Allow restricted trait impls in macros with min_specialization

Implementing traits marked with `#[rustc_specialization_trait]` normally
requires (min-)specialization to be enabled for the enclosing crate.

With this change, that permission can also be granted by an
`allow_internal_unstable` attribute on the macro that generates the impl.

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -10,7 +10,7 @@ use rustc_errors::{codes::*, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
-use rustc_span::ErrorGuaranteed;
+use rustc_span::{sym, ErrorGuaranteed};
 use rustc_trait_selection::traits;
 
 mod builtin;
@@ -70,7 +70,11 @@ fn enforce_trait_manually_implementable(
     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable =
         tcx.trait_def(trait_def_id).specialization_kind
     {
-        if !tcx.features().specialization && !tcx.features().min_specialization {
+        if !tcx.features().specialization
+            && !tcx.features().min_specialization
+            && !impl_header_span.allows_unstable(sym::specialization)
+            && !impl_header_span.allows_unstable(sym::min_specialization)
+        {
             return Err(tcx.dcx().emit_err(errors::SpecializationTrait { span: impl_header_span }));
         }
     }

tests/ui/specialization/allow_internal_unstable.rs → tests/ui/specialization/min_specialization/allow_internal_unstable.rs

@@ -5,14 +5,21 @@
 #![allow(internal_features)]
 #![feature(allow_internal_unstable)]
 
+// aux-build:specialization-trait.rs
+extern crate specialization_trait;
+
 #[allow_internal_unstable(min_specialization)]
 macro_rules! test {
     () => {
         struct T<U>(U);
         trait Tr {}
         impl<U> Tr for T<U> {}
         impl Tr for T<u8> {}
-    }
+
+        impl<U> specialization_trait::SpecTrait for T<U> {
+            fn method(&self) {}
+        }
+    };
 }
 
 test! {}