Expand description
Why have a slice when you can have a loaf?
What this is
Sometimes you know that a slice must have at least one element in it,
but Rust forces you to do “last minute decision” by unwrap()ing Option
from for example first() for split_first() methods.
Loaf guarantees to have at least one element by its definition.
How it works
First, lets consider a simple slice
let x: &[u8] = &[10, 42, 0, 7, 91];&[u8] underneath is really just a pair of a pointer to buffer and its length
(a fat pointer)
[ ptr: *const u8 | len: usize = 5 ]
| |
| v
| | <- [u8] -> |
| +----+----+----+----+----+
+--->| 10 | 42 | 00 | 07 | 91 |
+----+----+----+----+----+Thats because size of [u8] can be known only at runtime.
Rust also allows to define a structure that has exactly one dynamically-sized type at the end of it.
struct LoafT<u8, 2> {
loaf: [u8; 2],
rest: [u8],
}let x: &[u8] = &[10, 42, 0, 7, 91];
let loaf: &LoafN<u8, 2> = abracadabra!(slice);In this case the len also contains the length of [u8]
[ ptr: *const ?? | len: usize = 3 ]
| |
| v
| | [u8; 2] | <- [u8] -> |
| +----+----+----+----+----+
+--->| 10 | 42 | 00 | 07 | 91 |
+----+----+----+----+----+ptr doesn’t have here a clear type, because *const LoafN<u8, 2> is
itself a fat pointer (because of the [u8] field).
The Hack
Rust does have a way to fiddle with fat pointer internals, but it
requires ptr_metadata feature, which are only avaliable on nightly.
The hack here is to create an *mut [T] as it was *mut Loaf<T> and then cast it.
See Loaf::from_slice source code for more details
Safety
As long as two arrays could be interpreted as one bigger and vice versa, everything should be alright