feature: gate repr(scalable) with repr_scalable

Add a `repr_scalable` feature gate and require its use for the
`repr(scalable(N)` attribute.

Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

Author: davidtwco

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -213,6 +213,15 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                 "SIMD types are experimental and possibly buggy"
                             );
                         }
+
+                        if item.has_name(sym::scalable) {
+                            gate!(
+                                &self,
+                                repr_scalable,
+                                attr.span,
+                                "scalable vector types are experimental"
+                            );
+                        }
                     }
                 }
             }

compiler/rustc_feature/src/unstable.rs

@@ -235,6 +235,8 @@ declare_features! (
     (internal, prelude_import, "1.2.0", None),
     /// Used to identify crates that contain the profiler runtime.
     (internal, profiler_runtime, "1.18.0", None),
+    /// Allows the use of `repr(scalable)`.
+    (unstable, repr_scalable, "CURRENT_RUSTC_VERSION", None),
     /// Allows using `rustc_*` attributes (RFC 572).
     (internal, rustc_attrs, "1.0.0", None),
     /// Introduces a hierarchy of `Sized` traits (RFC 3729).

compiler/rustc_span/src/symbol.rs

@@ -1764,6 +1764,7 @@ symbols! {
         repr_align,
         repr_align_enum,
         repr_packed,
+        repr_scalable,
         repr_simd,
         repr_transparent,
         require,

tests/ui/feature-gates/feature-gate-repr-scalable.rs

@@ -0,0 +1,8 @@
+#![feature(repr_simd)]
+
+#[repr(simd, scalable(16))] //~ ERROR: scalable vector types are experimental
+struct Foo { //~ ERROR: SIMD vector's only field must be an array
+    _ty: [i8],
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-repr-scalable.stderr

@@ -0,0 +1,21 @@
+error[E0658]: scalable vector types are experimental
+  --> $DIR/feature-gate-repr-scalable.rs:3:1
+   |
+LL | #[repr(simd, scalable(16))]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(repr_scalable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0076]: SIMD vector's only field must be an array
+  --> $DIR/feature-gate-repr-scalable.rs:4:1
+   |
+LL | struct Foo {
+   | ^^^^^^^^^^
+LL |     _ty: [i8],
+   |     --------- not an array
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0076, E0658.
+For more information about an error, try `rustc --explain E0076`.

tests/ui/simd/scalable/invalid.rs

@@ -2,6 +2,7 @@
 #![allow(internal_features, unused_imports, unused_macros)]
 #![feature(extern_types)]
 #![feature(gen_blocks)]
+#![feature(repr_scalable)]
 #![feature(repr_simd)]
 #![feature(stmt_expr_attributes)]
 #![feature(trait_alias)]