CODEWITHSUNDEEP
genericsrust
Paul C
Paul C

Reputation: 8457

How to tell the Rust compiler that type annotations are not actually needed?

struct MyStruct<T: MyTrait> {
    inner: T,
}

impl<T: MyTrait> MyStruct<T> {
    const MYCONST: u32 = 42;

    pub fn x() {
        println!("x!");
    }
}

pub trait MyTrait {}

Accessing either MYCONST or x() requires a trait. If none are provided, the compiler complains:

error[E0282]: type annotations needed
  --> src/main.rs:42:5
   |
42 |     MyStruct::x();
   |     ^^^^^^^^^^^ cannot infer type for `T`

or

error[E0282]: type annotations needed
  --> src/main.rs:42:20
   |
42 |     println!("{}", MyStruct::MYCONST);
   |                    ^^^^^^^^^^^^^^^^^ cannot infer type for `T`

Is there a way to tell the compiler to ignore the need for type annotations here?

Upvotes: 0

Views: 660

Answers (2)

Jmb
Jmb

Reputation: 23264

In answer to "Why is the type annotation needed?" consider the following code:

struct MyStruct<T> {
    inner: T,
}

impl MyStruct<i32> {
    const MYCONST: u32 = 42;

    pub fn x() {
        println!("i32!");
    }
}

impl MyStruct<i64> {
    const MYCONST: u32 = 64;

    pub fn x() {
        println!("i64!");
    }
}


fn main() {
    MyStruct::x();
}

Should it print i32 or i64?

Upvotes: 3

Shepmaster
Shepmaster

Reputation: 430554

The type annotations are needed — there is no type called MyStruct. This has nothing to do with traits, either:

struct MyStruct<T> {
    inner: T,
}

impl<T> MyStruct<T> {
    const MYCONST: u32 = 42;

    pub fn x() {
        println!("x!");
    }
}

fn main() {
    MyStruct::x();
}

error[E0282]: type annotations needed
  --> src/main.rs:14:6
   |
14 |      MyStruct::x();
   |      ^^^^^^^^^^^ cannot infer type for type parameter `T`

A type definition with a generic isn't actually a type, it's a type constructor. You must fill in the generics to get a type in order to access associated items.

Upvotes: 3

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