From eb63d3a49d7681d6ede7dc1bcac7bb1125ff7132 Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Sun, 14 Jan 2024 13:48:58 +0000
Subject: [PATCH] `allow_internal_unstable(min_specialization)` on
 `newtype_index`

---
 compiler/rustc_index_macros/src/lib.rs        |  3 ++
 .../src/traits/specialize/mod.rs              | 32 +++++++++++--------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_index_macros/src/lib.rs b/compiler/rustc_index_macros/src/lib.rs
index ac374a41eb6..b0bf4554ba8 100644
--- a/compiler/rustc_index_macros/src/lib.rs
+++ b/compiler/rustc_index_macros/src/lib.rs
@@ -36,6 +36,9 @@
     feature = "nightly",
     allow_internal_unstable(step_trait, rustc_attrs, trusted_step, spec_option_partial_eq)
 )]
+// FIXME: Remove the above comment about `min_specialization` once bootstrap is bumped,
+// and the corresponding one on SpecOptionPartialEq
+#[cfg_attr(all(feature = "nightly", not(bootstrap)), allow_internal_unstable(min_specialization))]
 pub fn newtype_index(input: TokenStream) -> TokenStream {
     newtype::newtype(input)
 }
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;
+            }
         }
     }