`--emit obj` with `--crate-type staticlib` doesn't work when `-C lto=thin` is set

[ChrisDenton]

I tried this code:

// answer.rs
pub fn answer() -> u64 {
    42
}

And compiled it with:

rustc answer.rs --crate-type staticlib --emit obj -C lto=thin

I expected to see an object file created. Instead it appears to succeed but no object file is produced.

Meta

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: x86_64-pc-windows-msvc
release: 1.82.0
LLVM version: 19.1.1

[bjorn3]

--emit obj copies a single object file out of the codegen results to the output location, but thinLTO produces a single object file per input llvm bitcode file, so at least one file per crate and unless -Ccodegen-units is used, by default 16 object files per crate. --emit obj doesn't know which of these object files you want.

[ChrisDenton]

If it's simply not possible to produce an object file, I'd suggest the combination of flags should be an error. Or more generally it should at least warn if no object file was produced, if not error.