`allow_internal_unstable(min_specialization)` on `newtype_index`

1 files changed, 19 insertions, 13 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index d3862451a01..b37d9714ddd 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -142,23 +142,29 @@ pub fn translate_args_with_cause<'tcx>(
 pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
     // The feature gate should prevent introducing new specializations, but not
     // taking advantage of upstream ones.
+    // If specialization is enabled for this crate then no extra checks are needed.
+    // If it's not, and either of the `impl`s is local to this crate, then this definitely
+    // isn't specializing - unless specialization is enabled for the `impl` span,
+    // e.g. if it comes from an `allow_internal_unstable` macro
     let features = tcx.features();
     let specialization_enabled = features.specialization || features.min_specialization;
-    if !specialization_enabled && impl1_def_id.is_local() {
-        let span = tcx.def_span(impl1_def_id);
-        if !span.allows_unstable(sym::specialization)
-            && !span.allows_unstable(sym::min_specialization)
-        {
-            return false;
+    if !specialization_enabled {
+        if impl1_def_id.is_local() {
+            let span = tcx.def_span(impl1_def_id);
+            if !span.allows_unstable(sym::specialization)
+                && !span.allows_unstable(sym::min_specialization)
+            {
+                return false;
+            }
         }
-    }
 
-    if !specialization_enabled && impl2_def_id.is_local() {
-        let span = tcx.def_span(impl2_def_id);
-        if !span.allows_unstable(sym::specialization)
-            && !span.allows_unstable(sym::min_specialization)
-        {
-            return false;
+        if impl2_def_id.is_local() {
+            let span = tcx.def_span(impl2_def_id);
+            if !span.allows_unstable(sym::specialization)
+                && !span.allows_unstable(sym::min_specialization)
+            {
+                return false;
+            }
         }
     }