rust - How is from_raw_parts_mut able to transmute between types of different sizes? -


i looking @ code of from_raw_parts_mut:

pub unsafe fn from_raw_parts_mut<'a, t>(p: *mut t, len: usize) -> &'a mut [t] {     mem::transmute(repr { data: p, len: len }) }

it uses transmute reinterpret repr &mut [t]. far understand, repr 128 bit struct. how transmute of differently sized types work?

mem::transmute() work when transmuting type of same size - means &mut[t] slice same size.

looking @ repr:

#[repr(c)] struct repr<t> {     pub data: *const t,     pub len: usize, }

it has pointer data , length. slice - pointer array of items (which might actual array, or owned vec<t>, etc.) length how many items valid.

the object passed around slice (under covers) repr looks like, though data refers can 0 many t fit memory.

in rust, references not implemented pointer in other languages. types "fat pointers". might not obvious @ first since, if familiar references/pointers in other languages! examples are:

  • slices &[t] , &mut [t], described above, pointer , length. length needed bounds checks. example, can pass slice corresponding part of array or vec function.
  • trait objects &trait or box<trait>, trait trait rather concrete type, pointer concrete type , pointer vtable — information needed call trait methods on object, given concrete type not known.

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