Pretty-print values in the REPL

[9999years]

Motivation

Pretty-print values in the REPL by printing each item in a list or attrset on a separate line. When possible, single-item lists and attrsets are printed on one line, as long as they don't contain a nested list, attrset, or thunk.

Before:

{ attrs = { a = { b = { c = { }; }; }; }; list = [ 1 ]; list' = [ 1 2 3 ]; }

After:

{
  attrs = {
    a = {
      b = {
        c = { };
      };
    };
  };
  list = [ 1 ];
  list' = [
    1
    2
    3
  ];
}

Here's how the output looks (with #9928 and #9926 applied as well):

Context

@Qyriad's prototype pretty-printer in Xil gave me the inspiration to do this. It turns out the patch is pretty small, so here it is!

In the future, users should be able to customize how much is printed for large values. See #9942.

Priorities and Process

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[9999years]

Needs tests & changelog entry.

[thufschmitt]

That's great!

Left a few comments, and I'll leave a bit of time for others to chime in, but a bit 👍 overall

[fricklerhandwerk]

A conceptual question on the side: couldn't we instead allow plugging a formatter as developed by @piegames and @infinisil? Otherwise we'd be duplicating effort.

[thufschmitt]

A conceptual question on the side: couldn't we instead allow plugging a formatter as developed by @piegames and @infinisil? Otherwise we'd be duplicating effort.

I briefly touched on that with @infinisil, but doing so would make the whole infra significantly more complex, and this formatting is only for values (and not arbitrary expressions), which makes it orders of magnitude simpler (the formatting change is around 60 extras loc)

[fricklerhandwerk]

This could have a release note though.

[9999years]

@fricklerhandwerk Good point, I've added a release note in #9943.