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Consider discriminant fields that are ordered before variant fields

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This commit is contained in:
Michael Goulet 2022-12-12 18:21:10 +00:00
parent ed620cf969
commit 5f5ae17f4e
3 changed files with 59 additions and 7 deletions
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30
compiler/rustc_ty_utils/src/layout.rs
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@ -919,7 +919,7 @@ fn variant_info_for_generator<'tcx>(
def_id: DefId,
substs: ty::SubstsRef<'tcx>,
) -> (Vec<VariantInfo>, Option<Size>) {
let Variants::Multiple { tag, ref tag_encoding, .. } = layout.variants else {
let Variants::Multiple { tag, ref tag_encoding, tag_field, .. } = layout.variants else {
return (vec![], None);
};
@ -975,12 +975,28 @@ fn variant_info_for_generator<'tcx>(
if variant_size == Size::ZERO {
variant_size = upvars_size;
}
// We need to add the discriminant size back into min_size, since it is subtracted
// later during printing.
variant_size += match tag_encoding {
TagEncoding::Direct => tag.size(cx),
_ => Size::ZERO,
};
// This `if` deserves some explanation.
//
// The layout code has a choice of where to place the discriminant of this generator.
// If the discriminant of the generator is placed early in the layout (before the
// variant's own fields), then it'll implicitly be counted towards the size of the
// variant, since we use the maximum offset to calculate size.
// (side-note: I know this is a bit problematic given upvars placement, etc).
//
// This is important, since the layout printing code always subtracts this discriminant
// size from the variant size if the struct is "enum"-like, so failing to account for it
// will either lead to numerical underflow, or an underreported variant size...
//
// However, if the discriminant is placed past the end of the variant, then we need
// to factor in the size of the discriminant manually. This really should be refactored
// better, but this "works" for now.
if layout.fields.offset(tag_field) >= variant_size {
variant_size += match tag_encoding {
TagEncoding::Direct => tag.size(cx),
_ => Size::ZERO,
};
}
VariantInfo {
name: Some(Symbol::intern(&ty::GeneratorSubsts::variant_name(variant_idx))),