compiletests: cover invalid runtime-array const bitcasts

Add a UI compile-fail test for casting a 3-byte constant allocation to `*const RuntimeArray<u16>`. This exercises the existing trailing-bytes diagnostic for unsupported unsized constants and guards the review concern that this shape must fail instead of silently lowering.

Author: LegNeato

tests/compiletests/ui/lang/consts/runtime-array-invalid-const-bitcast.rs

@@ -0,0 +1,19 @@
+// build-fail
+
+use spirv_std::{RuntimeArray, spirv};
+
+// Reinterpreting a 3-byte backing allocation as `RuntimeArray<u16>` leaves
+// one trailing byte that cannot form a whole element.
+const BYTES: &[u8; 3] = &[1, 2, 3];
+const BAD_RUNTIME_ARRAY_PTR: *const RuntimeArray<u16> =
+    BYTES as *const [u8; 3] as *const RuntimeArray<u16>;
+
+#[inline(never)]
+fn use_runtime_array_ptr(ptr: *const RuntimeArray<u16>) -> bool {
+    !ptr.is_null()
+}
+
+#[spirv(fragment)]
+pub fn main(output: &mut u32) {
+    *output = u32::from(use_runtime_array_ptr(BAD_RUNTIME_ARRAY_PTR));
+}

tests/compiletests/ui/lang/consts/runtime-array-invalid-const-bitcast.stderr

@@ -0,0 +1,16 @@
+error: unsupported unsized `[u16]` constant with 1 trailing bytes
+   |
+   = note: used by unnamed global (%87)
+note: used from within `runtime_array_invalid_const_bitcast::main`
+  --> <$DIR/runtime-array-invalid-const-bitcast.rs>:18:25
+   |
+LL |     *output = u32::from(use_runtime_array_ptr(BAD_RUNTIME_ARRAY_PTR));
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: called by `main`
+  --> <$DIR/runtime-array-invalid-const-bitcast.rs>:17:8
+   |
+LL | pub fn main(output: &mut u32) {
+   |        ^^^^
+
+error: aborting due to 1 previous error
+