Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0393: the type parameter `T` must be explicitly specified
What it meansA type parameter which references Self in its default value was not specified.
What rustc prints
error[E0393]: the type parameter `T` must be explicitly specified --> main.rs:3:47 | 1 | trait A<T = Self> {} | ----------------- type parameter `T` must be specified for this 2 | 3 | fn together_we_will_rule_the_galaxy(son: &dyn A) {} | ^ | = note: because the parameter default references `Self`, the parameter must be specified on the trait object type help: explicitly specify the type parameter | 3 | fn together_we_will_rule_the_galaxy(son: &dyn A</* T */>) {} | +++++++++
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
A type parameter which references Self in its default value was not specified.
Erroneous code example:
trait A<T = Self> {}
fn together_we_will_rule_the_galaxy(son: &dyn A) {}
// error: the type parameter `T` must be explicitly specifiedA trait object is defined over a single, fully-defined trait. With a regular default parameter, this parameter can just be instantiated 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 instantiated 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:
trait A<T = Self> {}
fn together_we_will_rule_the_galaxy(son: &dyn A<i32>) {} // Ok!From rustc --explain E0393 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
Related errors
- E0308mismatched types
- E0277the trait bound is not satisfied
- E0599no method named X found in the current scope
- E0433failed to resolve
- E0390cannot define inherent `impl` for primitive types
- E0391cycle detected when computing the super clauses of `FirstTrait`
- E0401can't use generic parameters from outer item
- E0403the name `T` is already used for a generic parameter in this item's generic parameters
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?