unused_must_use lint after `write!` fails to note its origin

[pnkfelix]

Consider the following code (play):

// Uncomment the `cfg` to see expected output noting `#[warn(unused_must_use)]`
// #[cfg(without_write)]
pub fn encode_json<T: std::fmt::Display>(val: &T, wr: &mut Vec<u8>) {
    use std::io::Write;
    write!(wr, "{}", val);
}

pub fn after_write_macro() {
    Result::Ok::<_, ()>(());
}

fn main() {
}

It generates the following warning:

warning: unused `std::result::Result` which must be used
 --> src/main.rs:9:5
  |
9 |     Result::Ok::<_, ()>(());
  |     ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this `Result` may be an `Err` variant, which should be handled

which is almost, but not quite, what I expect to see.

Namely, I expect to see this:

warning: unused `std::result::Result` which must be used
 --> src/main.rs:9:5
  |
9 |     Result::Ok::<_, ()>(());
  |     ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: this `Result` may be an `Err` variant, which should be handled

which includes the useful note: #[warn(unused_must_use)] on by default

As far as I can tell, the choice whether or not to include that note is based on the presence/absence of the earlier write! invocation.

(Hypothesis: perhaps there is an occurrence of an unused must_use within that macros expansion that we are silencing, but even though it is silenced, it is erroneously counted in whatever mechanism we are using to avoid printing out multiple redundant instances of notes like note: #[warn(unused_must_use)] on by default?)

[zackmdavis]

whatever mechanism we are using to avoid printing out multiple redundant instances of notes

(The deduplication is done by checking a one_time_diagnostics field on the Session that gets populated by appropriate methods.)

[Enselic]

Triage: Still reproduces despite various fixes related to this (see e.g. most recently linked issue).