librustc should not depend on librustc_llvm

[whitequark]

I'm adding something to librustc_llvm and changing it rebuilds a lot of rustc libraries. @eddyb said this should not be the case.

[eddyb]

Indeed, only librustc_trans actually needs LLVM, in fact I'd really love to combine the two, since LLVM is only used for the one backend, even if it happens to be our only official one.

[whitequark]

Wouldn't that degrade compile times on rebuild?

[eddyb]

Does librustc_llvm actually take a long time to build? Or is linking LLVM super slow?

EDIT For the latter we should use the -sys convention and no higher-level APIs.

[whitequark]

I think it's nontrivial, and statically linking LLVM (I dynamically link, personally) can take a lot of time with the default set of targets (I recall linking it for straight minutes).

[eddyb]

@whitequark Yeah we can still separate that out I think, but move anything that's not imports into what is currently librustc_trans - if we can have the crate empty, even better.

[arielb1]

Currently, librustc_metadata depends on LLVM for its archive reader, so we'll first have to split that out (unless the archive reader does not require configuring LLVM).

[eddyb]

@arielb1 Yeah I think we want to split it so the backend provides file -> metadata blob functionality, and librustc_metadata handles both finding the files and decoding the blob.

[arielb1]

@eddyb

Yeah. That would also be the Right Thing for targets with "unconventional linking".

[hanna-kruppe]

This impacts me a lot, as essentially all my rustc work for the next few weeks or more consists of modifying LLVM, occasionally librustc_llvm, and rarely librustc_trans. So I'd volunteer to fix it. I could use some details on what this "file -> metadata blob" API should look like, though. Please elaborate here or ping me on IRC.

[nagisa]

@rkruppe maybe something like this?

// in metadata
trait MetadataStore {
    fn get_metadata(&self) -> Result<MetadataBlob, String>;
    ...
}

// in trans or somewhere else
impl MetadataStore for LlvmMetadataStore { ... }

// in driver
session.metadata_store_impl: Box<MetadataStore> = Box::new(LlvmMetadataStore);  // or something like that