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# Single-Bit Volatile Write
This writes a bit into `dst` directly, using a volatile I/O intrinsic to
prevent compiler reördering or removal.
You should not use `bitvec` to perform any volatile I/O operations. You should
instead do volatile I/O work on integer values directly, or use a crate like
[`voladdress`][0] to perform I/O transactions, and use `bitvec` only on stack
locals that have no additional memory semantics.
## Original
[`ptr::write_volatile](core::ptr::write_volatile)
## Safety
Because this performs a dereference of memory, it inherits the original
`ptr::write_volatile`’s requirements:
- `dst` must be valid to write
- `dst` must be properly aligned. This is an invariant of the `BitPtr` type as
well as of the memory access.
Additionally, `dst` must point to an initialized value of `T`. Integers cannot
be initialized one bit at a time.
## Behavior
This is required to perform a read/modify/write cycle on the memory location.
LLVM *may or may not* emit a bit-write instruction on targets that have them in
the ISA, but this is not specified in any way.
## Examples
```rust
use bitvec::prelude::*;
use bitvec::ptr as bv_ptr;
let mut data = 0u8;
let ptr = BitPtr::<_, _, Lsb0>::from_mut(&mut data);
unsafe { bv_ptr::write_volatile(ptr.add(2), true); }
assert_eq!(data, 4);
```
[0]: https://docs.rs/voladdress/latest/voladdress
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