如何在Rust中定义关联的const或类型别名？

Question

Basically, I just want "shortcuts":

impl Foo {
    const c : i32 = 42;
    type V = i32;
    fn bar() -> V { c }
}

But the compiler complains with:

error: associated constants are experimental (see issue #29646)

Answer 1

As mentioned in the error message, associated constants ( const c: i32 = 42 ) are experimental.

This means that, unless you switch to a nightly compiler and enable the feature, you cannot have constants nested inside implementations or traits.

You can, however, have constants at module scope:

const C: i32 = 42;

If you fix this, you will get to the next error: associated types are only allowed in traits and their implementation, not in "inherent" implementations.

Once again, the work-around is to pull out the type alias at module scope:

type V = i32;

The following code compiles:

const C: i32 = 42;
type V = i32;

struct Foo;

impl Foo {
    fn bar() -> V { C }
}

and with C and V being private (only accessible within the module) they do not leak.