Add feature gate for Scalable SIMD types

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -261,6 +261,15 @@ 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 SIMD types are experimental and possibly buggy"
+                            );
+                        }
                     }
                 }
             }

compiler/rustc_feature/src/unstable.rs

@@ -199,6 +199,8 @@ declare_features! (
     (internal, prelude_import, "1.2.0", None, None),
     /// Used to identify crates that contain the profiler runtime.
     (internal, profiler_runtime, "1.18.0", None, None),
+    /// Allows the use of scalable SIMD types.
+    (unstable, repr_scalable, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows using `rustc_*` attributes (RFC 572).
     (internal, rustc_attrs, "1.0.0", None, None),
     /// Allows using the `#[stable]` and `#[unstable]` attributes.

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

@@ -0,0 +1,8 @@
+#![feature(repr_simd)]
+
+#[repr(simd, scalable(16))] //~ error: Scalable SIMD types are experimental
+struct Foo {
+    _ty: [i8; 0],
+}
+
+fn main() {}

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

@@ -0,0 +1,11 @@
+error[E0658]: Scalable SIMD types are experimental and possibly buggy
+  --> $DIR/feature-gate-repr-scalable.rs:3:1
+   |
+LL | #[repr(simd, scalable(16))]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(repr_scalable)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.