Compiler fails to mark main function as dead code in integration test

[PoignardAzur]

Code

fn foobar() {
    panic!();
}

fn main() {
    foobar();
}

Current output

Finished `test` profile [unoptimized + debuginfo] target(s) in 0.15s

Desired output

warning: function `foobar` is never used
 --> tests/foo.rs:1:4
  |
1 | fn foobar() {
  |    ^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `main` is never used
 --> tests/foo.rs:5:4
  |
5 | fn main() {
  |    ^^^^^

warning: `test-rust-test` (test "foo") generated 2 warnings

Rationale and extra context

In an integration tests (such as, by default, the files in a workspace's tests/ folder), the main function is usually replaced by the standard test harness.

When that happens, the main function should be marked as dead code, or more specifically, it shouldn't be included in the roots during liveness analysis.

(This is obviously different in cases where e.g. harness = false)

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

[bjorn3]

If rustc were to not mark main as root for --test, then unit testing executables would also cause dead code warnings about main not being called. Rustc can't distinguish between unit and integration tests. Cargo does an identical rustc invocation between the two.

[PoignardAzur]

Cargo does an identical rustc invocation between the two.

How hard would it be to have Cargo pass a different flag in the test case? Maybe a #![warn] or #![diagnostic] annotation? (I don't think if Cargo can do that yet, but you get my point.)

[fmease]

The test harness actually marks their generated main with the internal attribute #[rustc_main], so unless I'm missing something, dead code analysis should indeed be able to distinguish between a user-written main and a synthetic one.

[fmease]

Ah, I guess it would lead to user-written main being flagged in normal bin crates under --test, I see now.