Changing profile.*.lto in Cargo.toml shouldn't rebuild everything

[japaric]

Neither should adding, modifying or removing linker flags via -C link-arg or -C link-args in .cargo/config (build.rustflags or target.*.rustflags).

These only affect the compilation of the top crate.

[alexcrichton]

I'd prefer to keep Cargo from parsing rustflags and trying to learn about specific flags, but Cargo already has intrusive knowledge of lto to fixing that seems reasonable to me!

[bluss]

codegen-units is the same kind of thing, isn't it. Nothing needs to be rebuilt.

In rustbuild, I would like it to not rebuild anything when -Ccodegen-units=x is added in rustflags. (Maybe rustbuild has some specific option for this).

[jonas-schievink]

We just ran into this one again. It seems that adding lto = true to the dev profile means that switching between cargo build and cargo build --tests will now rebuild the entire crate graph every time, which is pretty unfortunate (and was also fairly hard to track down to this issue).

I think this should be considered a bug instead of a feature request.

[ehuss]

@jonas-schievink That sounds like a different issue. This issue is about changing the LTO setting. What you're mentioning sounds like a rebuild-detection issue when switching between commands. Can you file a new issue with a reproduction? Also, beware, there has been a recent change which might affect the behavior that will appear in the next nightly.

[jonas-schievink]

@ehuss Okay, filed #8669