Avoid alignment mismatch between ABI and layout for unions.

compiler/rustc_abi/src/layout.rs

@@ -729,42 +729,73 @@ pub trait LayoutCalculator {
             align = align.max(AbiAndPrefAlign::new(repr_align));
         }
 
-        let optimize = !repr.inhibit_union_abi_opt();
+        // If all the non-ZST fields have the same ABI and union ABI optimizations aren't
+        // disabled, we can use that common ABI for the union as a whole.
+        struct AbiMismatch;
+        let mut common_non_zst_abi_and_align = if repr.inhibit_union_abi_opt() {
+            // Can't optimize
+            Err(AbiMismatch)
+        } else {
+            Ok(None)
+        };
+
         let mut size = Size::ZERO;
-        let mut abi = Abi::Aggregate { sized: true };
         let only_variant = &variants[FIRST_VARIANT];
         for field in only_variant {
             assert!(field.0.is_sized());
+
             align = align.max(field.align());
+            size = cmp::max(size, field.size());
 
-            // If all non-ZST fields have the same ABI, forward this ABI
-            if optimize && !field.0.is_zst() {
+            if field.0.is_zst() {
+                // Nothing more to do for ZST fields
+                continue;
+            }
+
+            if let Ok(common) = common_non_zst_abi_and_align {
                 // Discard valid range information and allow undef
-                let field_abi = match field.abi() {
-                    Abi::Scalar(x) => Abi::Scalar(x.to_union()),
-                    Abi::ScalarPair(x, y) => Abi::ScalarPair(x.to_union(), y.to_union()),
-                    Abi::Vector { element: x, count } => {
-                        Abi::Vector { element: x.to_union(), count }
-                    }
-                    Abi::Uninhabited | Abi::Aggregate { .. } => Abi::Aggregate { sized: true },
-                };
+                let field_abi = field.abi().to_union();
 
-                if size == Size::ZERO {
-                    // first non ZST: initialize 'abi'
-                    abi = field_abi;
-                } else if abi != field_abi {
-                    // different fields have different ABI: reset to Aggregate
-                    abi = Abi::Aggregate { sized: true };
+                if let Some((common_abi, common_align)) = common {
+                    if common_abi != field_abi {
+                        // Different fields have different ABI: disable opt
+                        common_non_zst_abi_and_align = Err(AbiMismatch);
+                    } else {
+                        // Fields with the same non-Aggregate ABI should also
+                        // have the same alignment
+                        if !matches!(common_abi, Abi::Aggregate { .. }) {
+                            assert_eq!(
+                                common_align,
+                                field.align().abi,
+                                "non-Aggregate field with matching ABI but differing alignment"
+                            );
+                        }
+                    }
+                } else {
+                    // First non-ZST field: record its ABI and alignment
+                    common_non_zst_abi_and_align = Ok(Some((field_abi, field.align().abi)));
                 }
             }
-
-            size = cmp::max(size, field.size());
         }
 
         if let Some(pack) = repr.pack {
             align = align.min(AbiAndPrefAlign::new(pack));
         }
 
+        // If all non-ZST fields have the same ABI, we may forward that ABI
+        // for the union as a whole, unless otherwise inhibited.
+        let abi = match common_non_zst_abi_and_align {
+            Err(AbiMismatch) | Ok(None) => Abi::Aggregate { sized: true },
+            Ok(Some((abi, _))) => {
+                if abi.inherent_align(dl).map(|a| a.abi) != Some(align.abi) {
+                    // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
+                    Abi::Aggregate { sized: true }
+                } else {
+                    abi
+                }
+            }
+        };
+
         Some(LayoutS {
             variants: Variants::Single { index: FIRST_VARIANT },
             fields: FieldsShape::Union(NonZeroUsize::new(only_variant.len())?),

compiler/rustc_abi/src/lib.rs

@@ -1272,6 +1272,50 @@ impl Abi {
     pub fn is_scalar(&self) -> bool {
         matches!(*self, Abi::Scalar(_))
     }
+
+    /// Returns the fixed alignment of this ABI, if any is mandated.
+    pub fn inherent_align<C: HasDataLayout>(&self, cx: &C) -> Option<AbiAndPrefAlign> {
+        Some(match *self {
+            Abi::Scalar(s) => s.align(cx),
+            Abi::ScalarPair(s1, s2) => s1.align(cx).max(s2.align(cx)),
+            Abi::Vector { element, count } => {
+                cx.data_layout().vector_align(element.size(cx) * count)
+            }
+            Abi::Uninhabited | Abi::Aggregate { .. } => return None,
+        })
+    }
+
+    /// Returns the fixed size of this ABI, if any is mandated.
+    pub fn inherent_size<C: HasDataLayout>(&self, cx: &C) -> Option<Size> {
+        Some(match *self {
+            Abi::Scalar(s) => {
+                // No padding in scalars.
+                s.size(cx)
+            }
+            Abi::ScalarPair(s1, s2) => {
+                // May have some padding between the pair.
+                let field2_offset = s1.size(cx).align_to(s2.align(cx).abi);
+                (field2_offset + s2.size(cx)).align_to(self.inherent_align(cx)?.abi)
+            }
+            Abi::Vector { element, count } => {
+                // No padding in vectors, except possibly for trailing padding
+                // to make the size a multiple of align (e.g. for vectors of size 3).
+                (element.size(cx) * count).align_to(self.inherent_align(cx)?.abi)
+            }
+            Abi::Uninhabited | Abi::Aggregate { .. } => return None,
+        })
+    }
+
+    /// Discard validity range information and allow undef.
+    pub fn to_union(&self) -> Self {
+        assert!(self.is_sized());
+        match *self {
+            Abi::Scalar(s) => Abi::Scalar(s.to_union()),
+            Abi::ScalarPair(s1, s2) => Abi::ScalarPair(s1.to_union(), s2.to_union()),
+            Abi::Vector { element, count } => Abi::Vector { element: element.to_union(), count },
+            Abi::Uninhabited | Abi::Aggregate { .. } => Abi::Aggregate { sized: true },
+        }
+    }
 }
 
 #[derive(PartialEq, Eq, Hash, Clone, Debug)]

compiler/rustc_ty_utils/src/layout_sanity_check.rs

@@ -4,7 +4,7 @@ use rustc_middle::ty::{
 };
 use rustc_target::abi::*;
 
-use std::cmp;
+use std::assert_matches::assert_matches;
 
 /// Enforce some basic invariants on layouts.
 pub(super) fn sanity_check_layout<'tcx>(
@@ -68,21 +68,31 @@ pub(super) fn sanity_check_layout<'tcx>(
     }
 
     fn check_layout_abi<'tcx>(cx: &LayoutCx<'tcx, TyCtxt<'tcx>>, layout: &TyAndLayout<'tcx>) {
+        // Verify the ABI mandated alignment and size.
+        let align = layout.abi.inherent_align(cx).map(|align| align.abi);
+        let size = layout.abi.inherent_size(cx);
+        let Some((align, size)) = align.zip(size) else {
+            assert_matches!(
+                layout.layout.abi(),
+                Abi::Uninhabited | Abi::Aggregate { .. },
+                "ABI unexpectedly missing alignment and/or size in {layout:#?}"
+            );
+            return
+        };
+        assert_eq!(
+            layout.layout.align().abi,
+            align,
+            "alignment mismatch between ABI and layout in {layout:#?}"
+        );
+        assert_eq!(
+            layout.layout.size(),
+            size,
+            "size mismatch between ABI and layout in {layout:#?}"
+        );
+
+        // Verify per-ABI invariants
         match layout.layout.abi() {
-            Abi::Scalar(scalar) => {
-                // No padding in scalars.
-                let size = scalar.size(cx);
-                let align = scalar.align(cx).abi;
-                assert_eq!(
-                    layout.layout.size(),
-                    size,
-                    "size mismatch between ABI and layout in {layout:#?}"
-                );
-                assert_eq!(
-                    layout.layout.align().abi,
-                    align,
-                    "alignment mismatch between ABI and layout in {layout:#?}"
-                );
+            Abi::Scalar(_) => {
                 // Check that this matches the underlying field.
                 let inner = skip_newtypes(cx, layout);
                 assert!(
@@ -135,24 +145,6 @@ pub(super) fn sanity_check_layout<'tcx>(
                 }
             }
             Abi::ScalarPair(scalar1, scalar2) => {
-                // Sanity-check scalar pairs. Computing the expected size and alignment is a bit of work.
-                let size1 = scalar1.size(cx);
-                let align1 = scalar1.align(cx).abi;
-                let size2 = scalar2.size(cx);
-                let align2 = scalar2.align(cx).abi;
-                let align = cmp::max(align1, align2);
-                let field2_offset = size1.align_to(align2);
-                let size = (field2_offset + size2).align_to(align);
-                assert_eq!(
-                    layout.layout.size(),
-                    size,
-                    "size mismatch between ABI and layout in {layout:#?}"
-                );
-                assert_eq!(
-                    layout.layout.align().abi,
-                    align,
-                    "alignment mismatch between ABI and layout in {layout:#?}",
-                );
                 // Check that the underlying pair of fields matches.
                 let inner = skip_newtypes(cx, layout);
                 assert!(
@@ -189,8 +181,9 @@ pub(super) fn sanity_check_layout<'tcx>(
                         "`ScalarPair` layout for type with less than two non-ZST fields: {inner:#?}"
                     )
                 });
-                assert!(
-                    fields.next().is_none(),
+                assert_matches!(
+                    fields.next(),
+                    None,
                     "`ScalarPair` layout for type with at least three non-ZST fields: {inner:#?}"
                 );
                 // The fields might be in opposite order.
@@ -200,6 +193,10 @@ pub(super) fn sanity_check_layout<'tcx>(
                     (offset2, field2, offset1, field1)
                 };
                 // The fields should be at the right offset, and match the `scalar` layout.
+                let size1 = scalar1.size(cx);
+                let align1 = scalar1.align(cx).abi;
+                let size2 = scalar2.size(cx);
+                let align2 = scalar2.align(cx).abi;
                 assert_eq!(
                     offset1,
                     Size::ZERO,
@@ -213,10 +210,12 @@ pub(super) fn sanity_check_layout<'tcx>(
                     field1.align.abi, align1,
                     "`ScalarPair` first field with bad align in {inner:#?}",
                 );
-                assert!(
-                    matches!(field1.abi, Abi::Scalar(_)),
+                assert_matches!(
+                    field1.abi,
+                    Abi::Scalar(_),
                     "`ScalarPair` first field with bad ABI in {inner:#?}",
                 );
+                let field2_offset = size1.align_to(align2);
                 assert_eq!(
                     offset2, field2_offset,
                     "`ScalarPair` second field at bad offset in {inner:#?}",
@@ -229,27 +228,14 @@ pub(super) fn sanity_check_layout<'tcx>(
                     field2.align.abi, align2,
                     "`ScalarPair` second field with bad align in {inner:#?}",
                 );
-                assert!(
-                    matches!(field2.abi, Abi::Scalar(_)),
+                assert_matches!(
+                    field2.abi,
+                    Abi::Scalar(_),
                     "`ScalarPair` second field with bad ABI in {inner:#?}",
                 );
             }
-            Abi::Vector { count, element } => {
-                // No padding in vectors, except possibly for trailing padding to make the size a multiple of align.
-                let size = element.size(cx) * count;
-                let align = cx.data_layout().vector_align(size).abi;
-                let size = size.align_to(align); // needed e.g. for vectors of size 3
+            Abi::Vector { element, .. } => {
                 assert!(align >= element.align(cx).abi); // just sanity-checking `vector_align`.
-                assert_eq!(
-                    layout.layout.size(),
-                    size,
-                    "size mismatch between ABI and layout in {layout:#?}"
-                );
-                assert_eq!(
-                    layout.layout.align().abi,
-                    align,
-                    "alignment mismatch between ABI and layout in {layout:#?}"
-                );
                 // FIXME: Do some kind of check of the inner type, like for Scalar and ScalarPair.
             }
             Abi::Uninhabited | Abi::Aggregate { .. } => {} // Nothing to check.

compiler/rustc_ty_utils/src/lib.rs

@@ -5,6 +5,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(assert_matches)]
 #![feature(iterator_try_collect)]
 #![feature(let_chains)]
 #![feature(never_type)]

tests/ui/layout/debug.rs

@@ -1,8 +1,9 @@
 // normalize-stderr-test "pref: Align\([1-8] bytes\)" -> "pref: $$PREF_ALIGN"
-#![feature(never_type, rustc_attrs, type_alias_impl_trait)]
+#![feature(never_type, rustc_attrs, type_alias_impl_trait, repr_simd)]
 #![crate_type = "lib"]
 
 #[rustc_layout(debug)]
+#[derive(Copy, Clone)]
 enum E { Foo, Bar(!, i32, i32) } //~ ERROR: layout_of
 
 #[rustc_layout(debug)]
@@ -17,6 +18,51 @@ type Test = Result<i32, i32>; //~ ERROR: layout_of
 #[rustc_layout(debug)]
 type T = impl std::fmt::Debug; //~ ERROR: layout_of
 
+#[rustc_layout(debug)]
+pub union V { //~ ERROR: layout_of
+    a: [u16; 0],
+    b: u8,
+}
+
+#[rustc_layout(debug)]
+pub union W { //~ ERROR: layout_of
+    b: u8,
+    a: [u16; 0],
+}
+
+#[rustc_layout(debug)]
+pub union Y { //~ ERROR: layout_of
+    b: [u8; 0],
+    a: [u16; 0],
+}
+
+#[rustc_layout(debug)]
+#[repr(packed(1))]
+union P1 { x: u32 } //~ ERROR: layout_of
+
+#[rustc_layout(debug)]
+#[repr(packed(1))]
+union P2 { x: (u32, u32) } //~ ERROR: layout_of
+
+#[repr(simd)]
+#[derive(Copy, Clone)]
+struct F32x4(f32, f32, f32, f32);
+
+#[rustc_layout(debug)]
+#[repr(packed(1))]
+union P3 { x: F32x4 } //~ ERROR: layout_of
+
+#[rustc_layout(debug)]
+#[repr(packed(1))]
+union P4 { x: E } //~ ERROR: layout_of
+
+#[rustc_layout(debug)]
+#[repr(packed(1))]
+union P5 { zst: [u16; 0], byte: u8 } //~ ERROR: layout_of
+
+#[rustc_layout(debug)]
+type X = std::mem::MaybeUninit<u8>; //~ ERROR: layout_of
+
 fn f() -> T {
     0i32
 }