Add first-class support for binary crates

[boringcactus]

fixes #1630

Essentially, "if nothing was tagged #[wasm_bindgen(start)] and there is an exported fn main(i32, i32) -> i32 then process it as though it were tagged that way."

open questions before i'm confident this implementation is ready:

• should it refuse to autodiscover a main that is already tagged for wasm_bindgen in some other way?
• can the shim_idx field on the function descriptor be safely set to 0?
• can the arg names be justifiably set to argc and argv?
• should there be an automated test for this behavior somewhere? if so, how and where should that test be written?

[alexcrichton]

Thanks for this! I agree that pattern matching and using inference here is the way to go. I think though what we need to do here is to perhaps inject a new function to be start which calls main with zero'd out arguments?

Currently this I believe will export a binding for main but we want to be sure to hook it up to all the start infrastructure to make sure it's started automatically

[boringcactus]

considering that #[wasm_bindgen(start)] emits a scary warning if applied to a function with arguments, hacking the planet to get the same effect attached to a function with two arguments is probably not ideal.

if i manually write #[wasm_bindgen(start)] pub fn start() { main(); } in Rust and then ask walrus what its body is, i get

function Id { idx: 22 } (Some("start"))
  GlobalGet(GlobalGet { global: Id { idx: 0 } })
  LocalSet(LocalSet { local: Id { idx: 182 } })
  Const(Const { value: I32(16) })
  LocalSet(LocalSet { local: Id { idx: 183 } })
  LocalGet(LocalGet { local: Id { idx: 182 } })
  LocalGet(LocalGet { local: Id { idx: 183 } })
  Binop(Binop { op: I32Sub })
  LocalSet(LocalSet { local: Id { idx: 184 } })
  LocalGet(LocalGet { local: Id { idx: 184 } })
  GlobalSet(GlobalSet { global: Id { idx: 0 } })
  Call(Id { idx: 21 } (Some("bin_crate_without_a_bundler::start::h2a2091f59a0e039a")))
  Call(Id { idx: 888 } (Some("<T as wasm_bindgen::convert::traits::ReturnWasmAbi>::return_abi::h4fdec286c4868b5e")))
  Const(Const { value: I32(16) })
  LocalSet(LocalSet { local: Id { idx: 185 } })
  LocalGet(LocalGet { local: Id { idx: 184 } })
  LocalGet(LocalGet { local: Id { idx: 185 } })
  Binop(Binop { op: I32Add })
  LocalSet(LocalSet { local: Id { idx: 186 } })
  LocalGet(LocalGet { local: Id { idx: 186 } })
  GlobalSet(GlobalSet { global: Id { idx: 0 } })
  Return(Return)
function Id { idx: 21 } (Some("bin_crate_without_a_bundler::start::h2a2091f59a0e039a"))
  Call(Id { idx: 20 } (Some("bin_crate_without_a_bundler::main::ha40db6ddc3c404a3")))
  Return(Return)

so hopefully replicating that verbatim is not actually necessary (especially bc a few pieces would be difficult or impossible to reliably replicate). zeroing out the arguments and dropping the return value is, unsurprisingly, far easier.

[alexcrichton]

I think you'll want to avoid calling self.program here and synthesize a new function to place in the start function with walrus, and I think that'd help cut down on the verbosity here as well?

Additionally can you be sure to add some tests for this change?

[boringcactus]

I'm already synthesizing a replacement with walrus, but per your suggestion I've skipped program and gone straight to export since that's the only part of program that actually runs anyway.

i wasn't quite sure where to write tests, but

wasm-bindgen/crates/cli/tests/wasm-bindgen/main.rs

Lines 8 to 9 in 2e03961

	//! Assertions about errors in `wasm-bindgen` or assertions about the output of
	//! `wasm-bindgen` should all be placed in this test suite, however. Currently

pointed me in the right direction.

[alexcrichton]

Oh right sorry good point! I think though that none of export or program should be needed here, it should be possible to just directly modify the state of the local context to reflect that the main function was found? Either that or this could share infrastructure with the detection of #[wasm_bindgen(start)]?

And yeah that tests directory should be good to house tests!

[alexcrichton]

Could this use a match with an early return to de-indent the rest of this body?

[boringcactus]

it could indeed!

[boringcactus]

export is the detection of #[wasm_bindgen(start)], but it turns out only one line from export is actually needed for the new test to pass, so just using that one line cleans up a lot of other stuff.

[alexcrichton]

Perfect, thanks!