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Clarify layout of tuples when last field is unsized #1952

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Clarify layout of tuples when last field is unsized #1952

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4 changes: 3 additions & 1 deletion src/type-layout.md
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Expand Up @@ -91,7 +91,9 @@ r[layout.tuple]
## Tuple layout

r[layout.tuple.def]
Tuples are laid out according to the [`Rust` representation][`Rust`].
Tuples are laid out according to the same rules as structs using the [`Rust` representation][`Rust`].

They are laid out like a struct without generic parameters. This means that the last field of the tuple is not guaranteed to be stored at the end.

@RalfJung RalfJung Jul 8, 2026 •
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Do we document anything about the last field of structs? I don't think we should, we should just say that coercions are sound but we don't have to specify the means by which the compiler makes them sound.

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r[layout.tuple.unit]
The exception to this is the unit tuple (`()`), which is guaranteed as a [zero-sized type] to have a size of 0 and an alignment of 1.
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