Don't warn on use of a deprecated item in a deprecated item

[emberian]

For example:

#[deprecated]
fn foo() { }

#[deprecated]
fn bar() { foo() }

Should not be a warning.

[emberian]

#6875

[huonw]

(Similarly for experimental and unstable, etc.)

Should this also consider the case when bar is not marked deprecated but it is placed inside a module marked deprecated?

[emberian]

@huonw probably

[alexcrichton]

I like the idea of: if any parent of yours is unstable, you're allowed to make unstable calls. Equivalently for other stability indices.

[huonw]

Presumably it's the first parent with a stability annotation that's the stability we should be caring about? So foo can make experimental calls, but not deprecated ones in the following:

#[deprecated]
mod x {
  #[experimental]
  mod y {
    fn foo() {
      experimental(); // ok
      deprecated(); // "warning: use of deprecated item"
    }
  }
}

(The stability attributes checking could easily get very slow since it will have to traverse the tree upward and parse the attributes on each parent item for every single function/method call and uses of structs/enums etc etc; it's probably worth using some caches for this information.)

[alexcrichton]

OK, I'm ok with the closest-parent. I'd also be OK with not re-defining the stability index, or possibly not in some directions. deprecated => experimental doesn't really make sense to me...

In theory this can be done in two passes over the ast (with lots of space), one to collect the level of all items and one to process all of them, but that may be overkill.

[metajack]

Bug triage. This is very similar to the dead_code lint change that we made that considers code marked with #[allow(dead_code)] to be live. See #11471

[steveklabnik]

Triage: this system isn't really considered for external code anymore, so I wonder how important it is. Also, the original code sample does not warn for me today, which seems...odd.

[steveklabnik]

Updated code:

#![feature(staged_api)]
#![staged_api]

#[deprecated(since = "1.6.0", reason = "")]
#[stable(since = "1.6.0", feature="lol")]
fn foo() { }

#[deprecated(since = "1.6.0", reason = "")]
#[stable(since = "1.6.0", feature="lol")]
fn bar() { foo() }

fn main() {}

I agree with my own comment from ten months ago, and this hasn't had any non-triage activity for two years, so I'm just going to give a close.