const_eval and codegen: audit uses of is_zst

compiler/rustc_codegen_ssa/src/base.rs

@@ -202,7 +202,7 @@ pub fn unsize_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             (src, unsized_info(bx, a, b, old_info))
         }
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
-            assert_eq!(def_a, def_b);
+            assert_eq!(def_a, def_b); // implies same number of fields
             let src_layout = bx.cx().layout_of(src_ty);
             let dst_layout = bx.cx().layout_of(dst_ty);
             if src_ty == dst_ty {
@@ -211,7 +211,8 @@ pub fn unsize_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             let mut result = None;
             for i in 0..src_layout.fields.count() {
                 let src_f = src_layout.field(bx.cx(), i);
-                if src_f.is_zst() {
+                if src_f.is_1zst() {
+                    // We are looking for the one non-1-ZST field; this is not it.
                     continue;
                 }
 
@@ -272,13 +273,14 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         }
 
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
-            assert_eq!(def_a, def_b);
+            assert_eq!(def_a, def_b); // implies same number of fields
 
             for i in def_a.variant(FIRST_VARIANT).fields.indices() {
                 let src_f = src.project_field(bx, i.as_usize());
                 let dst_f = dst.project_field(bx, i.as_usize());
 
                 if dst_f.layout.is_zst() {
+                    // No data here, nothing to copy/coerce.
                     continue;
                 }
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -933,8 +933,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         {
                             for i in 0..op.layout.fields.count() {
                                 let field = op.extract_field(bx, i);
-                                if !field.layout.is_zst() {
-                                    // we found the one non-zero-sized field that is allowed
+                                if !field.layout.is_1zst() {
+                                    // we found the one non-1-ZST field that is allowed
                                     // now find *its* non-zero-sized field, or stop if it's a
                                     // pointer
                                     op = field;
@@ -975,10 +975,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         {
                             for i in 0..op.layout.fields.count() {
                                 let field = op.extract_field(bx, i);
-                                if !field.layout.is_zst() {
-                                    // we found the one non-zero-sized field that is allowed
-                                    // now find *its* non-zero-sized field, or stop if it's a
-                                    // pointer
+                                if !field.layout.is_1zst() {
+                                    // We found the one non-1-ZST field that is allowed. (The rules
+                                    // for `DispatchFromDyn` ensure there's exactly one such field.)
+                                    // Now find *its* non-zero-sized field, or stop if it's a
+                                    // pointer.
                                     op = field;
                                     continue 'descend_newtypes;
                                 }

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -145,7 +145,7 @@ impl<'tcx, V: CodegenObject> LocalRef<'tcx, V> {
         if layout.is_zst() {
             // Zero-size temporaries aren't always initialized, which
             // doesn't matter because they don't contain data, but
-            // we need something in the operand.
+            // we need something sufficiently aligned in the operand.
             LocalRef::Operand(OperandRef::zero_sized(layout))
         } else {
             LocalRef::PendingOperand

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -50,7 +50,8 @@ pub enum OperandValue<V> {
     /// from [`ConstMethods::const_poison`].
     ///
     /// An `OperandValue` *must* be this variant for any type for which
-    /// `is_zst` on its `Layout` returns `true`.
+    /// `is_zst` on its `Layout` returns `true`. Note however that
+    /// these values can still require alignment.
     ZeroSized,
 }
 

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -114,7 +114,8 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
                     bx.struct_gep(ty, self.llval, 1)
                 }
                 Abi::Scalar(_) | Abi::ScalarPair(..) | Abi::Vector { .. } if field.is_zst() => {
-                    // ZST fields are not included in Scalar, ScalarPair, and Vector layouts, so manually offset the pointer.
+                    // ZST fields (even some that require alignment) are not included in Scalar,
+                    // ScalarPair, and Vector layouts, so manually offset the pointer.
                     bx.gep(bx.cx().type_i8(), self.llval, &[bx.const_usize(offset.bytes())])
                 }
                 Abi::Scalar(_) | Abi::ScalarPair(..) => {

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -1004,6 +1004,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::Rvalue::Aggregate(..) => {
                 let ty = rvalue.ty(self.mir, self.cx.tcx());
                 let ty = self.monomorphize(ty);
+                // For ZST this can be `OperandValueKind::ZeroSized`.
                 self.cx.spanned_layout_of(ty, span).is_zst()
             }
         }