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Guarantee char layout #1401

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Oct 30, 2023
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Guarantee char layout #1401

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4 changes: 3 additions & 1 deletion src/types/textual.md
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Expand Up @@ -17,7 +17,9 @@ is valid UTF-8. Calling a `str` method with a non-UTF-8 buffer can cause
Since `str` is a [dynamically sized type], it can only be instantiated through a
pointer type, such as `&str`.

## Bit validity
## Layout and bit validity

`char` is guaranteed to have the same size and alignment as `u32` on all platforms.
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Suggested change
`char` is guaranteed to have the same size and alignment as `u32` on all platforms.
`char` is guaranteed to have the same size and alignment as `u32` on all platforms.
`Option<char>` is guaranteed to have the same size and alignment as `char` on all platforms.

(Unless we want to put this somewhere else.)

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This suggestion should probably be handled separately, not in this PR.

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Filed #1426 to track this.

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TBH I was surprised this was to the reference directly, not to core like the other guarantees have been done. We currently have no guarantees written for specific alignment of anything, as far as I could find.

I was going to suggest something like

/// ```
///     use std::alloc::Layout;
///     assert_eq!(Layout::new::<char>(), Layout::new::<u32>());
///     assert_eq!(Layout::new::<Option<char>>(), Layout::new::<u32>());
/// ```

but then this wasn't a docs change...

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@joshlf joshlf Oct 18, 2023 •
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No opposition to this going in core as well of course, but we already have text in the Reference about the sizes of primitive types: https://doc.rust-lang.org/reference/type-layout.html#primitive-data-layout

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Put up a PR to add the same text to core: rust-lang/rust#116894


Every byte of a `char` is guaranteed to be initialized (in other words,
`transmute::<char, [u8; size_of::<char>()]>(...)` is always sound -- but since
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