nightly `unused_assignments` false positives for `miette::Diagnostic` fields

[adr1anh]

Starting with rustc 1.92.0-nightly (2300c2a, 2025-10-12), the compiler’s unused_assignments lint now emits warnings in the following snippet which uses miette and thiserror

Code

I tried this code:

#![warn(unused)]

use miette::{Diagnostic, NamedSource, SourceSpan};

#[derive(Debug, thiserror::Error, Diagnostic)]
enum MyError {
    #[error("malformed input")]
    Example {
        #[source_code]
        source_code: NamedSource<String>, // WARNING: value assigned to `source_code` is never read
        #[label]
        span: SourceSpan, // WARNING: value assigned to `span` is never read
    },
}

fn main() {
    let _error = MyError::Example {
        source_code: NamedSource::new("example", "fn main() {}".to_owned()),
        span: SourceSpan::from((0, 5)),
    };
}

I expected to see this happen: The code compiles without any error

Instead, this happened:

warning: value assigned to `source_code` is never read
  --> src/main.rs:10:9
   |
10 |         source_code: NamedSource<String>, // WARNING: value assigned to `source_code` is never read
   |         ^^^^^^^^^^^
   |
   = help: maybe it is overwritten before being read?
note: the lint level is defined here
  --> src/main.rs:1:9
   |
 1 | #![warn(unused)]
   |         ^^^^^^
   = note: `#[warn(unused_assignments)]` implied by `#[warn(unused)]`

warning: value assigned to `span` is never read
  --> src/main.rs:12:9
   |
12 |         span: SourceSpan, // WARNING: value assigned to `span` is never read
   |         ^^^^
   |
   = help: maybe it is overwritten before being read?

warning: `clippy-error` (bin "clippy-error") generated 2 warnings

Version it worked on

It most recently worked on:

rustc 1.92.0-nightly (be0ade2b6 2025-10-11)
binary: rustc
commit-hash: be0ade2b602bdfe37a3cc259fcc79e8624dcba94
commit-date: 2025-10-11
host: aarch64-apple-darwin
release: 1.92.0-nightly
LLVM version: 21.1.2

Version with regression

rustc --version --verbose:

rustc 1.92.0-nightly (2300c2aef 2025-10-12)
binary: rustc
commit-hash: 2300c2aef7dbc2a7bbbeaa9894d07d459abd9bc6
commit-date: 2025-10-12
host: aarch64-apple-darwin
release: 1.92.0-nightly
LLVM version: 21.1.2

Backtrace

Backtrace

<backtrace>

[apiraino]

bisected to commit ff6dc92 cc @cjgillot @davidtwco

Are these new warnings intended in #142390 (as I suspect they are)?

[cjgillot]

The assignments in the derived code are indeed unused. But we should not emit lints inside a proc-macro code, so this is a bug.

[kpreid]

I just encountered a similar issue with displaydoc. I don't know about what miette does, but in displaydoc's case, it is producing the wrong diagnostic, unused_assignments rather than unused_variables. Specifically, given this test program:

#[derive(displaydoc::Display)]
pub enum Error {
    /// Foo
    Foo { x: String },
}

there is an unused_assignments warning, but if I copy-paste the expansion into the program unchanged:

// #[derive(displaydoc::Display)]
pub enum Error {
    /// Foo
    Foo { x: String },
}

#[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
const _: () = {
    impl ::core::fmt::Display for Error {
        fn fmt(&self, formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
            #[allow(unused_variables)]
            match self {
                Self::Foo { x } => {
                    write!(formatter, "Foo")
                }
            }
        }
    }
};

then there is no warning (except for a suppressed unused_variables). Thus, something odd is going on here.

[sgued]

This also shows up with zeroize: RustCrypto/utils#1241