1.64 - Unexpected `dead_code` warning from inside macro expansion.

[Themayu]

I am getting an unexpected dead_code warning from inside a procedural macro's output. This warning did not appear in the previous version of the Rust Compiler. This is not present in the changelog for 1.64, so I do not believe it to be an expected change.

Code

I tried this code:

#[pin_project(project = Merge2Proj)]
#[derive(Debug)]
pub struct Merge2
{
	// struct contents...
}

// impl that only makes use of Merge2Proj::project...

I expected to see this happen: I expected the code to compile without warnings, as was true for the previous compiler version.

Instead, this happened: I get a dead_code warning from inside the pin_project macro expansion, telling me that Merge2Proj::project_ref is unused. This warning is impossible for me to remove, as I have no need for project_ref and there is no way to tell pin_project to not generate it.

Version it worked on

It most recently worked on: 1.63

Version with regression

rustc --version --verbose:

rustc 1.64.0 (a55dd71d5 2022-09-19)
binary: rustc
commit-hash: a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52
commit-date: 2022-09-19
host: x86_64-pc-windows-msvc
release: 1.64.0
LLVM version: 14.0.6

[inquisitivecrystal]

Thanks for the report!

This should be fixed by pin_project version 1.0.11. I'm having a lot of trouble reproducing the warning, even on older versions of pin_project; I think I'd need a complete example of the code that triggers the warning to have a chance of doing so.

[Themayu]

This warning came from pin_project version 1.0.8; I wasn't aware that there were updates available that prevented it. With that in mind, it should be reproducible by the following std::future::Future custom implementation:

# Cargo.toml
[package]
name = "warn_test"
description = ""
authors = ["Jamie Ridding"]
license = "MIT OR Apache-2.0"
version = "0.0.0"

[dependencies]
pin-project = "=1.0.8"

// main.rs
use pin_project::pin_project;
use std::future::Future;
use std::task::{Context, Poll};
use std::pin::Pin;

#[pin_project(project = PinProjectTestProj)]
#[derive(Debug)]
pub struct PinProjectTest<Fut>
where
	Fut: Future<Output = ()>,
{
	#[pin]
	fut: Fut,
}

impl<Fut> PinProjectTest<Fut>
where
	Fut: Future<Output = ()>,
{
	pub fn new(fut: Fut) -> Self {
		PinProjectTest {
			fut,
		}
	}
}

impl<Fut> Future for PinProjectTest<Fut>
where
	Fut: Future<Output = ()>,
{
	type Output = Fut::Output;

	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
		let PinProjectTestProj { fut } = self.project();

		fut.poll(cx)
	}
}

#[allow(unused_must_use)]
fn main() {
	let fut = async {};
	let test = PinProjectTest::new(fut);

	async {
		test.await;
	};
}

cargo check:

PS C:\Users\jamie\test> cargo clean
PS C:\Users\jamie\test> cargo check      
   Compiling proc-macro2 v1.0.43
   Compiling quote v1.0.21
   Compiling unicode-ident v1.0.4
   Compiling syn v1.0.100
   Compiling pin-project-internal v1.0.8
    Checking pin-project v1.0.8
    Checking warn_test v0.1.0 (C:\Users\jamie\test)
warning: associated function `project_ref` is never used
 --> src\main.rs:7:1
  |
7 | #[pin_project(project = PinProjectTestProj)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `warn_test` (bin "warn_test") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 6.06s

PS C:\Users\jamie\test>

cargo +1.63 check:

PS C:\Users\jamie\test> cargo clean
PS C:\Users\jamie\test> cargo +1.63 check
   Compiling proc-macro2 v1.0.43
   Compiling unicode-ident v1.0.4
   Compiling quote v1.0.21
   Compiling syn v1.0.100
   Compiling pin-project-internal v1.0.8
    Checking pin-project v1.0.8
    Checking warn_test v0.1.0 (C:\Users\jamie\test)
    Finished dev [unoptimized + debuginfo] target(s) in 7.54s

PS C:\Users\jamie\test>

(Sorry for the huge amount of edits, I am terrible at remembering to include information. And writing, apparently.)

[inquisitivecrystal]

Thanks! This bisects to #98402. CC @cjgillot.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium -E-needs-mcve