Understanding Rust Error E0128
Rust Error E0128 Explanation
In Rust, error E0128 occurs when a type parameter with a default value is using a forward declared identifier. This means the default value of the type parameter refers to another type parameter that appears later in the list, which is not allowed.
Erroneous Code Example
```rust
#![allow(unused)]
fn main() {
struct Foo<T = U, U = ()> {
field1: T,
field2: U,
}
// error: generic parameters with a default cannot use forward declared
// identifiers
}
```In this example, the error occurs because the default value of the type parameter 'T' is set to 'U', which appears later in the list of type parameters. This is not allowed because type parameter defaults can only refer to parameters that occur before them.
Solution
To fix this issue, you can rearrange the type parameters to ensure that a default value always refers to a type parameter that comes before it in the list. Here's the corrected code:
```rust
#![allow(unused)]
fn main() {
struct Foo<U = (), T = U> {
field1: T,
field2: U,
}
}
```In this example, the type parameter 'U' is correctly positioned before the type parameter 'T' that uses it as the default value.
Name-Clash Consideration
When you encounter Rust Error E0128, make sure to check if the type parameter naming causes a name-clash. If so, renaming the type parameter may resolve the error.