Add a --target option target different environment (nodejs, browser)

[djfarly]

Implements #75

I had some time in the train – I hope it's okay.
Some lines of documentation and a few line breaks for readability included.

Thoughts?

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed and have your
  cloned directory set to nightly

$ rustup override set nightly
$ rustup component add rustfmt-preview --toolchain nightly

• You ran rustfmt on the code base before submitting
• You reference which issue is being closed in the PR text

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[djfarly]

i'll look at rustfmt in a minute

[mgattozzi]

This LGTM, I think we just need you to format it with the latest rustfmt :)

[ashleygwilliams]

hi! so thanks for starting on this. one of the reasons i had assigned this to myself tho is that i have a very specific design for it. wasm-bindgen has several non-compatible flags that affect the js output (https://github.com/rustwasm/wasm-bindgen#cli-reference). i dont personally care for this design because it allows you to pass several flags even tho they do not work together at all. i would like the design to be something like --module or --output that takes a single argument, one of the flags listed in the link.

[djfarly]

Mea culpa — sorry for the steal.
I'm fine to drop this if you prefer to tackle it yourself, no hard feelings. 🙃

I see where you're going and like the idea.

Some bikeshedding:
I think --module is confusing as a name. --output describes it quite well. Personally I'd go for --target in the style of webpack.

What would be the valid options currently? I find the bindgen docs a bit confusing.

I guess currently only those?

• --target nodejs
• --target browser

/edit rollup uses format which also seems fine to me ;)

[djfarly]

Does the update meet your expectations a bit more closely?

As per TeXitoi/structopt#104, TeXitoi/structopt#110 I don't see a simple way to allow a list of possible values (an enum?).

[djfarly]

I'm not quite sure why travis fails - I tried updating rustfmt…

The log says:

0.10s$ cargo fmt -- --write-mode diff
Unrecognized option: 'write-mode'

🤔

[ashleygwilliams]

@djfarly - no worries on "stealing", i meant it more as an apology for filing a very vague feature issue 😅

--target sounds great to me, it's both analogous to rust and webpack, which is perfect IMO.

FYI the travis failure was a breaking change in rustfmt that we(lol, actually YOU! https://github.com/ashleygwilliams/wasm-pack/pull/128) fixed.

as for what should be supported- you can see the flags here: https://github.com/rustwasm/wasm-bindgen#cli-reference. i agree that the docs are a bit unwiedly to navigate- which is partially why wrapping wasm-bindgen up into wasm-pack will hopefully make it an easier user experience!

i'm gonna take a slightly closer look at this- but i think we should probably be able to merge this today, or at least in the next few days! thank you so much for your efforts on this!!

[ashleygwilliams]

slurped this up, rebased it, added docs in https://github.com/ashleygwilliams/wasm-pack/pull/132.