bjoernager/rust
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Rollup merge of #133681 - RalfJung:niches, r=wesleywiser

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improve TagEncoding::Niche docs, sanity check, and UB checks

Turns out the `niche_variants` range can actually contain the `untagged_variant`. We should report this as UB in Miri, so this PR implements that.

Also rename `partially_check_layout` to `layout_sanity_check` for better consistency with how similar functions are called in other parts of the compiler.

Turns out my adjustments to the transmutation logic also fix https://github.com/rust-lang/rust/issues/126267.
This commit is contained in:
Matthias Krüger 2024-12-03 21:55:26 +01:00 • committed by GitHub
commit 6e87eb58ed
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13 changed files with 177 additions and 92 deletions
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2
compiler/rustc_ty_utils/src/layout.rs
View file

@ -81,7 +81,7 @@ fn layout_of<'tcx>(
record_layout_for_printing(&cx, layout);
}
invariant::partially_check_layout(&cx, &layout);
invariant::layout_sanity_check(&cx, &layout);
Ok(layout)
}

16
compiler/rustc_ty_utils/src/layout/invariant.rs
View file

@ -1,11 +1,11 @@
use std::assert_matches::assert_matches;
use rustc_abi::{BackendRepr, FieldsShape, Scalar, Size, Variants};
use rustc_abi::{BackendRepr, FieldsShape, Scalar, Size, TagEncoding, Variants};
use rustc_middle::bug;
use rustc_middle::ty::layout::{HasTyCtxt, LayoutCx, TyAndLayout};
/// Enforce some basic invariants on layouts.
pub(super) fn partially_check_layout<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayout<'tcx>) {
pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayout<'tcx>) {
let tcx = cx.tcx();
// Type-level uninhabitedness should always imply ABI uninhabitedness.
@ -241,7 +241,17 @@ pub(super) fn partially_check_layout<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLa
check_layout_abi(cx, layout);
if let Variants::Multiple { variants, .. } = &layout.variants {
if let Variants::Multiple { variants, tag, tag_encoding, .. } = &layout.variants {
if let TagEncoding::Niche { niche_start, untagged_variant, niche_variants } = tag_encoding {
let niche_size = tag.size(cx);
assert!(*niche_start <= niche_size.unsigned_int_max());
for (idx, variant) in variants.iter_enumerated() {
// Ensure all inhabited variants are accounted for.
if !variant.is_uninhabited() {
assert!(idx == *untagged_variant || niche_variants.contains(&idx));
}
}
}
for variant in variants.iter() {
// No nested "multiple".
assert_matches!(variant.variants, Variants::Single { .. });