warning that uninhabited struct is never constructed

[ijackson]

Steps

git clone https://gitlab.torproject.org/Diziet/rust-derive-deftly -b derive-deftly-1.2.0
cd rust-derive-deftly
cargo +beta clippy -p derive-deftly-macros --all-features

The code contains this, where Void is from void:

pub struct MetaUnsupported(Void);

Expected behaviour

It lints without complaint.

Actual behaviour

warning: struct `MetaUnsupported` is never constructed
   --> macros/meta.rs:746:12
    |
746 | pub struct MetaUnsupported(Void);
    |            ^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

Discussion

I don't think it ever makes sense to warn that an uninhabited type is never constructed. That is normal. Presumably the uninhabited type has some function in the code, or might become inhabited with a different configuration.

Version it worked on

It works on 1.88.0.

Version with regression

rustc --version --verbose:

rustc 1.90.0-beta.3 (630a5f1d3 2025-08-16)
binary: rustc
commit-hash: 630a5f1d3cda0e54d878f3f6bf9be435ee4c68a5
commit-date: 2025-08-16
host: x86_64-unknown-linux-gnu
release: 1.90.0-beta.3
LLVM version: 20.1.8

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

[workingjubilee]

...I suppose the existing warning is, at least, technically correct.

[workingjubilee]

Regression in 56835d7

It took me a while of struggling with cargo bisect-rustc before I realized: I do not think the perf builds include clippy, so we cannot automatically bisect "into" them, like we can with things that do not involve extended toolchain components.

This is fine, but means we're not sure where in the rollup the regression is.

[jieyouxu]

I wonder if it is #143519 but haven't had the time to do a precise bisection.

[jieyouxu]

Ah, it doesn't need clippy, because dead_code was emitted by rustc.

********************************************************************************
Regression in 56835d7ac14da9f966e1ff39fd9ffd2e29b764d1
********************************************************************************

Attempting to search unrolled perf builds
Found commits ["cc6f9a4f", "eca1577d", "c014e31a", "ad23b634", "cb87a244", "5af0d5ca", "5344c98b", "c2e70f0a", "02ccb8e7", "8d3a8594", "351d616f"]
installing cc6f9a4f1c437c02fcaff182aaab53efaa8d5c8b
rust-std-nightly-x86_64-unknown-linux-gnu: 31.17 MB / 31.17 MB [==============================================] 100.00 % 2.55 MB/s testing...
RESULT: cc6f9a4f1c437c02fcaff182aaab53efaa8d5c8b, ===> Successfully compiled
uninstalling cc6f9a4f1c437c02fcaff182aaab53efaa8d5c8b

installing eca1577d274fa023ef5db02d9c7a432325b11781
rust-std-nightly-x86_64-unknown-linux-gnu: 31.15 MB / 31.15 MB [==============================================] 100.00 % 2.58 MB/s testing...
RESULT: eca1577d274fa023ef5db02d9c7a432325b11781, ===> Compile error
uninstalling eca1577d274fa023ef5db02d9c7a432325b11781

Regression in https://github.com/rust-lang/rust/commit/eca1577d274fa023ef5db02d9c7a432325b11781. Note that if it is a legacy rollup build, it might be available in https://github.com/rust-lang-ci/rust/commit/eca1577d274fa023ef5db02d9c7a432325b11781.
The PR introducing the regression in this rollup is #143519: Check assoc consts and tys later like assoc fns