = true;
             } else if let Some(sugg) = suggest_impl_trait(tcx, ret_ty, ty.span, def_id) {
                 diag.span_suggestion(
                     ty.span,
@@ -1181,9 +1179,20 @@ fn infer_return_ty_for_fn_sig<'tcx>(
                      https://doc.rust-lang.org/book/ch13-01-closures.html",
                 );
             }
-            diag.emit();
 
-            ty::Binder::dummy(fn_sig)
+            let guar = diag.emit();
+
+            if should_recover {
+                ty::Binder::dummy(fn_sig)
+            } else {
+                ty::Binder::dummy(tcx.mk_fn_sig(
+                    fn_sig.inputs().iter().copied(),
+                    tcx.ty_error(guar),
+                    fn_sig.c_variadic,
+                    fn_sig.unsafety,
+                    fn_sig.abi,
+                ))
+            }
         }
         None => icx.astconv().ty_of_fn(
             hir_id,

tests/ui/typeck/bad-recursive-type-sig-infer.rs

@@ -0,0 +1,11 @@
+fn a() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    &a
+}
+
+fn b() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    &a
+}
+
+fn main() {}

tests/ui/typeck/bad-recursive-type-sig-infer.stderr

@@ -0,0 +1,15 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/bad-recursive-type-sig-infer.rs:1:11
+   |
+LL | fn a() -> _ {
+   |           ^ not allowed in type signatures
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/bad-recursive-type-sig-infer.rs:6:11
+   |
+LL | fn b() -> _ {
+   |           ^ not allowed in type signatures
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.