What #[derive(AsMut)] generates

Deriving AsMut generates one or more implementations of AsMut, each corresponding to one of the fields of the decorated type. This allows types which contain some T to be passed anywhere that an AsMut<T> is accepted.

1 Newtypes and Structs with One Field

When AsMut is derived for a newtype or struct with one field, a single implementation is generated to expose the underlying field.

#[derive(AsMut)]
struct MyWrapper(String);

Generates:

impl AsMut<String> for MyWrapper {
    fn as_mut(&mut self) -> &mut String {
        &mut self.0
    }
}

It's also possible to use the #[as_mut(forward)] attribute to forward to the as_mut implementation of the field. So here SigleFieldForward implements all AsMut for all types that Vec<i32> implements AsMut for.

#[derive(AsMut)]
#[as_mut(forward)]
struct SingleFieldForward(Vec<i32>);

fn main() {
    let mut item = SingleFieldForward(vec![]);
    let _: &mut [i32] = (&mut item).as_mut();
}

This generates:

impl<__AsMutT: ?::core::marker::Sized> ::core::convert::AsMut<__AsMutT> for SingleFieldForward
where
    Vec<i32>: ::core::convert::AsMut<__AsMutT>,
{
    #[inline]
    fn as_mut(&mut self) -> &mut __AsMutT {
        <Vec<i32> as ::core::convert::AsMut<__AsMutT>>::as_mut(&mut self.0)
    }
}

2 Structs with Multiple Fields

When AsMut is derived for a struct with more than one field (including tuple structs), you must also mark one or more fields with the #[as_mut] attribute. An implementation will be generated for each indicated field. You can also exclude a specific field by using #[as_mut(ignore)].

#[derive(AsMut)]
struct MyWrapper {
    #[as_mut]
    name: String,
    #[as_mut]
    num: i32,
    valid: bool,
}

Generates:

impl AsMut<String> for MyWrapper {
    fn as_mut(&mut self) -> &mut String {
        &mut self.name
    }
}

impl AsMut<i32> for MyWrapper {
    fn as_mut(&mut self) -> &mut i32 {
        &mut self.num
    }
}

Note that AsMut<T> may only be implemented once for any given type T. This means any attempt to mark more than one field of the same type with #[as_mut] will result in a compilation error.

This example deliberately fails to compile
// Error! Conflicting implementations of AsMut<String>
#[derive(AsMut)]
struct MyWrapper {
    #[as_mut]
    str1: String,
    #[as_mut]
    str2: String,
}

3 Enums

Deriving AsMut for enums is not supported.