Error code E0393
A type parameter which references Self
in its default value was not specified.
Erroneous code example:
#![allow(unused)] fn main() { trait A<T=Self> {} fn together_we_will_rule_the_galaxy(son: &A) {} // error: the type parameter `T` must be explicitly specified in an // object type because its default value `Self` references the // type `Self` }
A trait object is defined over a single, fully-defined trait. With a regular
default parameter, this parameter can just be substituted in. However, if the
default parameter is Self
, the trait changes for each concrete type; i.e.
i32
will be expected to implement A<i32>
, bool
will be expected to
implement A<bool>
, etc... These types will not share an implementation of a
fully-defined trait; instead they share implementations of a trait with
different parameters substituted in for each implementation. This is
irreconcilable with what we need to make a trait object work, and is thus
disallowed. Making the trait concrete by explicitly specifying the value of the
defaulted parameter will fix this issue. Fixed example:
#![allow(unused)] fn main() { trait A<T=Self> {} fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok! }