When the write! macro's result is chained stability warnings show up, and the expansion site is not reported

[renato-zannon]

Example:

fn main() {
 let mut buffer = Vec::new();
 writeln!(&mut buffer, "foo").unwrap();
}

This errors as:

<std macros>:2:20: 2:66 warning: use of unstable library feature 'io'
<std macros>:2 ( & mut * $ dst ) . write_fmt ( format_args ! ( $ ( $ arg ) * ) ) )
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<std macros>:2:20: 2:66 help: add #![feature(io)] to the crate attributes to silence this warning
<std macros>:2 ( & mut * $ dst ) . write_fmt ( format_args ! ( $ ( $ arg ) * ) ) )
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Contrast that with the error when the .unwrap() is removed:

<std macros>:2:1: 2:66 warning: unused result which must be used, #[warn(unused_must_use)] on by default
<std macros>:2 ( & mut * $ dst ) . write_fmt ( format_args ! ( $ ( $ arg ) * ) ) )
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<std macros>:1:1: 2:68 note: in expansion of write!
<std macros>:2:1: 2:46 note: expansion site
<std macros>:1:1: 4:66 note: in expansion of writeln!
src/main.rs:3:5: 3:34 note: expansion site

This can be pretty hairy to find if the project has many crates (my project euler repo, which is where I observed this, has 60+ executables).

[rprichard]

Related to #23459

[gbersac]

Still a problem with the 1.2 rustc compiler.

[pmarcelll]

Triage: we have nice error messages now.

[petrochenkov]

Current diagnostics:

rustc 1.17.0-nightly (b1e31766d 2017-03-03)
error: no method named `write_fmt` found for type `&mut std::vec::Vec<_>` in the current scope
 --> <anon>:3:5
 |
3 | writeln!(&mut buffer, "foo").unwrap();
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 |
 = help: items from traits can only be used if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
 = help: candidate #1: `use std::io::Write;`
 = note: this error originates in a macro outside of the current crate