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.
1 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 SigleFieldForward
implements all AsRef
for all types that Vec<i32>
implements AsRef
for.
#[derive(AsRef)] #[as_ref(forward)] struct SingleFieldForward(Vec<i32>); fn main() { 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) } }
2 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, }
3 Enums
Deriving AsRef
for enums is not supported.