Why Rust don't use default generic parameter type

Question

I want to create generic structure with default type. But Rust compiler still requires me to specify explicit type when creating my structure.

struct A {}

struct C<T = A> {
    t: Option<T>
}

fn main() {
    let c = C { t: None };
}

Rust compiler shows this error:

error[E0282]: type annotations needed for `C<T>`
 --> src/main.rs:8:9
  |
8 | let c = C { t: None };
  |     -   ^ cannot infer type for `T`
  |     |
  |     consider giving `c` the explicit type `C<T>`, where the type parameter `T` is specified

How can I allow user of my code to omit generic parameter?

Answer 1

When you don't precise a type in a variable binding (left of the assignment), the compiler must infer it.

Here, the value isn't precise enough (None could be anything).

A solution is to declare a type in the binding. You don't have to give a type to T, if you write just C, the default type for T is applied:

let c:C = C { t: None };

It's debatable

• whether it's a compiler bug or not (I don't think it is but it can be argued that a human sees no ambiguity here)
• whether it should be fixed or not (I don't think it should as more complex cases could be ambiguous or hard to decipher when there are multiple inference locations)

Note that in c:C, there's no type inference at all: Omitting <_> or <SomeType> means the default type applies.