Support rich compiler error messages

[ferd]

This is rather straightforward as an implementation: given any error message that contains column information, read the source file and find that line. Display it, and indent a cursor underneath it that points to the column.

Then we can just show the same error message as before.

In practice, given the module:

-module(fake_mod).

-export([diagnostic/1]).

diagnostic(A) ->
    X = add(5 / 0),
    {X,X}.

add(X) -> X.

add(X, Y) -> X + Y.

Calling rebar3 compile yields:

...
===> Compiling apps/rebar/src/fake_mod.erl failed
   ┌─ apps/rebar/src/fake_mod.erl:
   |
 5 |  diagnostic(A) ->
   |             ^-- variable 'A' is unused

   ┌─ apps/rebar/src/fake_mod.erl:
   |
 6 |      X = add(5 / 0),
   |                ^-- evaluation of operator '/'/2 will fail with a 'badarith' exception

    ┌─ apps/rebar/src/fake_mod.erl:
    |
 11 |  add(X, Y) -> X + Y.
    |  ^-- function add/2 is unused

and in a terminal supporting color output:

This is friendlier than what we had before for sure. Adding color could be an extension to this if we find the format useful.

By default, this format is turned off, but can be turned on optionally by configuring values with

{compiler_error_format, rich}.

Compilers that want to support it must switch their calls from rebar_compiler:error_tuple(Source, Errors, Warnings, Opts) to rebar_compiler:error_tuple(Source, Errors, Warnings, Config, Opts), which allows the configuration to work and go for non-default values.

The intent of this PR is to put the initial scaffolding in place to gradually get nicer and richer error formats for users, without necessarily harming the ability of tools to still work with them (via config switches).

[tsloughter]

Nice, much more straightforward than I expected.

[ferd]

Do you think it's worth driving this forward as-is or we'd like to have fancier output?

I also imagine we'll need a switch for machine parsing to reuse the old context-free messages

[ferd]

Just pushed an extra commit that attempts a format closer to the original issue and also colorizes the output

[eproxus]

Has there been any consideration using more fancy box drawing or unicode characters?

Rust uses carets for the whole error column range:

2 |     vec![(), ()].iter().sum::<i32>();
  |     ^^^^^^^^^^^^^^^^^^^ --- required by a bound introduced by this call

I think that looks slightly better and it is easier to see what exactly is marked as a problem.

[eproxus]

Here's an example with box drawing characters (looks even better in a terminal):

    ┌╴ apps/rebar/src/fake_mod.erl:
    │
 25 │      A = foo,
    │      ╰── variable 'A' is unused

[ferd]

I could use box drawing chars easily, but I wouldn't use the carets for the whole expression, only because I have no width information about the error, just the column it starts on.

The Color highlight is done with a sort of messy regex trick, and it's unlikely to carry well to other compilers we support. If the Color is a bit wrong but the arrow is good, it sort of limits the scope of error.

I'll update the PR to use the fancier characters.

[ferd]

the arrow won't go bold but that's okay