CODEWITHSUNDEEP
rust
Alberto Centelles
Alberto Centelles

Reputation: 1253

Leave associated type unspecified in trait implementation

I want to leave an associated type specification in a trait implementation for later, as it requires some work. Is there any way I can tell the compiler to ignore the missing associated type?

impl A for B {
    type C = todo()!;
}

It throws this error

error: non-type macro in type position: ::core::panic

Upvotes: 0

Views: 130

Answers (1)

Xavier Denis
Xavier Denis

Reputation: 426

Rust macros like todo!() aren't magic, they must expand to valid Rust code. In the case of todo() it expands to panic!("not yet implemented: ....") (see src).

If you use a macro in a type, then it must expand to a valid type. Unfortunately, that means you can't have it expand to the equivalent of todo!(): it wouldn't really make sense!

However, instead, you can use () as a placeholder, or even define a custom type alias: type Todo = ().

Upvotes: 0

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