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Rollup merge of #136985 - zachs18:backend-repr-remove-uninhabited, r=workingjubilee

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Do not ignore uninhabited types for function-call ABI purposes. (Remove BackendRepr::Uninhabited)

Accepted MCP: https://github.com/rust-lang/compiler-team/issues/832

Fixes #135802

Do not consider the inhabitedness of a type for function call ABI purposes.

* Remove the [`rustc_abi::BackendRepr::Uninhabited`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/enum.BackendRepr.html) variant
  * Instead calculate the `BackendRepr` of uninhabited types "normally" (as though they were not uninhabited "at the top level", but still considering inhabitedness of variants to determine enum layout, etc)
* Add an `uninhabited: bool` field to [`rustc_abi::LayoutData`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_abi/struct.LayoutData.html) so inhabitedness of a `LayoutData` can still be queried when necessary (e.g. when determining if an enum variant needs a tag value allocated to it).

This should not affect type layouts (size/align/field offset); this should only affect function call ABI, and only of uninhabited types.

cc ``@RalfJung``
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Jubilee 2025-02-20 14:58:18 -08:00 • committed by GitHub
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12
crates/hir-ty/src/layout.rs
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@ -194,6 +194,7 @@ fn layout_of_simd_ty(
fields,
backend_repr: BackendRepr::Vector { element: e_abi, count: e_len },
largest_niche: e_ly.largest_niche,
uninhabited: false,
size,
align,
max_repr_align: None,
@ -297,20 +298,17 @@ pub fn layout_of_ty_query(
.checked_mul(count, dl)
.ok_or(LayoutError::BadCalc(LayoutCalculatorError::SizeOverflow))?;
let backend_repr =
if count != 0 && matches!(element.backend_repr, BackendRepr::Uninhabited) {
BackendRepr::Uninhabited
} else {
BackendRepr::Memory { sized: true }
};
let backend_repr = BackendRepr::Memory { sized: true };
let largest_niche = if count != 0 { element.largest_niche } else { None };
let uninhabited = if count != 0 { element.uninhabited } else { false };
Layout {
variants: Variants::Single { index: struct_variant_idx() },
fields: FieldsShape::Array { stride: element.size, count },
backend_repr,
largest_niche,
uninhabited,
align: element.align,
size,
max_repr_align: None,
@ -325,6 +323,7 @@ pub fn layout_of_ty_query(
fields: FieldsShape::Array { stride: element.size, count: 0 },
backend_repr: BackendRepr::Memory { sized: false },
largest_niche: None,
uninhabited: false,
align: element.align,
size: Size::ZERO,
max_repr_align: None,
@ -337,6 +336,7 @@ pub fn layout_of_ty_query(
fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
backend_repr: BackendRepr::Memory { sized: false },
largest_niche: None,
uninhabited: false,
align: dl.i8_align,
size: Size::ZERO,
max_repr_align: None,