#[deprecated] fn using format_args!() triggers "use of deprecated item" warning

[shahn]

This code:

#[deprecated]
fn foo() {
    format_args!("bla");
}

gives this warning:

warning: use of deprecated item, #[warn(deprecated)] on by default
 --> <anon>:3:18
3 |>     format_args!("bla");
  |>                  ^^^^^
<anon>:3:5: 3:25: note: in this expansion of format_args!

in current stable and nightly

[cengiz-io]

Hello.

What's the real issue here? Are you concerned about the extra error message?

Because when I try this in playground, I get two errors and one of them is correct behaviour.

warning: use of deprecated item, #[warn(deprecated)] on by default
 --> <anon>:3:18
3 |>     format_args!("bla");
  |>                  ^^^^^
<anon>:3:5: 3:25: note: in this expansion of format_args!

warning: use of deprecated item, #[warn(deprecated)] on by default
 --> <anon>:7:5
7 |>     foo();
  |>     ^^^

If it's so, I think you should update the title to "Deprecation warning getting incorrectly recursed into builtin macro calls" or something like that.

Cheers!

[TimNN]

@cengizio: I believe the current title is pretty accurate:

• only the format_args! macro is affected (as far as I can tell, stringify!, concat! and try! don't work for example)
• the error is triggered, even if foo is never used

See: https://is.gd/ua1Lyz

[cengiz-io]

@TimNN Thanks for clarifying that for me.

I traced source to librustc_lint/builtin.rs#L577 but can't seem to find something special for macros.

[TimNN]

Alright, I did some more tests and basically the following happens:

• #[deprecated] is applied recursively, so a #[deprecated] mod foo {} will deprecate anything inside that module as well.
• format_args! (probably) expands (among other things) to something like const BLA: &'static str = "bla"; ...; do_somthing(BLA);
• since foo is deprecated, BLA is deprecated, so using BLA in do_something(BLA) triggers a deprecation warning

Example:

#[deprecated]
fn _foo() {
    static FOO: &'static str = "abc";

    let _ = FOO; //~ WARN use of deprecated item
}

While this specific case is indeed very surprising, the more general case ("deprecated use inside of deprecated item") is already tracked at #16490.

[cengiz-io]

@TimNN Nice catch! Here's the full expanded version

#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std as std;
#[deprecated]
fn _foo() {
    ::std::fmt::Arguments::new_v1({
                                      static __STATIC_FMTSTR: &'static [&'static str] = &["bla"];
                                      __STATIC_FMTSTR
                                  },
                                  &match () {
                                      () => [],
                                  });
}

fn main() {}