arrays-and-slices.md

Layout of Rust array types and slices

Layout of Rust array types

Array types, [T; N], store N values of type T with a stride that is equal to the size of T. Here, stride is the distance between each pair of consecutive values within the array.

The offset of the first array element is 0, that is, a pointer to the array and a pointer to its first element both point to the same memory address.

The alignment of array types is greater or equal to the alignment of its element type. If the element type is repr(C) the layout of the array is guaranteed to be the same as the layout of a C array with the same element type.

Note: the type of array arguments in C function signatures, e.g., void foo(T x[N]), decays to a pointer. That is, these functions do not take arrays as an arguments, they take a pointer to the first element of the array instead. Array types are therefore improper C types (not C FFI safe) in Rust foreign function declarations, e.g., extern { fn foo(x: [T; N]) -> [U; M]; }. Pointers to arrays are fine: extern { fn foo(x: *const [T; N]) -> *const [U; M]; }, and structs and unions containing arrays are also fine.

Arrays of zero-size

Arrays [T; N] have zero size if and only if their count N is zero or their element type T is zero-sized.

Layout compatibility with packed SIMD vectors

The layout of packed SIMD vector types ¹ requires the size and alignment of the vector elements to match. That is, types with packed SIMD vector layout are layout compatible with arrays having the same element type and the same number of elements as the vector.

Layout of Rust slices

The layout of a slice [T] of length N is the same as that of a [T; N] array.

Footnotes

1. The packed SIMD vector layout is the layout of repr(simd) types like __m128. ↩