Ban projecting into SIMD types [MCP838]

Author: scottmcm

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -329,20 +329,11 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
         let offset = self.layout.fields.offset(i);
 
         if !bx.is_backend_ref(self.layout) && bx.is_backend_ref(field) {
-            if let BackendRepr::SimdVector { count, .. } = self.layout.backend_repr
-                && let BackendRepr::Memory { sized: true } = field.backend_repr
-                && count.is_power_of_two()
-            {
-                assert_eq!(field.size, self.layout.size);
-                // This is being deprecated, but for now stdarch still needs it for
-                // Newtype vector of array, e.g. #[repr(simd)] struct S([i32; 4]);
-                let place = PlaceRef::alloca(bx, field);
-                self.val.store(bx, place.val.with_type(self.layout));
-                return bx.load_operand(place);
-            } else {
-                // Part of https://github.com/rust-lang/compiler-team/issues/838
-                bug!("Non-ref type {self:?} cannot project to ref field type {field:?}");
-            }
+            // Part of https://github.com/rust-lang/compiler-team/issues/838
+            span_bug!(
+                fx.mir.span,
+                "Non-ref type {self:?} cannot project to ref field type {field:?}",
+            );
         }
 
         let val = if field.is_zst() {

compiler/rustc_mir_transform/src/validate.rs

@@ -721,6 +721,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             );
                         }
 
+                        if adt_def.repr().simd() {
+                            self.fail(
+                                location,
+                                format!(
+                                    "Projecting into SIMD type {adt_def:?} is banned by MCP#838"
+                                ),
+                            );
+                        }
+
                         let var = parent_ty.variant_index.unwrap_or(FIRST_VARIANT);
                         let Some(field) = adt_def.variant(var).fields.get(f) else {
                             fail_out_of_bounds(self, location);

tests/codegen/simd/project-to-simd-array-field.rs

@@ -0,0 +1,18 @@
+// Optimized MIR shouldn't have critical call edges
+//
+//@ build-fail
+//@ edition: 2021
+//@ compile-flags: --crate-type=lib
+//@ failure-status: 101
+//@ dont-check-compiler-stderr
+
+#![feature(repr_simd)]
+
+#[repr(simd)]
+pub struct U32x4([u32; 4]);
+
+pub fn f(a: U32x4) -> [u32; 4] {
+    a.0
+    //~^ ERROR broken MIR in Item
+    //~| ERROR Projecting into SIMD type U32x4 is banned by MCP#838
+}

tests/ui/mir/validate/project-into-simd.rs

@@ -10,7 +10,9 @@ struct i32x4([i32; 4]);
 
 #[inline(always)]
 fn to_array(a: i32x4) -> [i32; 4] {
-    a.0
+    // This was originally just `a.0`, but that ended up being annoying enough
+    // that it was banned by <https://github.com/rust-lang/compiler-team/issues/838>
+    unsafe { std::mem::transmute(a) }
 }
 
 fn main() {