Is isize always pointer sized?

[chrisd]

Are isize and usize guaranteed to be the same byte width as pointers? Or is there a possibility they can differ? E.g. when pointer widths are larger than than the addressable memory.

[Yandros]

Here is the current stance of the Unsafe Code Guidelines Reference:

@https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html#isize-and-usize

isize and usize

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 requires uintptr_t and intptr_t to be at least 16-bit wide. All platforms we currently support have a C platform, and as a consequence, isize / usize are at least 16-bit wide for all of them.

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.

Note : in the current Rust implementation, the layouts of isize and usize determine the following:

• the maximum size of Rust allocations is limited to isize::max_value() . The LLVM getelementptr instruction uses signed-integer field offsets. Rust calls getelementptr with the inbounds flag which assumes that field offsets do not overflow,
• the maximum number of elements in an array is usize::max_value() ( [T; N: usize] . Only ZST arrays can probably be this large in practice, non-ZST arrays are bound by the maximum size of Rust values,
• the maximum value in bytes by which a pointer can be offseted using ptr.add or ptr.offset is isize::max_value() .

These limits have not gone through the RFC process and are not guaranteed to hold.