[WIP] Use --message-format=json to fix #333

[konstin]

Parse the --message-format=json output with cargo_metadata to get the location of the wasm file from cargo.

This PR is marked WIP because it depends on oli-obk/cargo_metadata#53. 7484faf is because I otherwise had test failures about wrong versions (same as for @alexcrichton in #336 (comment)). With that commit it_can_run_browser_tests is still failing though. I guess this will be fixed before merging and I can rebase the commit away.

Adding wasm_file to Build is a bit of an ugly solution, but I couldn't come up with something better without rewriting the whole steps code. Currently I'm also ignoring the diagnostics messages and instead return the captured stderr in case of an error. (Not capturing stderr and setting -q would fix #288 and with the other messages we could easily do #287. We'd only need to make sure how this interacts with indicatif)

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed and have your
  cloned directory set to nightly (well, 2018-09-16)
• You ran rustfmt on the code base before submitting
• You reference which issue is being closed in the PR text

Closes #333
Fixes #305
Relevant for #339

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

[alexcrichton]

For this I believe it will need to pessimistically spawn a new thread to read stderr. Othrewise the blocking reads on stdout could cause wasm-pack to deadlock when it's waiting for info on stdout but the child is waiting to write info to stderr.

[konstin]

Afaik we're spawning a new process here and stderr is new pipe from the os, so the child can write to stderr no matter what the parent process does (except broken pipe errors and the like). Not an expert though.

[alexcrichton]

Pipes have a fixed capacity, however, and won't infinitely buffer. If Cargo dumps a lot of information on stderr it'll eventually block, and then it won't make progress until the pipe is read, which wasm-pack doesn't do until the process has otherwise finished.

[konstin]

Ah thanks, I didn't think about that! I've replaced the pipe with a tempfile.

[konstin]

I've rebased onto master. I've realized that rustc and therefore cargo can't do both human and json messages at the same time, so we need to get the human messages out of the json.

Because both PBAR and Logger errors aren't shown to the user, I simply eprintln'd the compiler messages. This also means the user is now shown the warning even if the compilation succeeds.

  [1/9] Checking `rustc` version...
  [2/9] Checking crate configuration...
  [3/9] Adding WASM target...
| [4/9] Compiling to WASM...
error[E0425]: cannot find function `fail` in this scope
 --> src/lib.rs:6:5
  |
6 |     fail();
  |     ^^^^ not found in this scope


warning: unused import: `std::ptr`
 --> src/lib.rs:1:5
  |
1 | use std::ptr;
  |     ^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

\ [4/9] Compiling to WASM...
warning: unused import: `std::mem::replace`
 --> src/lib.rs:2:5
  |
2 | use std::mem::replace;
  |     ^^^^^^^^^^^^^^^^^


error: aborting due to previous error


For more information about this error, try `rustc --explain E0425`.

Compilation of your program failed. stderr:

   Compiling a v0.1.0 (/home/konsti/wasm-pack/a)
error: Could not compile `a`.

To learn more, run the command again with --verbose.

[konstin]

This was made impossible by #392.