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Make struct layout not depend on unsizeable tail

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This commit is contained in:
Lukas Markeffsky
2023-05-29 14:54:48 +02:00
parent 70e04bd88d
commit 381b778d27
2 changed files with 31 additions and 2 deletions
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8
compiler/rustc_abi/src/layout.rs
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@@ -828,6 +828,7 @@ fn univariant(
if optimize && fields.len() > 1 {
let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
let optimizing = &mut inverse_memory_index.raw[..end];
let fields_excluding_tail = &fields.raw[..end];
// If `-Z randomize-layout` was enabled for the type definition we can shuffle
// the field ordering to try and catch some code making assumptions about layouts
@@ -844,8 +845,11 @@ fn univariant(
}
// Otherwise we just leave things alone and actually optimize the type's fields
} else {
let max_field_align = fields.iter().map(|f| f.align().abi.bytes()).max().unwrap_or(1);
let largest_niche_size = fields
// To allow unsizing `&Foo<Type>` -> `&Foo<dyn Trait>`, the layout of the struct must
// not depend on the layout of the tail.
let max_field_align =
fields_excluding_tail.iter().map(|f| f.align().abi.bytes()).max().unwrap_or(1);
let largest_niche_size = fields_excluding_tail
.iter()
.filter_map(|f| f.largest_niche())
.map(|n| n.available(dl))