Generate a $start function when compiling a binary.

[lachlansneff]

Currently, rust doesn't not generate a $start function when compiling a binary to wasm. This should be added at some point.

[mgattozzi]

This is part of what's being discussed in #16

[alexcrichton]

This was also the subject of https://github.com/rust-lang-nursery/rust-wasm/issues/18 which has a lot of detailed discussion about start and such. I believe the overall conclusion is that we'd want an opt-in to using the start symbol, and bundlers may adopt a convention of auto-running a main symbol at an appropriate time.

[lachlansneff]

Okay, an opt-in solution sounds good. I'm not sure how libstd would support this, though, since it seems to have a single entry point for all architectures in rt.rs.

[alexcrichton]

@lachlansneff oh the main symbol itself is manufactured in the compiler so I don't think it'll be too too difficult here. We should probably go ahead and change the main symbol to not have any arguments on wasm.

[lachlansneff]

Is there any progress on this? Nebulet is very close to being ready for application and driver development and it waiting on this.

[alexcrichton]

Currently, no, but a champion to design and implement it would be most welcome!

[lachlansneff]

Sure, I'd be happy to! I'll need some guidance though; I haven't delved into the rustc internals before.

[lachlansneff]

Also, I'm not so sure about opt-in. Seems to me that having a fn main() in your program counts as opt-in to me.

[lachlansneff]

This was discussed in the last meeting and it was decided that rustc should generate a start function.