Rollup merge of rust-lang#158666 - scottmcm:scalar-but-packed, r=workingjubilee

Carry the `b_offset` inside `BackendRepr::ScalarPair`

Inspired by rust-lang/compiler-team#1007 but doesn't actually change any of the layout rules just yet.

This turned out to be a nice change even if we didn't use the extra flexibility, IMHO, because it allowed so many things like

```diff
@@ -222,12 +224,12 @@ fn from_const_alloc<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
                 let val = read_scalar(offset, size, s, bx.immediate_backend_type(layout));
                 OperandRef { val: OperandValue::Immediate(val), layout, move_annotation: None }
             }
-            BackendRepr::ScalarPair(
-                a @ abi::Scalar::Initialized { .. },
-                b @ abi::Scalar::Initialized { .. },
-            ) => {
+            BackendRepr::ScalarPair {
+                a: a @ abi::Scalar::Initialized { .. },
+                b: b @ abi::Scalar::Initialized { .. },
+                b_offset,
+            } => {
                 let (a_size, b_size) = (a.size(bx), b.size(bx));
-                let b_offset = (offset + a_size).align_to(b.default_align(bx).abi);
                 assert!(b_offset.bytes() > 0);
                 let a_val = read_scalar(
                     offset,
```

as *oh my* was that little magic incantation copy-pasted all over the place.

Apologies for the pretty-giant PR.  I tried to make it as direct a change as I could: if it was `(..)` before it's `{ .. }` now, if it was `(_, _)` before it's `{ a: _, b: _, b_offset: _ }` now.  I kept the names the same so the code lines were unchanged even if normally I might have just renamed things, etc.  I'll add some inline notes for places of particular interest.

r? @workingjubilee

Author: JonathanBrouwer

compiler/rustc_abi/src/callconv.rs

@@ -89,7 +89,7 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
                 unreachable!("`homogeneous_aggregate` should not be called for scalable vectors")
             }
 
-            BackendRepr::ScalarPair(..) | BackendRepr::Memory { sized: true } => {
+            BackendRepr::ScalarPair { .. } | BackendRepr::Memory { sized: true } => {
                 // Helper for computing `homogeneous_aggregate`, allowing a custom
                 // starting offset (used below for handling variants).
                 let from_fields_at =

compiler/rustc_abi/src/layout.rs

@@ -476,7 +476,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             Err(AbiMismatch) | Ok(None) => BackendRepr::Memory { sized: true },
             Ok(Some((repr, _))) => match repr {
                 // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
-                BackendRepr::Scalar(_) | BackendRepr::ScalarPair(_, _)
+                BackendRepr::Scalar(_) | BackendRepr::ScalarPair { .. }
                     if repr.scalar_platform_align(dl).unwrap() != align =>
                 {
                     BackendRepr::Memory { sized: true }
@@ -489,7 +489,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 }
                 // the alignment tests passed and we can use this
                 BackendRepr::Scalar(..)
-                | BackendRepr::ScalarPair(..)
+                | BackendRepr::ScalarPair { .. }
                 | BackendRepr::SimdVector { .. }
                 | BackendRepr::SimdScalableVector { .. }
                 | BackendRepr::Memory { .. } => repr,
@@ -558,7 +558,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             };
             match &mut st.backend_repr {
                 BackendRepr::Scalar(scalar) => hide_niches(scalar),
-                BackendRepr::ScalarPair(a, b) => {
+                BackendRepr::ScalarPair { a, b, b_offset: _ } => {
                     hide_niches(a);
                     hide_niches(b);
                 }
@@ -701,13 +701,21 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                     // When the total alignment and size match, we can use the
                     // same ABI as the scalar variant with the reserved niche.
                     BackendRepr::Scalar(_) => BackendRepr::Scalar(niche_scalar),
-                    BackendRepr::ScalarPair(first, second) => {
+                    BackendRepr::ScalarPair { a: first, b: second, b_offset } => {
                         // Only the niche is guaranteed to be initialised,
                         // so use union layouts for the other primitive.
                         if niche_offset == Size::ZERO {
-                            BackendRepr::ScalarPair(niche_scalar, second.to_union())
+                            BackendRepr::ScalarPair {
+                                a: niche_scalar,
+                                b: second.to_union(),
+                                b_offset,
+                            }
                         } else {
-                            BackendRepr::ScalarPair(first.to_union(), niche_scalar)
+                            BackendRepr::ScalarPair {
+                                a: first.to_union(),
+                                b: niche_scalar,
+                                b_offset,
+                            }
                         }
                     }
                     _ => BackendRepr::Memory { sized: true },
@@ -1037,7 +1045,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         // If we pick a "clever" (by-value) ABI, we might have to adjust the ABI of the
         // variants to ensure they are consistent. This is because a downcast is
         // semantically a NOP, and thus should not affect layout.
-        if matches!(abi, BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..)) {
+        if matches!(abi, BackendRepr::Scalar(..) | BackendRepr::ScalarPair { .. }) {
             for variant in &mut layout_variants {
                 // We only do this for variants with fields; the others are not accessed anyway.
                 // Also do not overwrite any already existing "clever" ABIs.
@@ -1354,7 +1362,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                             }
                             // But scalar pairs are Rust-specific and get
                             // treated as aggregates by C ABIs anyway.
-                            BackendRepr::ScalarPair(..) => {
+                            BackendRepr::ScalarPair { .. } => {
                                 abi = field.backend_repr;
                             }
                             _ => {}

compiler/rustc_abi/src/layout/simple.rs

@@ -110,7 +110,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
                 offsets: [Size::ZERO, b_offset].into(),
                 in_memory_order: [FieldIdx::new(0), FieldIdx::new(1)].into(),
             },
-            backend_repr: BackendRepr::ScalarPair(a, b),
+            backend_repr: BackendRepr::ScalarPair { a, b, b_offset },
             largest_niche,
             uninhabited: false,
             align: AbiAlign::new(align),

compiler/rustc_abi/src/lib.rs

@@ -1796,14 +1796,18 @@ impl IntoDiagArg for NumScalableVectors {
 pub enum BackendRepr {
     Scalar(Scalar),
     /// The data contained in this type can be entirely represented by two scalars.
-    /// The two scalars are listed in *memory* order, so the first is at offset zero
-    /// and the second at a non-zero offset.
+    /// The two scalars are listed in *memory* order, so `a` is at offset zero
+    /// and `b` is at non-zero offset `b_offset`.
     /// These need not be `FieldIdx(0)` and `FieldIdx(1)`.
     ///
-    /// As of June 2026 the offset to the second scalar is the size of the first
-    /// scalar rounded up to the platform alignment of the second scalar.
+    /// As of June 2026 the `b_offset` is always the size of the `a`
+    /// scalar rounded up to the platform alignment of the `b` scalar.
     /// That may soon change, however; see MCP#1007.
-    ScalarPair(Scalar, Scalar),
+    ScalarPair {
+        a: Scalar,
+        b: Scalar,
+        b_offset: Size,
+    },
     SimdScalableVector {
         element: Scalar,
         count: u64,
@@ -1826,7 +1830,7 @@ impl BackendRepr {
     pub fn is_unsized(&self) -> bool {
         match *self {
             BackendRepr::Scalar(_)
-            | BackendRepr::ScalarPair(..)
+            | BackendRepr::ScalarPair { .. }
             // FIXME(rustc_scalable_vector): Scalable vectors are `Sized` while the
             // `sized_hierarchy` feature is not yet fully implemented. After `sized_hierarchy` is
             // fully implemented, scalable vectors will remain `Sized`, they just won't be
@@ -1875,7 +1879,7 @@ impl BackendRepr {
     pub fn scalar_platform_align<C: HasDataLayout>(&self, cx: &C) -> Option<Align> {
         match *self {
             BackendRepr::Scalar(s) => Some(s.default_align(cx).abi),
-            BackendRepr::ScalarPair(s1, s2) => {
+            BackendRepr::ScalarPair { a: s1, b: s2, b_offset: _ } => {
                 Some(s1.default_align(cx).max(s2.default_align(cx)).abi)
             }
             // The align of a Vector can vary in surprising ways
@@ -1893,8 +1897,7 @@ impl BackendRepr {
             // No padding in scalars.
             BackendRepr::Scalar(s) => Some(s.size(cx)),
             // May have some padding between the pair.
-            BackendRepr::ScalarPair(s1, s2) => {
-                let field2_offset = s1.size(cx).align_to(s2.default_align(cx).abi);
+            BackendRepr::ScalarPair { a: _, b: s2, b_offset: field2_offset } => {
                 let size = (field2_offset + s2.size(cx)).align_to(
                     self.scalar_platform_align(cx)
                         // We absolutely must have an answer here or everything is FUBAR.
@@ -1913,8 +1916,8 @@ impl BackendRepr {
     pub fn to_union(&self) -> Self {
         match *self {
             BackendRepr::Scalar(s) => BackendRepr::Scalar(s.to_union()),
-            BackendRepr::ScalarPair(s1, s2) => {
-                BackendRepr::ScalarPair(s1.to_union(), s2.to_union())
+            BackendRepr::ScalarPair { a: s1, b: s2, b_offset } => {
+                BackendRepr::ScalarPair { a: s1.to_union(), b: s2.to_union(), b_offset }
             }
             BackendRepr::SimdVector { element, count } => {
                 BackendRepr::SimdVector { element: element.to_union(), count }
@@ -1939,8 +1942,13 @@ impl BackendRepr {
                 BackendRepr::SimdVector { element: element_l, count: count_l },
                 BackendRepr::SimdVector { element: element_r, count: count_r },
             ) => element_l.primitive() == element_r.primitive() && count_l == count_r,
-            (BackendRepr::ScalarPair(l1, l2), BackendRepr::ScalarPair(r1, r2)) => {
-                l1.primitive() == r1.primitive() && l2.primitive() == r2.primitive()
+            (
+                BackendRepr::ScalarPair { a: l1, b: l2, b_offset: l_offset },
+                BackendRepr::ScalarPair { a: r1, b: r2, b_offset: r_offset },
+            ) => {
+                l1.primitive() == r1.primitive()
+                    && l2.primitive() == r2.primitive()
+                    && l_offset == r_offset
             }
             // Everything else must be strictly identical.
             _ => self == other,
@@ -2180,7 +2188,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
             BackendRepr::Scalar(_)
             | BackendRepr::SimdVector { .. }
             | BackendRepr::SimdScalableVector { .. } => false,
-            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => true,
+            BackendRepr::ScalarPair { .. } | BackendRepr::Memory { .. } => true,
         }
     }
 
@@ -2294,7 +2302,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
     pub fn is_zst(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_)
-            | BackendRepr::ScalarPair(..)
+            | BackendRepr::ScalarPair { .. }
             | BackendRepr::SimdScalableVector { .. }
             | BackendRepr::SimdVector { .. } => false,
             BackendRepr::Memory { sized } => sized && self.size.bytes() == 0,

compiler/rustc_codegen_cranelift/src/abi/mod.rs

@@ -220,7 +220,7 @@ fn make_local_place<'tcx>(
         );
     }
     let place = if is_ssa {
-        if let BackendRepr::ScalarPair(_, _) = layout.backend_repr {
+        if let BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } = layout.backend_repr {
             CPlace::new_var_pair(fx, local, layout)
         } else {
             CPlace::new_var(fx, local, layout)

compiler/rustc_codegen_cranelift/src/abi/pass_mode.rs

@@ -112,7 +112,7 @@ impl<'tcx> ArgAbiExt<'tcx> for ArgAbi<'tcx, Ty<'tcx>> {
                 _ => unreachable!("{:?}", self.layout.backend_repr),
             },
             PassMode::Pair(attrs_a, attrs_b) => match self.layout.backend_repr {
-                BackendRepr::ScalarPair(a, b) => {
+                BackendRepr::ScalarPair { a, b, b_offset: _ } => {
                     let a = scalar_to_clif_type(tcx, a);
                     let b = scalar_to_clif_type(tcx, b);
                     smallvec![
@@ -167,7 +167,7 @@ impl<'tcx> ArgAbiExt<'tcx> for ArgAbi<'tcx, Ty<'tcx>> {
                 _ => unreachable!("{:?}", self.layout.backend_repr),
             },
             PassMode::Pair(attrs_a, attrs_b) => match self.layout.backend_repr {
-                BackendRepr::ScalarPair(a, b) => {
+                BackendRepr::ScalarPair { a, b, b_offset: _ } => {
                     let a = scalar_to_clif_type(tcx, a);
                     let b = scalar_to_clif_type(tcx, b);
                     (

compiler/rustc_codegen_cranelift/src/base.rs

@@ -695,7 +695,7 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                         }
                         UnOp::PtrMetadata => match layout.backend_repr {
                             BackendRepr::Scalar(_) => CValue::zst(dest_layout),
-                            BackendRepr::ScalarPair(_, _) => {
+                            BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } => {
                                 CValue::by_val(operand.load_scalar_pair(fx).1, dest_layout)
                             }
                             _ => bug!("Unexpected `PtrToMetadata` operand: {operand:?}"),

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -572,7 +572,9 @@ fn codegen_regular_intrinsic_call<'tcx>(
             let layout = fx.layout_of(generic_args.type_at(0));
             // Note: Can't use is_unsized here as truly unsized types need to take the fixed size
             // branch
-            let meta = if let BackendRepr::ScalarPair(_, _) = ptr.layout().backend_repr {
+            let meta = if let BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } =
+                ptr.layout().backend_repr
+            {
                 Some(ptr.load_scalar_pair(fx).1)
             } else {
                 None
@@ -586,7 +588,9 @@ fn codegen_regular_intrinsic_call<'tcx>(
             let layout = fx.layout_of(generic_args.type_at(0));
             // Note: Can't use is_unsized here as truly unsized types need to take the fixed size
             // branch
-            let meta = if let BackendRepr::ScalarPair(_, _) = ptr.layout().backend_repr {
+            let meta = if let BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } =
+                ptr.layout().backend_repr
+            {
                 Some(ptr.load_scalar_pair(fx).1)
             } else {
                 None

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -55,8 +55,7 @@ fn codegen_field<'tcx>(
     }
 }
 
-fn scalar_pair_calculate_b_offset(tcx: TyCtxt<'_>, a_scalar: Scalar, b_scalar: Scalar) -> Offset32 {
-    let b_offset = a_scalar.size(&tcx).align_to(b_scalar.default_align(&tcx).abi);
+fn scalar_pair_convert_b_offset(b_offset: Size) -> Offset32 {
     Offset32::new(b_offset.bytes().try_into().unwrap())
 }
 
@@ -159,11 +158,11 @@ impl<'tcx> CValue<'tcx> {
         let layout = self.1;
         match self.0 {
             CValueInner::ByRef(ptr, None) => {
-                let (a_scalar, b_scalar) = match layout.backend_repr {
-                    BackendRepr::ScalarPair(a, b) => (a, b),
+                let (a_scalar, b_scalar, b_offset) = match layout.backend_repr {
+                    BackendRepr::ScalarPair { a, b, b_offset } => (a, b, b_offset),
                     _ => unreachable!("load_scalar_pair({:?})", self),
                 };
-                let b_offset = scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
+                let b_offset = scalar_pair_convert_b_offset(b_offset);
                 let clif_ty1 = scalar_to_clif_type(fx.tcx, a_scalar);
                 let clif_ty2 = scalar_to_clif_type(fx.tcx, b_scalar);
                 let mut flags = MemFlags::new();
@@ -189,7 +188,7 @@ impl<'tcx> CValue<'tcx> {
         match self.0 {
             CValueInner::ByVal(_) => unreachable!(),
             CValueInner::ByValPair(val1, val2) => match layout.backend_repr {
-                BackendRepr::ScalarPair(_, _) => {
+                BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } => {
                     let val = match field.as_u32() {
                         0 => val1,
                         1 => val2,
@@ -580,7 +579,7 @@ impl<'tcx> CPlace<'tcx> {
             }
             CPlaceInner::VarPair(_local, var1, var2) => {
                 let (data1, data2) = match from.1.backend_repr {
-                    BackendRepr::ScalarPair(_, _) => {
+                    BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } => {
                         CValue(from.0, dst_layout).load_scalar_pair(fx)
                     }
                     _ => {
@@ -607,9 +606,8 @@ impl<'tcx> CPlace<'tcx> {
                         to_ptr.store(fx, val, flags);
                     }
                     CValueInner::ByValPair(val1, val2) => match from.layout().backend_repr {
-                        BackendRepr::ScalarPair(a_scalar, b_scalar) => {
-                            let b_offset =
-                                scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
+                        BackendRepr::ScalarPair { a: _, b: _, b_offset } => {
+                            let b_offset = scalar_pair_convert_b_offset(b_offset);
                             to_ptr.store(fx, val1, flags);
                             to_ptr.offset(fx, b_offset).store(fx, val2, flags);
                         }
@@ -627,9 +625,8 @@ impl<'tcx> CPlace<'tcx> {
                                 to_ptr.store(fx, val, flags);
                                 return;
                             }
-                            BackendRepr::ScalarPair(a_scalar, b_scalar) => {
-                                let b_offset =
-                                    scalar_pair_calculate_b_offset(fx.tcx, a_scalar, b_scalar);
+                            BackendRepr::ScalarPair { a: _, b: _, b_offset } => {
+                                let b_offset = scalar_pair_convert_b_offset(b_offset);
                                 let (val1, val2) = from.load_scalar_pair(fx);
                                 to_ptr.store(fx, val1, flags);
                                 to_ptr.offset(fx, b_offset).store(fx, val2, flags);

compiler/rustc_codegen_cranelift/src/vtable.rs

@@ -56,13 +56,14 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
         }
     }
 
-    let (ptr, vtable) = if let BackendRepr::ScalarPair(_, _) = arg.layout().backend_repr {
-        let (ptr, vtable) = arg.load_scalar_pair(fx);
-        (Pointer::new(ptr), vtable)
-    } else {
-        let (ptr, vtable) = arg.try_to_ptr().unwrap();
-        (ptr, vtable.unwrap())
-    };
+    let (ptr, vtable) =
+        if let BackendRepr::ScalarPair { a: _, b: _, b_offset: _ } = arg.layout().backend_repr {
+            let (ptr, vtable) = arg.load_scalar_pair(fx);
+            (Pointer::new(ptr), vtable)
+        } else {
+            let (ptr, vtable) = arg.try_to_ptr().unwrap();
+            (ptr, vtable.unwrap())
+        };
 
     let usize_size = fx.layout_of(fx.tcx.types.usize).size.bytes();
     let func_ref = fx.bcx.ins().load(