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A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lintsC-bugCategory: This is a bug.Category: This is a bug.P-lowLow priorityLow priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.
Description
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
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
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Labels
A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lintsC-bugCategory: This is a bug.Category: This is a bug.P-lowLow priorityLow priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.