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

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

Reproduce the warning difference using the integration test example in tests/foo.rs and the reported rustc version. Trace how the standard test harness treats main during liveness analysis, then verify that warnings appear for both foobar and main while harness = false retains its normal behavior.