write! is somehow masking the #[must_use] on its result

[pnkfelix]

Hat tip to @shepmaster for finding this

play

fn main() {
    use std::fmt::Write;
    let mut example = String::new();
    write!(&mut example, "{}", 42);
}

is not triggering the #[must_use] lint on Result.

[shepmaster]

This code compiles with warnings in Rust 1.28.0, but no warnings in Rust 1.29.0

fn main() {
    use std::fmt::Write;
    let mut example = String::new();
    write!(&mut example, "{}", 42);
}

$ cargo +1.28.0 build
   Compiling xz v0.1.0 (file:///private/tmp/xz)
warning: unused `std::result::Result` which must be used
 --> src/main.rs:4:5
  |
4 |     write!(&mut example, "{}", 42);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: this `Result` may be an `Err` variant, which should be handled
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

    Finished dev [unoptimized + debuginfo] target(s) in 1.02s

$ cargo clean
$ cargo +1.29.0 build
   Compiling xz v0.1.0 (file:///private/tmp/xz)
    Finished dev [unoptimized + debuginfo] target(s) in 1.09s

Potentially related to #54288.

[shepmaster]

Expanding the macro and compiling does have the warning:

fn main() {
    use std::fmt::Write;
    let mut example = String::new();
    (&mut example).write_fmt(format_args!("{}", 42));
}

$ cargo clean
$ cargo +1.29.0 build
   Compiling xz v0.1.0 (file:///private/tmp/xz)
warning: unused `std::result::Result` which must be used
 --> src/main.rs:4:5
  |
4 |     (&mut example).write_fmt(format_args!("{}", 42));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: this `Result` may be an `Err` variant, which should be handled

    Finished dev [unoptimized + debuginfo] target(s) in 0.56s

[ehuss]

Seems related to #52467.

[shepmaster]

related to #52467.

/cc @alexcrichton

[alexcrichton]

Ah oops, definitely a mistake! I believe a fix for this would be to set report_in_external_macro (like in this macro) for the definition of the unused must use lint as well

[varkor]

Will #55240 also be fixed by this?