[rustc.git] / src / doc / unstable-book / src / language-features / intrinsics.md

# `intrinsics`

The tracking issue for this feature is: None.

Intrinsics are never intended to be stable directly, but intrinsics are often
exported in some sort of stable manner. Prefer using the stable interfaces to
the intrinsic directly when you can.

------------------------


These are imported as if they were FFI functions, with the special
`rust-intrinsic` ABI. For example, if one was in a freestanding
context, but wished to be able to `transmute` between types, and
perform efficient pointer arithmetic, one would import those functions
via a declaration like

```rust
#![feature(intrinsics)]
# fn main() {}

extern "rust-intrinsic" {
    fn transmute<T, U>(x: T) -> U;

    fn offset<T>(dst: *const T, offset: isize) -> *const T;
}
```

As with any other FFI functions, these are always `unsafe` to call.
Commit	Line	Data
8bb4bdeb XL	1	# `intrinsics`
	2
	3	The tracking issue for this feature is: None.
	4
	5	Intrinsics are never intended to be stable directly, but intrinsics are often
	6	exported in some sort of stable manner. Prefer using the stable interfaces to
	7	the intrinsic directly when you can.
	8
	9	------------------------
c34b1796	10
c34b1796 AL	11
	12	These are imported as if they were FFI functions, with the special
	13	`rust-intrinsic` ABI. For example, if one was in a freestanding
	14	context, but wished to be able to `transmute` between types, and
	15	perform efficient pointer arithmetic, one would import those functions
	16	via a declaration like
	17
62682a34	18	```rust
c1a9b12d	19	#![feature(intrinsics)]
c34b1796 AL	20	# fn main() {}
	21
	22	extern "rust-intrinsic" {
	23	fn transmute<T, U>(x: T) -> U;
	24
	25	fn offset<T>(dst: const T, offset: isize) -> const T;
	26	}
	27	```
	28
	29	As with any other FFI functions, these are always `unsafe` to call.