Need to be able to link against other `.wasm` files

[fitzgen]

There are places I'd like to write WAT directly, compile it into a .wasm in build.rs (or maybe just check in the compiled .wasm file into the repo since it is architecture independent), and then link that .wasm into my top level cdylib crate.

[pepyakin]

Sounds like job for wasm-link from wabt.

[fitzgen]

My understanding is that when rustc starts using lld as a linker, and lld finishes its .wasm support, this issue should go away.

However, using wasm-link for now works OK. Its usability is compromised by the lack of a post_build.rs hook in cargo, especially for crates that are intended to be used as a library and want to link some .wasm, rather than the root cdylib that creates the top .wasm file.

[mgattozzi]

Given the discussion over in cargo about post build scripts I doubt this will be the route to go. I think getting lld linker support in sooner would be the most bang for our buck as having linker support helps with other issues as well. wasm support has already been moved to upstream lld so I think this is only a matter of time till we can get it integrated into rustc.

[est31]

yeah sadly post build scripts have been rejected which would have made many things a bit easier :/

For this feature we should probably wait until we can integrate LLD into rustc or ship lld together with rustc or similar.

[alexcrichton]

@fitzgen I agree yeah that when we have lld this will probably just go away. At that point there will be a semi-standard format (I think) of "wasm object files", and I'd imagine at that point you'd just use archives and #[link] or -l flags as you do for other platforms today.

[alexcrichton]

With LLD I believe this is now possible. Currently the foreign files need to just be compiled with clang 6 and otherwise we should be good to go.

Now that being said I think there's still a long way to go here. I'm not sure if the "object file format" is well documented or if it's easy to write a "wat" file by hand. Furthermore crates like cc don't have support for compiling "wat" at the moment as well. In any case those concerns are probably best covered in follow-up issues, so closing.