rustc complains about unused code if item is used in `#[automatically_derived]` context

[aumetra]

Code

I tried this code:

use std::fmt::{Debug, Display};

struct Identity<T>(T);

impl<T> Display for Identity<T>
where
    T: Display,
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

struct Thing {}

#[automatically_derived]
impl Debug for Thing {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", Identity("Thing"))
    }
}

fn main() {
    let hello = Thing {};
    println!("{hello:?}");
}

I expected to see this happen: The code compiles without any warnings since, the Identity struct is clearly used.

Instead, this happened: The compiler emits a dead_code lint, telling me that the struct is never constructed

Example where this can occur: when using the derive_more crate to derive a specialized Debug implementation, and the usage of derive_more::Debug utilizes a formatter struct to avoid allocating into an intermediate heap allocated buffer

Real-world example

Version it worked on

It most recently worked on: Rust 1.77.2

Version with regression

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[aumetra]

Okay, I submitted it under the wrong category. Apologies for that. Clearly a diagnostic issue.

[aumetra]

@rustbot modify labels: -regression-from-stable-to-stable +A-diagnostics

[aumetra]

@rustbot modify labels: -I-prioritize +T-compiler

[apiraino]

Actually I think you were right, seems to be a diagnostic regression introduced in c69fda7 (so my bisection reports):

cc @mu001999 since author of #121752 and @pnkfelix (reviewer of said patch)

@rustbot label +regression-from-stable-to-stable +P-medium -needs-triage