LTO, alloc and debug compilation problem

[kaczmarczyck]

If you combine LTO, anything that uses alloc and debug compilation mode, the compiler fails.
The minimum example to reproduce is this src/lib.rs:

extern crate alloc;

#[cfg(test)]
mod tests {
    #[test]
    fn something_alloc() {
        assert_eq!(Vec::<u32>::new(), Vec::<u32>::new());
    }
}

With this Cargo.toml:

[package]
name = "lto_bug"
version = "0.0.1"
edition = "2018"

[profile.dev]
lto = true

[profile.release]
lto = true

To reproduce, run (or just use +nightly):

cargo +nightly-2023-03-26 test

You should see this error:

error: Cannot represent a difference across sections

The above error does not reproduce for other compiler versions or options, all of the below work:

cargo +nightly test --release
cargo +nightly-2023-03-25 test
cargo +stable test

I found #109730 reporting something similar, but for them --release is throwing the error, so I opened a new issue instead.

[nikic]

Initial reduction for llc -filetype=obj: https://gist.github.com/nikic/6f60958ed4295f0bcd48797b5e541aba

[nikic]

Filed llvm/llvm-project#61932.

[cuviper]

In https://reviews.llvm.org/D147620, dblaikie suggests that rustc ought to separate the type-method declaration from the function definition, and I think I have that working. It passes the reproducer here, at least...

@rustbot claim