1 files changed, 73 insertions, 0 deletions
diff --git a/compiler/rustc_traits/src/normalize_erasing_regions.rs b/compiler/rustc_traits/src/normalize_erasing_regions.rs
new file mode 100644
index 000000000..5d394ed22
--- /dev/null
+++ b/compiler/rustc_traits/src/normalize_erasing_regions.rs
@@ -0,0 +1,73 @@
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_middle::traits::query::NoSolution;
+use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt, TypeFoldable};
+use rustc_trait_selection::traits::query::normalize::AtExt;
+use rustc_trait_selection::traits::{Normalized, ObligationCause};
+use std::sync::atomic::Ordering;
+
+pub(crate) fn provide(p: &mut Providers) {
+    *p = Providers {
+        try_normalize_generic_arg_after_erasing_regions: |tcx, goal| {
+            debug!("try_normalize_generic_arg_after_erasing_regions(goal={:#?}", goal);
+
+            tcx.sess
+                .perf_stats
+                .normalize_generic_arg_after_erasing_regions
+                .fetch_add(1, Ordering::Relaxed);
+
+            try_normalize_after_erasing_regions(tcx, goal)
+        },
+        try_normalize_mir_const_after_erasing_regions: |tcx, goal| {
+            try_normalize_after_erasing_regions(tcx, goal)
+        },
+        ..*p
+    };
+}
+
+fn try_normalize_after_erasing_regions<'tcx, T: TypeFoldable<'tcx> + PartialEq + Copy>(
+    tcx: TyCtxt<'tcx>,
+    goal: ParamEnvAnd<'tcx, T>,
+) -> Result<T, NoSolution> {
+    let ParamEnvAnd { param_env, value } = goal;
+    tcx.infer_ctxt().enter(|infcx| {
+        let cause = ObligationCause::dummy();
+        match infcx.at(&cause, param_env).normalize(value) {
+            Ok(Normalized { value: normalized_value, obligations: normalized_obligations }) => {
+                // We don't care about the `obligations`; they are
+                // always only region relations, and we are about to
+                // erase those anyway:
+                debug_assert_eq!(
+                    normalized_obligations.iter().find(|p| not_outlives_predicate(p.predicate)),
+                    None,
+                );
+
+                let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
+                // It's unclear when `resolve_vars` would have an effect in a
+                // fresh `InferCtxt`. If this assert does trigger, it will give
+                // us a test case.
+                debug_assert_eq!(normalized_value, resolved_value);
+                let erased = infcx.tcx.erase_regions(resolved_value);
+                debug_assert!(!erased.needs_infer(), "{:?}", erased);
+                Ok(erased)
+            }
+            Err(NoSolution) => Err(NoSolution),
+        }
+    })
+}
+
+fn not_outlives_predicate<'tcx>(p: ty::Predicate<'tcx>) -> bool {
+    match p.kind().skip_binder() {
+        ty::PredicateKind::RegionOutlives(..) | ty::PredicateKind::TypeOutlives(..) => false,
+        ty::PredicateKind::Trait(..)
+        | ty::PredicateKind::Projection(..)
+        | ty::PredicateKind::WellFormed(..)
+        | ty::PredicateKind::ObjectSafe(..)
+        | ty::PredicateKind::ClosureKind(..)
+        | ty::PredicateKind::Subtype(..)
+        | ty::PredicateKind::Coerce(..)
+        | ty::PredicateKind::ConstEvaluatable(..)
+        | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::TypeWellFormedFromEnv(..) => true,
+    }
+}