Expand description
Used to do a cheap mutable-to-mutable reference conversion.
This trait is similar to AsRef but used for converting between mutable
references. If you need to do a costly conversion it is better to
implement From with type &mut T or write a custom function.
Note: This trait must not fail. If the conversion can fail, use a
dedicated method which returns an Option<T> or a Result<T, E>.
Generic Implementations
AsMut auto-dereferences if the inner type is a mutable reference
(e.g.: foo.as_mut() will work the same if foo has type &mut Foo or &mut &mut Foo).
Note that due to historic reasons, the above currently does not hold generally for all
mutably dereferenceable types, e.g. foo.as_mut() will not work the same as
Box::new(foo).as_mut(). Instead, many smart pointers provide an as_mut implementation which
simply returns a reference to the pointed-to value (but do not perform a cheap
reference-to-reference conversion for that value). However, AsMut::as_mut should not be
used for the sole purpose of mutable dereferencing; instead ‘Deref coercion’ can be used:
let mut x = Box::new(5i32);
// Avoid this:
// let y: &mut i32 = x.as_mut();
// Better just write:
let y: &mut i32 = &mut x;Types which implement DerefMut should consider to add an implementation of AsMut<T> as
follows:
impl<T> AsMut<T> for SomeType
where
<SomeType as Deref>::Target: AsMut<T>,
{
fn as_mut(&mut self) -> &mut T {
self.deref_mut().as_mut()
}
}Reflexivity
Ideally, AsMut would be reflexive, i.e. there would be an impl<T: ?Sized> AsMut<T> for T
with as_mut simply returning its argument unchanged.
Such a blanket implementation is currently not provided due to technical restrictions of
Rust’s type system (it would be overlapping with another existing blanket implementation for
&mut T where T: AsMut<U> which allows AsMut to auto-dereference, see “Generic
Implementations” above).
A trivial implementation of AsMut<T> for T must be added explicitly for a particular type T
where needed or desired. Note, however, that not all types from std contain such an
implementation, and those cannot be added by external code due to orphan rules.
Examples
Using AsMut as trait bound for a generic function, we can accept all mutable references that
can be converted to type &mut T. Unlike dereference, which has a single target type,
there can be multiple implementations of AsMut for a type. In particular, Vec<T> implements
both AsMut<Vec<T>> and AsMut<[T]>.
In the following, the example functions caesar and null_terminate provide a generic
interface which work with any type that can be converted by cheap mutable-to-mutable conversion
into a byte slice ([u8]) or byte vector (Vec<u8>), respectively.
struct Document {
info: String,
content: Vec<u8>,
}
impl<T: ?Sized> AsMut<T> for Document
where
Vec<u8>: AsMut<T>,
{
fn as_mut(&mut self) -> &mut T {
self.content.as_mut()
}
}
fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
for byte in data.as_mut() {
*byte = byte.wrapping_add(key);
}
}
fn null_terminate<T: AsMut<Vec<u8>>>(data: &mut T) {
// Using a non-generic inner function, which contains most of the
// functionality, helps to minimize monomorphization overhead.
fn doit(data: &mut Vec<u8>) {
let len = data.len();
if len == 0 || data[len-1] != 0 {
data.push(0);
}
}
doit(data.as_mut());
}
fn main() {
let mut v: Vec<u8> = vec![1, 2, 3];
caesar(&mut v, 5);
assert_eq!(v, [6, 7, 8]);
null_terminate(&mut v);
assert_eq!(v, [6, 7, 8, 0]);
let mut doc = Document {
info: String::from("Example"),
content: vec![17, 19, 8],
};
caesar(&mut doc, 1);
assert_eq!(doc.content, [18, 20, 9]);
null_terminate(&mut doc);
assert_eq!(doc.content, [18, 20, 9, 0]);
}Note, however, that APIs don’t need to be generic. In many cases taking a &mut [u8] or
&mut Vec<u8>, for example, is the better choice (callers need to pass the correct type then).