Substitute erased lifetimes on bad placeholder type

Fix #82455.

compiler/rustc_typeck/src/collect.rs

@@ -371,6 +371,11 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
         span: Span,
     ) -> &'tcx Const<'tcx> {
         bad_placeholder_type(self.tcx(), vec![span]).emit();
+        // Typeck doesn't expect erased regions to be returned from `type_of`.
+        let ty = self.tcx.fold_regions(ty, &mut false, |r, _| match r {
+            ty::ReErased => self.tcx.lifetimes.re_static,
+            _ => r,
+        });
         self.tcx().const_error(ty)
     }
 
@@ -1647,6 +1652,12 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
             match get_infer_ret_ty(&sig.decl.output) {
                 Some(ty) => {
                     let fn_sig = tcx.typeck(def_id).liberated_fn_sigs()[hir_id];
+                    // Typeck doesn't expect erased regions to be returned from `type_of`.
+                    let fn_sig = tcx.fold_regions(fn_sig, &mut false, |r, _| match r {
+                        ty::ReErased => tcx.lifetimes.re_static,
+                        _ => r,
+                    });
+
                     let mut visitor = PlaceholderHirTyCollector::default();
                     visitor.visit_ty(ty);
                     let mut diag = bad_placeholder_type(tcx, visitor.0);
@@ -1675,6 +1686,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                         }
                     }
                     diag.emit();
+
                     ty::Binder::bind(fn_sig)
                 }
                 None => AstConv::ty_of_fn(

src/test/ui/typeck/typeck_type_placeholder_item.rs

@@ -208,3 +208,15 @@ impl Qux for Struct {
     const D: _ = 42;
     //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
 }
+
+fn map<T>(_: fn() -> Option<&'static T>) -> Option<T> {
+    None
+}
+
+fn value() -> Option<&'static _> {
+//~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    Option::<&'static u8>::None
+}
+
+const _: Option<_> = map(value);
+//~^ ERROR the type placeholder `_` is not allowed within types on item signatures

src/test/ui/typeck/typeck_type_placeholder_item.stderr

@@ -158,7 +158,7 @@ LL | fn test11(x: &usize) -> &_ {
    |                         -^
    |                         ||
    |                         |not allowed in type signatures
-   |                         help: replace with the correct return type: `&&usize`
+   |                         help: replace with the correct return type: `&'static &'static usize`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item.rs:52:52
@@ -410,6 +410,24 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
 LL | type Y = impl Trait<_>;
    |                     ^ not allowed in type signatures
 
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:216:31
+   |
+LL | fn value() -> Option<&'static _> {
+   |               ----------------^-
+   |               |               |
+   |               |               not allowed in type signatures
+   |               help: replace with the correct return type: `Option<&'static u8>`
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:221:10
+   |
+LL | const _: Option<_> = map(value);
+   |          ^^^^^^^^^
+   |          |
+   |          not allowed in type signatures
+   |          help: replace `_` with the correct type: `Option<u8>`
+
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item.rs:140:31
    |
@@ -614,7 +632,7 @@ LL |     const D: _ = 42;
    |              not allowed in type signatures
    |              help: replace `_` with the correct type: `i32`
 
-error: aborting due to 67 previous errors
+error: aborting due to 69 previous errors
 
 Some errors have detailed explanations: E0121, E0282, E0403.
 For more information about an error, try `rustc --explain E0121`.