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Make struct layout not depend on unsizeable tail #112062

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Jun 14, 2023
Merged

Make struct layout not depend on unsizeable tail #112062

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381b778
Make struct layout not depend on unsizeable tail
May 29, 2023
3003d05
disable some layout optimizations for unsizable structs
the8472 May 29, 2023
b982f3a
Add test case for unsizing with niche
wesleywiser Jun 13, 2023
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8 changes: 6 additions & 2 deletions compiler/rustc_abi/src/layout.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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
Comment on lines +854 to +858
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@the8472 the8472 May 29, 2023 •
edited
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These changes are necessary to fix it but I'm not sure if they're sufficient.

If an unsizing coercion could change which niche is considered the largest or how many bytes it spans that could alter the chosen layout in LayoutCalculator::univariant.
I haven't constructed an example yet where that actually happens and I'm not sure if it can happen (changing available niches between coercions seems iffy). But if you want to be conservative then disabling the alt-layout branch for unsized structs would be a safe choice.

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I found a case that still fails with this PR:

#![feature(offset_of)]

#[derive(Clone)]
struct WideptrField<T: ?Sized> {
  first: usize,
  second: usize,
  niche: NicheAtEnd,
  tail: T,
}

#[derive(Clone)]
#[repr(C)]
struct NicheAtEnd {
  arr: [u8; 7],
  b: bool,
}

type Tail = [bool; 8];

fn main() {
  let sized = Box::new(WideptrField {
    first: 0xdead,
    second: 0xbeef,
    niche: NicheAtEnd {
      arr: [0; 7],
      b: false,
    },
    tail: [false; 8] as Tail,
  });

  let unsized_value: Box<WideptrField<dyn Send>> = sized.clone();

  println!("offset in niche, sized   = {}", core::mem::offset_of!(WideptrField<Tail>, niche));
  println!("offset in niche, unsized = {}", core::mem::offset_of!(WideptrField<dyn Send>, niche));
}

prints

offset in niche, sized   = 0
offset in niche, unsized = 16

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I have pushed a change that fixes this case. Feel free to force-push over my commit if you already have your own.

.iter()
.filter_map(|f| f.largest_niche())
.map(|n| n.available(dl))
Expand Down
25 changes: 25 additions & 0 deletions tests/ui/layout/issue-112048-unsizing-field-order.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
// run-pass

// Check that unsizing doesn't reorder fields.

#![allow(dead_code)]

use std::fmt::Debug;

#[derive(Debug)]
struct GcNode<T: ?Sized> {
gets_swapped_with_next: usize,
next: Option<&'static GcNode<dyn Debug>>,
tail: T,
}

fn main() {
let node: Box<GcNode<dyn Debug>> = Box::new(GcNode {
gets_swapped_with_next: 42,
next: None,
tail: Box::new(1),
});

assert_eq!(node.gets_swapped_with_next, 42);
assert!(node.next.is_none());
}