add a canary test for complex repr(simd)

compiler/rustc_middle/src/ty/sty.rs

@@ -1966,7 +1966,13 @@ impl<'tcx> TyS<'tcx> {
                 let f0_ty = variant.fields[0].ty(tcx, substs);
 
                 match f0_ty.kind() {
-                    Array(f0_elem_ty, f0_len) => (f0_len.eval_usize(tcx, ParamEnv::empty()) as u64, f0_elem_ty),
+                    Array(f0_elem_ty, f0_len) => {
+                        // FIXME(repr_simd): https://github.com/rust-lang/rust/pull/78863#discussion_r522784112
+                        // The way we evaluate the `N` in `[T; N]` here only works since we use
+                        // `simd_size_and_type` post-monomorphization. It will probably start to ICE
+                        // if we use it in generic code. See the `simd-array-trait` ui test.
+                        (f0_len.eval_usize(tcx, ParamEnv::empty()) as u64, f0_elem_ty)
+                    }
                     _ => (variant.fields.len() as u64, f0_ty),
                 }
             }

src/test/ui/simd/simd-array-trait.rs

@@ -0,0 +1,40 @@
+// Figuring out the size of a vector type that depends on traits doesn't ICE
+
+#![allow(dead_code)]
+
+// pretty-expanded FIXME #23616
+
+#![feature(repr_simd, platform_intrinsics, const_generics)]
+#![allow(non_camel_case_types, incomplete_features)]
+
+pub trait Simd {
+    type Lane: Clone + Copy;
+    const SIZE: usize;
+}
+
+pub struct i32x4;
+impl Simd for i32x4 {
+    type Lane = i32;
+    const SIZE: usize = 4;
+}
+
+#[repr(simd)]
+#[derive(Copy, Clone)]
+pub struct T<S: Simd>([S::Lane; S::SIZE]); //~ ERROR constant expression depends on a generic parameter
+
+extern "platform-intrinsic" {
+    fn simd_insert<T, E>(x: T, idx: u32, y: E) -> T;
+    fn simd_extract<T, E>(x: T, idx: u32) -> E;
+}
+
+pub fn main() {
+    let mut t = T::<i32x4>([0; 4]);
+    unsafe {
+        for i in 0_i32..4 {
+            t = simd_insert(t, i as u32, i);
+        }
+        for i in 0_i32..4 {
+            assert_eq!(i, simd_extract(t, i as u32));
+        }
+    }
+}

src/test/ui/simd/simd-array-trait.stderr

@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/simd-array-trait.rs:23:23
+   |
+LL | pub struct T<S: Simd>([S::Lane; S::SIZE]);
+   |                       ^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+