add helper method for finding the one non-1-ZST field

5 files changed, 49 insertions, 89 deletions

diff --git a/compiler/rustc_codegen_cranelift/src/vtable.rs b/compiler/rustc_codegen_cranelift/src/vtable.rs
index 7598c6eee03..41ea0b122de 100644
--- a/compiler/rustc_codegen_cranelift/src/vtable.rs
+++ b/compiler/rustc_codegen_cranelift/src/vtable.rs
@@ -48,19 +48,12 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
 ) -> (Pointer, Value) {
     let (ptr, vtable) = 'block: {
         if let Abi::Scalar(_) = arg.layout().abi {
-            'descend_newtypes: while !arg.layout().ty.is_unsafe_ptr() && !arg.layout().ty.is_ref() {
-                for i in 0..arg.layout().fields.count() {
-                    let field = arg.value_field(fx, FieldIdx::new(i));
-                    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
-                        arg = field;
-                        continue 'descend_newtypes;
-                    }
-                }
-
-                bug!("receiver has no non-zero-sized fields {:?}", arg);
+            while !arg.layout().ty.is_unsafe_ptr() && !arg.layout().ty.is_ref() {
+                let (idx, _) = arg
+                    .layout()
+                    .non_1zst_field(fx)
+                    .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type");
+                arg = arg.value_field(fx, FieldIdx::new(idx));
             }
         }
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index d78a0c49107..d8f6b4ed836 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -928,21 +928,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         // we get a value of a built-in pointer type.
                         //
                         // This is also relevant for `Pin<&mut Self>`, where we need to peel the `Pin`.
-                        'descend_newtypes: while !op.layout.ty.is_unsafe_ptr()
-                            && !op.layout.ty.is_ref()
-                        {
-                            for i in 0..op.layout.fields.count() {
-                                let field = op.extract_field(bx, i);
-                                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;
-                                    continue 'descend_newtypes;
-                                }
-                            }
-
-                            span_bug!(span, "receiver has no non-zero-sized fields {:?}", op);
+                        while !op.layout.ty.is_unsafe_ptr() && !op.layout.ty.is_ref() {
+                            let (idx, _) = op.layout.non_1zst_field(bx).expect(
+                                "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
+                            );
+                            op = op.extract_field(bx, idx);
                         }
 
                         // now that we have `*dyn Trait` or `&dyn Trait`, split it up into its
@@ -970,22 +960,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     }
                     Immediate(_) => {
                         // See comment above explaining why we peel these newtypes
-                        'descend_newtypes: while !op.layout.ty.is_unsafe_ptr()
-                            && !op.layout.ty.is_ref()
-                        {
-                            for i in 0..op.layout.fields.count() {
-                                let field = op.extract_field(bx, i);
-                                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;
-                                }
-                            }
-
-                            span_bug!(span, "receiver has no non-zero-sized fields {:?}", op);
+                        while !op.layout.ty.is_unsafe_ptr() && !op.layout.ty.is_ref() {
+                            let (idx, _) = op.layout.non_1zst_field(bx).expect(
+                                "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
+                            );
+                            op = op.extract_field(bx, idx);
                         }
 
                         // Make sure that we've actually unwrapped the rcvr down
diff --git a/compiler/rustc_const_eval/src/interpret/terminator.rs b/compiler/rustc_const_eval/src/interpret/terminator.rs
index 7e22981542e..e15499bc68d 100644
--- a/compiler/rustc_const_eval/src/interpret/terminator.rs
+++ b/compiler/rustc_const_eval/src/interpret/terminator.rs
@@ -269,19 +269,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match layout.ty.kind() {
             ty::Adt(adt_def, _) if adt_def.repr().transparent() && may_unfold(*adt_def) => {
                 assert!(!adt_def.is_enum());
-                // Find the non-1-ZST field.
-                let mut non_1zst_fields = (0..layout.fields.count()).filter_map(|idx| {
-                    let field = layout.field(self, idx);
-                    if field.is_1zst() { None } else { Some(field) }
-                });
-                let first = non_1zst_fields.next().expect("`unfold_transparent` called on 1-ZST");
-                assert!(
-                    non_1zst_fields.next().is_none(),
-                    "more than one non-1-ZST field in a transparent type"
-                );
-
-                // Found it!
-                self.unfold_transparent(first, may_unfold)
+                // Find the non-1-ZST field, and recurse.
+                let (_, field) = layout.non_1zst_field(self).unwrap();
+                self.unfold_transparent(field, may_unfold)
             }
             // Not a transparent type, no further unfolding.
             _ => layout,
@@ -797,25 +787,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         _ => {
                             // Not there yet, search for the only non-ZST field.
                             // (The rules for `DispatchFromDyn` ensure there's exactly one such field.)
-                            let mut non_zst_field = None;
-                            for i in 0..receiver.layout.fields.count() {
-                                let field = self.project_field(&receiver, i)?;
-                                let zst = field.layout.is_1zst();
-                                if !zst {
-                                    assert!(
-                                        non_zst_field.is_none(),
-                                        "multiple non-1-ZST fields in dyn receiver type {}",
-                                        receiver.layout.ty
-                                    );
-                                    non_zst_field = Some(field);
-                                }
-                            }
-                            receiver = non_zst_field.unwrap_or_else(|| {
-                                panic!(
-                                    "no non-1-ZST fields in dyn receiver type {}",
-                                    receiver.layout.ty
-                                )
-                            });
+                            let (idx, _) = receiver.layout.non_1zst_field(self).expect(
+                                "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
+                            );
+                            receiver = self.project_field(&receiver, idx)?;
                         }
                     }
                 };
diff --git a/compiler/rustc_target/src/abi/mod.rs b/compiler/rustc_target/src/abi/mod.rs
index dd435dbb0a3..a335585dbf3 100644
--- a/compiler/rustc_target/src/abi/mod.rs
+++ b/compiler/rustc_target/src/abi/mod.rs
@@ -144,4 +144,25 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
 
         offset
     }
+
+    /// Finds the one field that is not a 1-ZST.
+    /// Returns `None` if there are multiple non-1-ZST fields or only 1-ZST-fields.
+    pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(usize, Self)>
+    where
+        Ty: TyAbiInterface<'a, C> + Copy,
+    {
+        let mut found = None;
+        for field_idx in 0..self.fields.count() {
+            let field = self.field(cx, field_idx);
+            if field.is_1zst() {
+                continue;
+            }
+            if found.is_some() {
+                // More than one non-1-ZST field.
+                return None;
+            }
+            found = Some((field_idx, field));
+        }
+        found
+    }
 }
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index 3ffe670d87a..16183403d67 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -588,19 +588,11 @@ fn make_thin_self_ptr<'tcx>(
         // To get the type `*mut RcBox<Self>`, we just keep unwrapping newtypes until we
         // get a built-in pointer type
         let mut fat_pointer_layout = layout;
-        'descend_newtypes: while !fat_pointer_layout.ty.is_unsafe_ptr()
-            && !fat_pointer_layout.ty.is_ref()
-        {
-            for i in 0..fat_pointer_layout.fields.count() {
-                let field_layout = fat_pointer_layout.field(cx, i);
-
-                if !field_layout.is_1zst() {
-                    fat_pointer_layout = field_layout;
-                    continue 'descend_newtypes;
-                }
-            }
-
-            bug!("receiver has no non-1-ZST fields {:?}", fat_pointer_layout);
+        while !fat_pointer_layout.ty.is_unsafe_ptr() && !fat_pointer_layout.ty.is_ref() {
+            fat_pointer_layout = fat_pointer_layout
+                .non_1zst_field(cx)
+                .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type")
+                .1
         }
 
         fat_pointer_layout.ty