Structured errors

[nrc]

As discussed here

r? @nikomatsakis or anyone else on the @rust-lang/compiler team

[jonas-schievink]

Travis fails because there's a double where is the parser: https://github.com/nrc/rust/blob/5b7665010cb0a36e377c08d535db442d0a3ce203/src/libsyntax/parse/parser.rs#L752-L753

[nrc]

Whoops, forgot the new commit

[aturon]

cc @rust-lang/compiler

[nrc]

Addressed the doc comment thing.

The problem with the Option thing was that using the StructErr was painful - lots of weird maps and as_mut stuff. Using the dummy thing is much cleaner to use, although a little bit less clear what is going on.

[nrc]

@bors: r=nagisa

[bors]

📌 Commit 6fd8a9c has been approved by nagisa

[nrc]

@bors: r= nagisa

[bors]

📌 Commit 489aa40 has been approved by ``

[nrc]

@bors: r=nagisa

[bors]

📌 Commit 489aa40 has been approved by nagisa

[nrc]

@bors: r=nagisa

[bors]

📌 Commit 04d9729 has been approved by nagisa

[bors]

⌛ Testing commit 04d9729 with merge 176ee34...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt