If you're writing a library, and you'd like to deprecate something, you can use the deprecated attribute:

#![allow(unused_variables)]
fn main() {
#[deprecated(
    since = "0.2.1",
    note = "Please use the bar function instead"
)]
pub fn foo() {
    // ...
}
}

This will give your users a warning if they use the deprecated functionality:

   Compiling playground v0.0.1 (file:///playground)
warning: use of deprecated item 'foo': Please use the bar function instead
  --> src/main.rs:10:5
   |
10 |     foo();
   |     ^^^
   |
   = note: #[warn(deprecated)] on by default

Both since and note are optional.

since can be in the future; you can put whatever you'd like, and what's put in there isn't checked.