"Struct is never constructed" false positive (dead_code)

[ijackson]

Code

git clone https://salsa.debian.org/dgit-team/tag2upload-service-manager.git -b 2025-10-26-unused-false-positive
# commitid a52ede5c5ed44708c53e509ba26cfa8cd1c5eaee
cd tag2upload-service-manager/
env -u http_proxy cargo +stable test --locked --all-features --all-targets --workspace -- t_comprehensive
# Note, there will be a *great deal* of output; you'll maybe want:
env -u http_proxy cargo +stable test --locked --all-features --all-targets --workspace -- t_comprehensive >log 2>&1

Current output

warning: struct `Earlier` is never constructed
   --> src/db_workflow.rs:463:12
    |
463 |     struct Earlier {
    |            ^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default
 
[ much other output ]

thread 'test::t_comprehensive::comprehensive' panicked at src/db_workflow.rs:526:9:
Earlier *was* constructed Earlier { jid: JobId(1), status: Uploaded }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[ much other output ]

Desired output

It shouldn't print the compiler warning.

Rationale and extra context

This struct very definitely is constructed, as demonstrated by the deliberate crash.

Other cases

Rust Version

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8

Anything else?

1.88.0 correctly omits this warning. I haven't bisected it.

The bug is still present in today's beta and nightly

(bookworm)rustcargo@zealot:/volatile/rustcargo/d/tag2upload-service-manager$ rustc +beta -vV
rustc 1.91.0-beta.10 (f2f881bb9 2025-10-25)
binary: rustc
commit-hash: f2f881bb99cf03bca0c54b4a0b1209b40b8cc383
commit-date: 2025-10-25
host: x86_64-unknown-linux-gnu
release: 1.91.0-beta.10
LLVM version: 21.1.2
(bookworm)rustcargo@zealot:/volatile/rustcargo/d/tag2upload-service-manager$ rustc +nightly -vV
rustc 1.93.0-nightly (34f954f9b 2025-10-25)
binary: rustc
commit-hash: 34f954f9b7cbdb5e9b408bac1c4ff1e88b5f2719
commit-date: 2025-10-25
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.3
(bookworm)rustcargo@zealot:/volatile/rustcargo/d/tag2upload-service-manager$ 

[JonathanBrouwer]

MCVE:

#[derive(Default)]
struct Test {

}

fn main() {
    if let Some::<Test>(test) = magic() {

    }
}


fn magic<T: Default>() -> Option<T> {
    Some(T::default())
}