tsc: Display source line and mark position where error occurred

[darrylring]

It would be super helpful to see the source line with the position of the error marked in the output from tsc. I've done a very quick, naive implementation of this (darrylring/TypeScript@0dfcc3ad7d664f622fcd5f139759bd6801d50b5d) and it seems to work nicely:

$ node built/local/tsc.js tests/cases/compiler/errorOnContextuallyTypedReturnType.ts 
tests/cases/compiler/errorOnContextuallyTypedReturnType.ts(1,5): error TS2322: Type '() => void' is not assignable to type '() => boolean'.
  Type 'void' is not assignable to type 'boolean'.
    >>> var n1: () => boolean = function () { }; // expect an error here
            ^~
tests/cases/compiler/errorOnContextuallyTypedReturnType.ts(2,37): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value or consist of a single 'throw' statement.
    >>> var n2: () => boolean = function ():boolean { }; // expect an error here
                                            ^~~~~~~

[danquirk]

I believe @DanielRosenwasser was looking at this kind of thing.

[darrylring]

Would a pull request be welcome on this?

[mhegazy]

@DanielRosenwasser can you share your work with @darrylring?

[adidahiya]

+1 better compiler UX

[RyanCavanaugh]

👍 @DanielRosenwasser can you link your PR?

Feedback from meeting was to colorize instead of squiggle, and cap multi-line error spans at one line (not sure if this means one line leading and trailing, or just one total)

[DanielRosenwasser]

#5140

Give it a try with --pretty when tonight's nightly build goes out.