improve NullOp::FieldOffset handling of non-field types

Author: BennoLossin

compiler/rustc_mir_transform/src/gvn.rs

@@ -494,9 +494,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     NullOp::AlignOf => arg_layout.align.abi.bytes(),
                     NullOp::FieldOffset => {
                         let &ty::Field(container, field_path) = arg_ty.kind() else {
-                            bug!(
-                                "FIXME(field_projections): should we really bug here, or return `None`?"
-                            )
+                            return None;
                         };
                         let layout = self.ecx.layout_of(container).ok()?;
                         self.ecx

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -627,9 +627,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                     NullOp::AlignOf => op_layout.align.abi.bytes(),
                     NullOp::FieldOffset => {
                         let &ty::Field(container, field_path) = ty.kind() else {
-                            bug!(
-                                "FIXME(field_projections): should we really bug here, or return `None`?"
-                            )
+                            return None;
                         };
                         let layout = self.ecx.layout_of(container).ok()?;
                         self.tcx

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -452,13 +452,10 @@ impl<'tcx> Validator<'_, 'tcx> {
                 self.validate_operand(operand)?;
             }
 
-            Rvalue::NullaryOp(op, ty) => match op {
+            Rvalue::NullaryOp(op, _) => match op {
                 NullOp::SizeOf => {}
                 NullOp::AlignOf => {}
-                NullOp::FieldOffset => match ty.kind() {
-                    ty::Field(..) => {}
-                    _ => bug!("FIXME(field_projections): should we report an error here?"),
-                },
+                NullOp::FieldOffset => {}
                 NullOp::UbChecks => {}
                 NullOp::ContractChecks => {}
             },

compiler/rustc_mir_transform/src/validate.rs

@@ -1343,7 +1343,10 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             }
             Rvalue::NullaryOp(NullOp::FieldOffset, ty) => {
                 let ty::Field(container, field_path) = ty.kind() else {
-                    bug!("FIXME(field_projections): report error?")
+                    span_bug!(
+                        self.body.span,
+                        "FIXME(field_projections): expected ty::Field, found {ty:?}"
+                    )
                 };
                 let fail_out_of_bounds = |this: &mut Self, location, field, ty| {
                     this.fail(location, format!("Out of bounds field {field:?} for {ty}"));