Type parameters in functions and methods are universal. This means that for all trait implementers, Trait::method<T>
must be implemented for any T
with the exact same constraints as those indicated by the trait (in this case, the constraint on T
is only the implicit Sized
).
The compiler’s error message that you indicated suggests that it was still expecting the parameter type T
. Instead, your Struct
implementation is assuming that T = u8
, which is incorrect. The type parameter is decided by the caller of the method rather than the implementer, so T
might not always be u8
.
If you wish to let the implementer choose a specific type, that has to be materialized in an associated type instead.
trait Trait {
type Output;
fn method(&self) -> Self::Output;
}
struct Struct;
impl Trait for Struct {
type Output = u8;
fn method(&self) -> u8 {
16
}
}
Read also this section of The Rust Programming Language: Specifying placeholder types in trait definitions with associated types.
See also: