More precisely detect cycle errors from type_of on opaque

compiler/rustc_middle/src/query/erase.rs

@@ -1,4 +1,5 @@
 use crate::mir;
+use crate::query::CyclePlaceholder;
 use crate::traits;
 use crate::ty::{self, Ty};
 use std::mem::{size_of, transmute_copy, MaybeUninit};
@@ -142,6 +143,10 @@ impl EraseType for Result<&'_ ty::List<Ty<'_>>, ty::util::AlwaysRequiresDrop> {
         [u8; size_of::<Result<&'static ty::List<Ty<'static>>, ty::util::AlwaysRequiresDrop>>()];
 }
 
+impl EraseType for Result<ty::EarlyBinder<Ty<'_>>, CyclePlaceholder> {
+    type Result = [u8; size_of::<Result<ty::EarlyBinder<Ty<'_>>, CyclePlaceholder>>()];
+}
+
 impl<T> EraseType for Option<&'_ T> {
     type Result = [u8; size_of::<Option<&'static ()>>()];
 }

compiler/rustc_middle/src/query/mod.rs

@@ -26,7 +26,7 @@ use crate::mir::interpret::{
 use crate::mir::interpret::{LitToConstError, LitToConstInput};
 use crate::mir::mono::CodegenUnit;
 use crate::query::erase::{erase, restore, Erase};
-use crate::query::plumbing::{query_ensure, query_get_at, DynamicQuery};
+use crate::query::plumbing::{query_ensure, query_get_at, CyclePlaceholder, DynamicQuery};
 use crate::thir;
 use crate::traits::query::{
     CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,
@@ -243,6 +243,16 @@ rustc_queries! {
         feedable
     }
 
+    /// Specialized instance of `type_of` that detects cycles that are due to
+    /// revealing opaque because of an auto trait bound. Unless `CyclePlaceholder` needs
+    /// to be handled separately, call `type_of` instead.
+    query type_of_opaque(key: DefId) -> Result<ty::EarlyBinder<Ty<'tcx>>, CyclePlaceholder> {
+        desc { |tcx|
+            "computing type of opaque `{path}`",
+            path = tcx.def_path_str(key),
+        }
+    }
+
     query collect_return_position_impl_trait_in_trait_tys(key: DefId)
         -> Result<&'tcx FxHashMap<DefId, ty::EarlyBinder<Ty<'tcx>>>, ErrorGuaranteed>
     {

compiler/rustc_middle/src/query/plumbing.rs

@@ -630,3 +630,6 @@ impl<'tcx> TyCtxtAt<'tcx> {
             .unwrap_or_else(|| bug!("def_kind: unsupported node: {:?}", def_id))
     }
 }
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct CyclePlaceholder(pub ErrorGuaranteed);

compiler/rustc_middle/src/values.rs

@@ -1,4 +1,5 @@
 use crate::dep_graph::DepKind;
+use crate::query::plumbing::CyclePlaceholder;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{pluralize, struct_span_err, Applicability, MultiSpan};
 use rustc_hir as hir;
@@ -24,6 +25,16 @@ impl<'tcx> Value<TyCtxt<'tcx>, DepKind> for Ty<'_> {
     }
 }
 
+impl<'tcx> Value<TyCtxt<'tcx>, DepKind> for Result<ty::EarlyBinder<Ty<'_>>, CyclePlaceholder> {
+    fn from_cycle_error(
+        _tcx: TyCtxt<'tcx>,
+        _: &[QueryInfo<DepKind>],
+        guar: ErrorGuaranteed,
+    ) -> Self {
+        Err(CyclePlaceholder(guar))
+    }
+}
+
 impl<'tcx> Value<TyCtxt<'tcx>, DepKind> for ty::SymbolName<'_> {
     fn from_cycle_error(
         tcx: TyCtxt<'tcx>,