Highlight code in `rustc --explain` (WIP)

[estebank]

Internals thread.

New output:

[rust-highfive]

r? @pnkfelix

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[estebank]

Not ready for merging yet: It doesn't check wether the output can actually support color codes. Piping to other tools will cause control characters to be displayed:

$ rustc --explain E0038 > file.txt && more file.txt
(...)
```
ESC[33mtraitESC(BESC[m ESC(BESC[mFooESC(BESC[m ESC(BESC[mESC[33mwhereESC(BESC[m ESC(BESC[mESC[36mSelfESC(BESC[m:ESC(BESC[m ESC(BESC[mSizedESC(BESC[m ESC(BESC[m{ESC(BESC[m

ESC(BESC[m}ESC(BESC[m
ESC(BESC[m```
(...)

[bors]

☔️ The latest upstream changes (presumably #39463) made this pull request unmergeable. Please resolve the merge conflicts.

[pnkfelix]

@estebank any status update here? Should I close this PR?

[estebank]

@pnkfelix I've been offline for two weeks and haven't had a chance to clean this up for submission (it's lacking detection of pipe vs stdout output). Regardless, could you advice on the changes that were made already, in order to know wether they're in the right track?

[pnkfelix]

@estebank I'm not clear on whether we can just add new dependencies to rustc; the last time I tried to just throw in some external crates to the Cargo.toml, @alexcrichton told me that our vendoring requirements make it difficult to add new crates.

[estebank]

@pnkfelix this PR didn't introduce any external crates, it moved existing code into its own crate.

I'll try to get back to this PR this Saturday to clean it up for merging.

[alexcrichton]

Closing due to inactivity, but feel free to resubmit with a rebase!

(seems like a nifty addition!)