warning that uninhabited struct is never constructed

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

Run cargo +beta clippy -p derive-deftly-macros --all-features in the reported rust-derive-deftly checkout and compare the behavior with Rust 1.88.0. Read macros/meta.rs around the MetaUnsupported(Void) definition, then trace the compiler's dead-code diagnostic entry point to determine why the warning changed. Done means the uninhabited struct no longer produces this warning without regressing ordinary dead-code diagnostics.