Actually generate an OpBitcast for illegal pointercasts.

crates/rustc_codegen_spirv/src/builder/builder_methods.rs

@@ -1267,7 +1267,7 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
         if let SpirvValueKind::LogicalPtrCast {
             original_ptr,
             original_pointee_ty,
-            zombie_target_undef: _,
+            bitcast_result_id: _,
         } = ptr.kind
         {
             let offset = match pointee_kind {
@@ -1533,7 +1533,7 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
             SpirvValueKind::LogicalPtrCast {
                 original_ptr,
                 original_pointee_ty,
-                zombie_target_undef: _,
+                bitcast_result_id: _,
             } => (
                 original_ptr.with_type(
                     SpirvType::Pointer {
@@ -1572,11 +1572,12 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
                 .with_type(dest_ty)
         } else {
             // Defer the cast so that it has a chance to be avoided.
+            let original_ptr = val.def(self);
             SpirvValue {
                 kind: SpirvValueKind::LogicalPtrCast {
-                    original_ptr: val.def(self),
+                    original_ptr,
                     original_pointee_ty: val_pointee,
-                    zombie_target_undef: self.undef(dest_ty).def(self),
+                    bitcast_result_id: self.emit().bitcast(dest_ty, None, original_ptr).unwrap(),
                 },
                 ty: dest_ty,
             }

crates/rustc_codegen_spirv/src/builder_spirv.rs

@@ -59,10 +59,12 @@ pub enum SpirvValueKind {
         /// Pointee type of `original_ptr`.
         original_pointee_ty: Word,
 
-        /// `OpUndef` of the right target pointer type, to attach zombies to.
-        // FIXME(eddyb) we should be using a real `OpBitcast` here, but we can't
-        // emit that on the fly during `SpirvValue::def`, due to builder locking.
-        zombie_target_undef: Word,
+        /// Result ID for the `OpBitcast` instruction representing the cast,
+        /// to attach zombies to.
+        //
+        // HACK(eddyb) having an `OpBitcast` only works by being DCE'd away,
+        // or by being replaced with a noop in `qptr::lower`.
+        bitcast_result_id: Word,
     },
 }
 
@@ -170,10 +172,10 @@ impl SpirvValue {
             SpirvValueKind::LogicalPtrCast {
                 original_ptr: _,
                 original_pointee_ty,
-                zombie_target_undef,
+                bitcast_result_id,
             } => {
                 cx.zombie_with_span(
-                    zombie_target_undef,
+                    bitcast_result_id,
                     span,
                     &format!(
                         "cannot cast between pointer types\
@@ -184,7 +186,7 @@ impl SpirvValue {
                     ),
                 );
 
-                zombie_target_undef
+                bitcast_result_id
             }
         }
     }

crates/rustc_codegen_spirv/src/linker/zombies.rs

@@ -102,8 +102,9 @@ impl Zombies {
         // No need to zombie defs within a function: If any def within a function is zombied, then the
         // whole function is zombied. But, we don't have to mark the defs within a function as zombie,
         // because the defs can't escape the function.
-        // HACK(eddyb) one exception to this is function-local variables, which may
-        // be unused and as such cannot be allowed to always zombie the function.
+        // HACK(eddyb) one exception to this is function-local variables, or the
+        // `OpBitcast`s of pointer casts, either of which which may actually be
+        // unused and as such cannot be allowed to always zombie the function.
         for func in &module.functions {
             let func_id = func.def_id().unwrap();
             if self.id_to_zombie_kind.contains_key(&func_id) {
@@ -126,7 +127,7 @@ impl Zombies {
                     _ => {}
                 }
 
-                if inst.class.opcode == Op::Variable {
+                if [Op::Variable, Op::Bitcast].contains(&inst.class.opcode) {
                     let result_id = inst.result_id.unwrap();
                     if self.id_to_zombie_kind.contains_key(&result_id) {
                         continue;

tests/ui/lang/core/ref/zst_member_ref_arg-broken.stderr

@@ -18,11 +18,11 @@ note: called by `main_scalar_scalar_pair_nested`
    | ^
 
 error: cannot cast between pointer types
-       from `*struct (usize, usize) { u32, u32 }`
+       from `*u32`
          to `*struct B {  }`
-  --> $DIR/zst_member_ref_arg-broken.rs:33:5
+  --> $DIR/zst_member_ref_arg-broken.rs:23:5
    |
-33 |     f(&s.y);
+23 |     f(&s.y);
    |     ^
    |
 note: used from within `zst_member_ref_arg_broken::main_scalar`
@@ -37,11 +37,11 @@ note: called by `main_scalar`
    | ^
 
 error: cannot cast between pointer types
-       from `*struct (usize, usize) { u32, u32 }`
+       from `*struct S<usize, usize> { u32, u32 }`
          to `*struct B {  }`
-  --> $DIR/zst_member_ref_arg-broken.rs:33:5
+  --> $DIR/zst_member_ref_arg-broken.rs:28:5
    |
-33 |     f(&s.y);
+28 |     f(&s.y);
    |     ^
    |
 note: used from within `zst_member_ref_arg_broken::main_scalar_pair`