Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0128: generic parameter defaults cannot reference parameters before they are declared
What it meansA type parameter with default value is using forward declared identifier.
What rustc prints
error[E0128]: generic parameter defaults cannot reference parameters before they are declared
--> main.rs:1:16
|
1 | struct Foo<T = U, U = ()> {
| ^ cannot reference `U` before it is declaredReal 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 with default value is using forward declared identifier.
Erroneous code example:
struct Foo<T = U, U = ()> {
field1: T,
field2: U,
}
// error: generic parameters with a default cannot use forward declared
// identifiersType parameter defaults can only use parameters that occur before them. Since type parameters are evaluated in-order, this issue could be fixed by doing:
struct Foo<U = (), T = U> {
field1: T,
field2: U,
}Please also verify that this wasn't because of a name-clash and rename the type parameter if so.
From rustc --explain E0128 (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
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- E0120the `Drop` trait may only be implemented for local structs, enums, and unions
- E0121the placeholder `_` is not allowed within types on item signatures for return types
- E0131The `main` function was defined with generic parameters
- E0158constant pattern cannot depend on generic parameters
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?