Rollup merge of #120870 - Zalathar:allow-min-spec, r=oli-obk

Allow restricted trait impls under `#[allow_internal_unstable(min_specialization)]`

This is a follow-up to #119963 and a companion to #120866, though it can land independently from the latter.

---

We have several compiler crates that only enable `#[feature(min_specialization)]` because it is required by their expansions of `newtype_index!`, in order to implement traits marked with `#[rustc_specialization_trait]`.

This PR allows those traits to be implemented internally by macros with `#[allow_internal_unstable(min_specialization)]`, without needing specialization to be enabled in the enclosing crate.

compiler/rustc_ast/src/lib.rs

@@ -15,7 +15,7 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(negative_impls)]
 #![feature(stmt_expr_attributes)]
 

compiler/rustc_errors/src/lib.rs

@@ -7,6 +7,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
@@ -16,7 +17,6 @@
 #![feature(error_reporter)]
 #![feature(extract_if)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_hir/src/lib.rs

@@ -5,7 +5,7 @@
 #![feature(associated_type_defaults)]
 #![feature(closure_track_caller)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(variant_count)]

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -10,7 +10,7 @@
 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 }));
         }
     }

compiler/rustc_hir_analysis/src/lib.rs

@@ -67,7 +67,7 @@
 #![feature(is_sorted)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(never_type)]
 #![feature(lazy_cell)]
 #![feature(slice_partition_dedup)]

compiler/rustc_hir_typeck/src/lib.rs

@@ -5,7 +5,7 @@
 #![feature(try_blocks)]
 #![feature(never_type)]
 #![feature(box_patterns)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(control_flow_enum)]
 
 #[macro_use]

compiler/rustc_infer/src/lib.rs

@@ -24,7 +24,7 @@
 #![feature(let_chains)]
 #![feature(if_let_guard)]
 #![feature(iterator_try_collect)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(try_blocks)]
 #![recursion_limit = "512"] // For rustdoc
 

compiler/rustc_mir_build/src/lib.rs

@@ -9,7 +9,7 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(try_blocks)]
 
 #[macro_use]

compiler/rustc_mir_dataflow/src/lib.rs

@@ -2,7 +2,7 @@
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(try_blocks)]
 
 #[macro_use]

compiler/rustc_mir_transform/src/lib.rs

@@ -8,7 +8,7 @@
 #![feature(is_sorted)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(never_type)]
 #![feature(option_get_or_insert_default)]
 #![feature(round_char_boundary)]

compiler/rustc_target/src/lib.rs

@@ -14,7 +14,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(rustc_attrs)]
 #![feature(step_trait)]
 #![allow(internal_features)]

compiler/rustc_trait_selection/src/lib.rs

@@ -24,7 +24,7 @@
 #![feature(option_take_if)]
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]

tests/ui/specialization/min_specialization/allow_internal_unstable.rs

@@ -5,6 +5,9 @@
 #![allow(internal_features)]
 #![feature(allow_internal_unstable)]
 
+// aux-build:specialization-trait.rs
+extern crate specialization_trait;
+
 #[allow_internal_unstable(min_specialization)]
 macro_rules! test {
     () => {
@@ -12,7 +15,11 @@ macro_rules! test {
         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! {}