Derive Macro derive_more::AsRef
source · #[derive(AsRef)]
{
// Attributes available to this derive:
#[as_ref]
}
as_ref
only.Expand description
What #[derive(AsRef)]
generates
Deriving AsRef
generates one or more implementations of AsRef
, each
corresponding to one of the fields of the decorated type.
This allows types which contain some T
to be passed anywhere that an
AsRef<T>
is accepted.
Newtypes and Structs with One Field
When AsRef
is derived for a newtype or struct with one field, a single
implementation is generated to expose the underlying field.
#[derive(AsRef)]
struct MyWrapper(String);
Generates:
impl AsRef<String> for MyWrapper {
fn as_ref(&self) -> &String {
&self.0
}
}
It’s also possible to use the #[as_ref(forward)]
attribute to forward
to the as_ref
implementation of the field. So here SingleFieldForward
implements all AsRef
for all types that Vec<i32>
implements AsRef
for.
#[derive(AsRef)]
#[as_ref(forward)]
struct SingleFieldForward(Vec<i32>);
let item = SingleFieldForward(vec![]);
let _: &[i32] = (&item).as_ref();
This generates:
impl<__AsRefT: ?::core::marker::Sized> ::core::convert::AsRef<__AsRefT> for SingleFieldForward
where
Vec<i32>: ::core::convert::AsRef<__AsRefT>,
{
#[inline]
fn as_ref(&self) -> &__AsRefT {
<Vec<i32> as ::core::convert::AsRef<__AsRefT>>::as_ref(&self.0)
}
}
Structs with Multiple Fields
When AsRef
is derived for a struct with more than one field (including tuple
structs), you must also mark one or more fields with the #[as_ref]
attribute.
An implementation will be generated for each indicated field.
You can also exclude a specific field by using #[as_ref(ignore)]
.
#[derive(AsRef)]
struct MyWrapper {
#[as_ref]
name: String,
#[as_ref]
num: i32,
valid: bool,
}
Generates:
impl AsRef<String> for MyWrapper {
fn as_ref(&self) -> &String {
&self.name
}
}
impl AsRef<i32> for MyWrapper {
fn as_ref(&self) -> &i32 {
&self.num
}
}
Note that AsRef<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_ref]
will result in a compilation error.
// Error! Conflicting implementations of AsRef<String>
#[derive(AsRef)]
struct MyWrapper {
#[as_ref]
str1: String,
#[as_ref]
str2: String,
}
Enums
Deriving AsRef
for enums is not supported.