[fluffything]

`auto` is just a keyword, that's used in D and C++ to implement many many different language features.

Rust does not have

    fn foo() -> let { ... }

where

    fn foo() -> let { 0_i32 }
    let x: i32 = foo();
    fn bar() -> let { 0_f32 }
    let y: f32 = bar();

That is, you can't have an opaque function return type, that's both opaque, but simultaneously the user can name and use all interfaces from.

If you change the implementation of `bar` with such a feature to return `i32` instead, all calling code of `bar` would break. And that's precisely why Rust doesn't have D/C++'s `auto` in return position.