38 lines
1.6 KiB
Rust
38 lines
1.6 KiB
Rust
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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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