Fix the size of niche enums with ZST alignment
For enums with an aligned ZST variant, like `[T; 0]`, the niche layout was not computing a sufficient size to be consistent with alignment. Now we pad that size up to the alignment, and also make sure to only use the niche variant's ABI when the size and alignment still match.
This commit is contained in:
Josh Stone
parent
5dda74a48c
commit
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3 changed files with 348 additions and 2 deletions
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@ -1231,11 +1231,15 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
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.collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
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let offset = st[i].fields().offset(field_index) + niche.offset;
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let size = st[i].size();
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// Align the total size to the largest alignment.
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let size = st[i].size().align_to(align.abi);
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let abi = if st.iter().all(|v| v.abi().is_uninhabited()) {
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Abi::Uninhabited
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} else {
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} else if align == st[i].align() && size == st[i].size() {
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// When the total alignment and size match, we can use the
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// same ABI as the scalar variant with the reserved niche.
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match st[i].abi() {
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Abi::Scalar(_) => Abi::Scalar(niche_scalar),
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Abi::ScalarPair(first, second) => {
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@ -1249,6 +1253,8 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
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}
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_ => Abi::Aggregate { sized: true },
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}
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} else {
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Abi::Aggregate { sized: true }
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};
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let largest_niche = Niche::from_scalar(dl, offset, niche_scalar);
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37
src/test/ui/layout/zero-sized-array-enum-niche.rs
Normal file
37
src/test/ui/layout/zero-sized-array-enum-niche.rs
Normal file
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@ -0,0 +1,37 @@
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// normalize-stderr-test "pref: Align\([1-8] bytes\)" -> "pref: $$PREF_ALIGN"
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#![feature(rustc_attrs)]
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#![crate_type = "lib"]
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// Various tests around the behavior of zero-sized arrays and
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// enum niches, especially that they have coherent size and alignment.
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// The original problem in #99836 came from ndarray's `TryFrom` for
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// `SliceInfo<[SliceInfoElem; 0], Din, Dout>`, where that returns
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// `Result<Self, ShapeError>` ~= `Result<AlignedZST, TypeWithNiche>`.
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// This is a close enough approximation:
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#[rustc_layout(debug)]
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type AlignedResult = Result<[u32; 0], bool>; //~ ERROR: layout_of
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// The bug gave that size 1 with align 4, but the size should also be 4.
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// It was also using the bool niche for the enum tag, which is fine, but
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// after the fix, layout decides to use a direct tagged repr instead.
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// Tagged repr is clever enough to grow tags to fill any padding, e.g.:
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// 1. `T_FF` (one byte of Tag, one byte of padding, two bytes of align=2 Field)
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// -> `TTFF` (Tag has expanded to two bytes, i.e. like `#[repr(u16)]`)
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// 2. `TFF` (one byte of Tag, two bytes of align=1 Field)
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// -> Tag has no room to expand!
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// (this outcome can be forced onto 1. by wrapping Field in `Packed<...>`)
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#[repr(packed)]
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struct Packed<T>(T);
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#[rustc_layout(debug)]
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type NicheLosesToTagged = Result<[u32; 0], Packed<std::num::NonZeroU16>>; //~ ERROR: layout_of
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// Should get tag_encoding: Direct, size == align == 4.
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#[repr(u16)]
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enum U16IsZero { _Zero = 0 }
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#[rustc_layout(debug)]
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type NicheWinsOverTagged = Result<[u32; 0], Packed<U16IsZero>>; //~ ERROR: layout_of
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// Should get tag_encoding: Niche, size == align == 4.
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303
src/test/ui/layout/zero-sized-array-enum-niche.stderr
Normal file
303
src/test/ui/layout/zero-sized-array-enum-niche.stderr
Normal file
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@ -0,0 +1,303 @@
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error: layout_of(std::result::Result<[u32; 0], bool>) = Layout {
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fields: Arbitrary {
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offsets: [
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Size(0 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Multiple {
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tag: Initialized {
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value: Int(
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I8,
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false,
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),
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valid_range: 0..=1,
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},
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tag_encoding: Direct,
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tag_field: 0,
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variants: [
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(4 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 0,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: None,
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(4 bytes),
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},
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(1 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 1,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(1 bytes),
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value: Int(
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I8,
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false,
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),
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valid_range: 0..=1,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(1 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(2 bytes),
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},
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],
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(0 bytes),
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value: Int(
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I8,
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false,
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),
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valid_range: 0..=1,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(4 bytes),
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}
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--> $DIR/zero-sized-array-enum-niche.rs:13:1
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LL | type AlignedResult = Result<[u32; 0], bool>;
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| ^^^^^^^^^^^^^^^^^^
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error: layout_of(std::result::Result<[u32; 0], Packed<std::num::NonZeroU16>>) = Layout {
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fields: Arbitrary {
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offsets: [
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Size(0 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Multiple {
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tag: Initialized {
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value: Int(
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I8,
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false,
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),
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valid_range: 0..=1,
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},
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tag_encoding: Direct,
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tag_field: 0,
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variants: [
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(4 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 0,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: None,
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(4 bytes),
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},
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(1 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 1,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(1 bytes),
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value: Int(
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I16,
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false,
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),
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valid_range: 1..=65535,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(1 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(3 bytes),
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},
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],
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(0 bytes),
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value: Int(
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I8,
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false,
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),
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valid_range: 0..=1,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(4 bytes),
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}
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--> $DIR/zero-sized-array-enum-niche.rs:29:1
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LL | type NicheLosesToTagged = Result<[u32; 0], Packed<std::num::NonZeroU16>>;
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| ^^^^^^^^^^^^^^^^^^^^^^^
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error: layout_of(std::result::Result<[u32; 0], Packed<U16IsZero>>) = Layout {
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fields: Arbitrary {
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offsets: [
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Size(0 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Multiple {
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tag: Initialized {
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value: Int(
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I16,
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false,
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),
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valid_range: 0..=1,
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},
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tag_encoding: Niche {
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dataful_variant: 1,
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niche_variants: 0..=0,
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niche_start: 1,
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},
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tag_field: 0,
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variants: [
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(0 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 0,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: None,
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(0 bytes),
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},
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Layout {
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fields: Arbitrary {
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offsets: [
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Size(0 bytes),
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],
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memory_index: [
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0,
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],
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},
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variants: Single {
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index: 1,
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(0 bytes),
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value: Int(
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I16,
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false,
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),
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valid_range: 0..=0,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(1 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(2 bytes),
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},
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],
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},
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abi: Aggregate {
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sized: true,
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},
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largest_niche: Some(
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Niche {
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offset: Size(0 bytes),
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value: Int(
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I16,
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false,
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),
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valid_range: 0..=1,
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},
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),
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align: AbiAndPrefAlign {
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abi: Align(4 bytes),
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pref: $PREF_ALIGN,
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},
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size: Size(4 bytes),
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}
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--> $DIR/zero-sized-array-enum-niche.rs:36:1
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LL | type NicheWinsOverTagged = Result<[u32; 0], Packed<U16IsZero>>;
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| ^^^^^^^^^^^^^^^^^^^^^^^^
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error: aborting due to 3 previous errors
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