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Document that isize/usize are at least 16-bit wide on all platforms we currently support #154

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Merged
merged 6 commits into from
Jul 25, 2019
Merged

Document that isize/usize are at least 16-bit wide on all platforms we currently support #154

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28673da
Guarantee that isize/usize are at least 16-bit wide
gnzlbg Jul 1, 2019
8128316
Document that libcore relies on this guarantee
gnzlbg Jul 1, 2019
3b1353f
Document current behavior and don't guarantee 16-bit wide isize/usize
gnzlbg Jul 1, 2019
5fc5e71
Reword
gnzlbg Jul 1, 2019
66a8182
Do not say that libcore relies on usize>=16bit
gnzlbg Jul 2, 2019
d8d7edb
Clarify that all platforms we support have a C platform
gnzlbg Jul 23, 2019
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6 changes: 6 additions & 0 deletions reference/src/layout/scalars.md
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Expand Up @@ -37,6 +37,12 @@ The `isize` and `usize` types are pointer-sized signed and unsigned integers.
They have the same layout as the [pointer types] for which the pointee is
`Sized`, and are layout compatible with C's `uintptr_t` and `intptr_t` types.

> **Note**: C99 [7.18.2.4](https://port70.net/~nsz/c/c99/n1256.html#7.18.2.4)
> requires `uintptr_t` and `intptr_t` to be at least 16-bit wide. That is, on
> all platforms that we currently support, which have a C platform,
> `isize`/`usize` are at least 16-bit wide. Currently, `libcore` unconditionally
> provides impls of `From<i16>`/`From<u16>` for `isize`/`usize`.

> **Note**: Rust's `usize` and C's `unsigned` types are **not** equivalent. C's
> `unsigned` is at least as large as a short, allowed to have padding bits, etc.
> but it is not necessarily pointer-sized.
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