Reintroduce the compile progress bar as an unstable feature (-Z compile-progress)

[kennytm]

This allows us to test the feature on-demand to see if there's any other bugs besides #5695.

Also, fixed the flickering #5697 (this was caused by build script emitting Stdout/Stderr events).

[alexcrichton]

@bors: r+

Sounds great to me!

[bors]

📌 Commit 9612be6 has been approved by alexcrichton

[bors]

⌛ Testing commit 9612be6 with merge a5617df...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing a5617df to master...

[konstin]

Is there an option to only show the progress bar? This would be really useful for tools which call cargo in a way similar to wasm-pack.

[kennytm]

Since the progress bar already expects nightly, I suggest an alternative:

1. Run cargo build -Zunstable-options --build-plan to get a list of jobs to be built.
2. Run cargo build --message-format json and construct your own tool's progress bar based on stdout (all the human readable progress will be printed to stderr).

(I assume the tool doesn't just run cargo and have its own progress bar implementation)

[konstin]

I already use --message-format json, so I guess I'll go with a custom progress bar then