Let's see what those opaque types actually are

compiler/rustc_feature/src/builtin_attrs.rs

@@ -825,6 +825,7 @@ pub struct BuiltinAttribute {
     rustc_attr!(TEST, rustc_strict_coherence, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_variance, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_variance_of_opaques, Normal, template!(Word), WarnFollowing),
+    rustc_attr!(TEST, rustc_hidden_type_of_opaques, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_layout, Normal, template!(List: "field1, field2, ..."), WarnFollowing),
     rustc_attr!(TEST, rustc_abi, Normal, template!(List: "field1, field2, ..."), WarnFollowing),
     rustc_attr!(TEST, rustc_regions, Normal, template!(Word), WarnFollowing),

compiler/rustc_hir_analysis/messages.ftl

@@ -339,6 +339,8 @@ hir_analysis_transparent_non_zero_sized_enum = the variant of a transparent {$de
     .label = needs at most one field with non-trivial size or alignment, but has {$field_count}
     .labels = this field has non-zero size or requires alignment
 
+hir_analysis_type_of = {$type_of}
+
 hir_analysis_typeof_reserved_keyword_used =
     `typeof` is a reserved keyword but unimplemented
     .suggestion = consider replacing `typeof(...)` with an actual type

compiler/rustc_hir_analysis/src/collect.rs

@@ -14,14 +14,11 @@
 //! At present, however, we do run collection across all items in the
 //! crate as a kind of pass. This should eventually be factored away.
 
-use crate::astconv::AstConv;
-use crate::check::intrinsic::intrinsic_operation_unsafety;
-use crate::errors;
-use hir::def::DefKind;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, StashKey};
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId, LocalModDefId};
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{GenericParamKind, Node};
@@ -40,6 +37,11 @@
 use std::iter;
 use std::ops::Bound;
 
+use crate::astconv::AstConv;
+use crate::check::intrinsic::intrinsic_operation_unsafety;
+use crate::errors;
+pub use type_of::test_opaque_hidden_types;
+
 mod generics_of;
 mod item_bounds;
 mod predicates_of;

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -11,6 +11,7 @@
 
 use super::ItemCtxt;
 use super::{bad_placeholder, is_suggestable_infer_ty};
+pub use opaque::test_opaque_hidden_types;
 
 mod opaque;
 

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -1,12 +1,25 @@
 use rustc_errors::StashKey;
-use rustc_hir::def_id::LocalDefId;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{self as hir, def, Expr, ImplItem, Item, Node, TraitItem};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
-use rustc_span::DUMMY_SP;
+use rustc_span::{sym, DUMMY_SP};
 
-use crate::errors::{TaitForwardCompat, UnconstrainedOpaqueType};
+use crate::errors::{TaitForwardCompat, TypeOf, UnconstrainedOpaqueType};
+
+pub fn test_opaque_hidden_types(tcx: TyCtxt<'_>) {
+    if tcx.has_attr(CRATE_DEF_ID, sym::rustc_hidden_type_of_opaques) {
+        for id in tcx.hir().items() {
+            if matches!(tcx.def_kind(id.owner_id), DefKind::OpaqueTy) {
+                let type_of = tcx.type_of(id.owner_id).instantiate_identity();
+
+                tcx.sess.emit_err(TypeOf { span: tcx.def_span(id.owner_id), type_of });
+            }
+        }
+    }
+}
 
 /// Checks "defining uses" of opaque `impl Trait` types to ensure that they meet the restrictions
 /// laid for "higher-order pattern unification".

compiler/rustc_hir_analysis/src/errors.rs

@@ -467,6 +467,14 @@ pub(crate) struct VariancesOf {
     pub variances_of: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_type_of)]
+pub(crate) struct TypeOf<'tcx> {
+    #[primary_span]
+    pub span: Span,
+    pub type_of: Ty<'tcx>,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_pass_to_variadic_function, code = "E0617")]
 pub(crate) struct PassToVariadicFunction<'tcx, 'a> {

compiler/rustc_hir_analysis/src/lib.rs

@@ -214,6 +214,10 @@ pub fn check_crate(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
         tcx.hir().for_each_module(|module| tcx.ensure().check_mod_item_types(module))
     });
 
+    if tcx.features().rustc_attrs {
+        tcx.sess.track_errors(|| collect::test_opaque_hidden_types(tcx))?;
+    }
+
     // Freeze definitions as we don't add new ones at this point. This improves performance by
     // allowing lock-free access to them.
     tcx.untracked().definitions.freeze();

compiler/rustc_span/src/symbol.rs

@@ -1369,6 +1369,7 @@
         rustc_evaluate_where_clauses,
         rustc_expected_cgu_reuse,
         rustc_has_incoherent_inherent_impls,
+        rustc_hidden_type_of_opaques,
         rustc_host,
         rustc_if_this_changed,
         rustc_inherit_overflow_checks,

tests/ui/impl-trait/erased-regions-in-hidden-ty.current.stderr

@@ -0,0 +1,14 @@
+error: {foo<ReEarlyBound(DefId(..), 0, 'a)>::{closure#0} closure_kind_ty=i8 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(()) upvar_tys=()}
+  --> $DIR/erased-regions-in-hidden-ty.rs:11:36
+   |
+LL | fn foo<'a: 'a>(x: &'a Vec<i32>) -> impl Fn() + 'static {
+   |                                    ^^^^^^^^^^^^^^^^^^^
+
+error: Opaque(DefId(..), [ReErased])
+  --> $DIR/erased-regions-in-hidden-ty.rs:17:13
+   |
+LL | fn bar() -> impl Fn() + 'static {
+   |             ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/impl-trait/erased-regions-in-hidden-ty.next.stderr

@@ -0,0 +1,14 @@
+error: {foo<ReEarlyBound(DefId(..), 0, 'a)>::{closure#0} closure_kind_ty=i8 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(()) upvar_tys=()}
+  --> $DIR/erased-regions-in-hidden-ty.rs:11:36
+   |
+LL | fn foo<'a: 'a>(x: &'a Vec<i32>) -> impl Fn() + 'static {
+   |                                    ^^^^^^^^^^^^^^^^^^^
+
+error: Opaque(DefId(..), [ReErased])
+  --> $DIR/erased-regions-in-hidden-ty.rs:17:13
+   |
+LL | fn bar() -> impl Fn() + 'static {
+   |             ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/impl-trait/erased-regions-in-hidden-ty.rs

@@ -1,14 +1,22 @@
 // revisions: current next
+// compile-flags: -Zverbose
 //[next] compile-flags: -Ztrait-solver=next
-// check-pass
+// normalize-stderr-test "DefId\([^\)]+\)" -> "DefId(..)"
+
+#![feature(rustc_attrs)]
+#![rustc_hidden_type_of_opaques]
 
 // Make sure that the compiler can handle `ReErased` in the hidden type of an opaque.
 
 fn foo<'a: 'a>(x: &'a Vec<i32>) -> impl Fn() + 'static {
+//~^ ERROR 0, 'a)>::{closure#0} closure_kind_ty=i8 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(()) upvar_tys=()}
+// Can't write whole type because of lack of path sanitization
     || ()
 }
 
 fn bar() -> impl Fn() + 'static {
+//~^ ERROR , [ReErased])
+// Can't write whole type because of lack of path sanitization
     foo(&vec![])
 }