Dead code not linked in beta or nightly

[xd009642]

I've noticed with beta and nightly the rust flag link-dead-code no longer includes unused source files when running cargo test. Testing it out on this test project, I used objdump -WL to get the decoded lines in the binary. With rustc stable both source files in the crate are linked into the test binary (lib.rs and unused.rs), however with stable and nightly only lib.rs is linked.

This is counter to what I'd expect link-dead-code to do. Is this a desired feature? And if so is there the means to obtain the old behaviour?

[Mark-Simulacrum]

Probably a regression related to ThinLTO work... Cc @alexcrichton

[alexcrichton]

My guess is this is #45575 (cc @michaelwoerister) rather than ThinLTO b/c the mention of cargo test here I think indicates debug mode?

[michaelwoerister]

My guess is this is #45575 (cc @michaelwoerister) rather than ThinLTO

Yes, that could well be. We still would need to decide whether link-dead-code is supposed to also generate dead code (as opposed to what the name implies in a strict sense).

[nikomatsakis]

Discussed in @rust-lang/compiler meeting. Consensus was that -Clink-dead-code should make compiler generate dead code, as it used to, since there isn't much value in just having the linker preserve dead code (but not having compiler generate it). We considered changing the name but decided that was more trouble than it's worth, and it'd be better to just adjust the -Chelp message to clarify.

[nikomatsakis]

triage: P-high