Matching: more debug

[gasche]

This is an update on the now-merged #12532, adding smaller, more specialized changes to the pattern-matching debug output that were useful to track #7241.

The notable change is in the first commit. Quoting the commit message:

Before this commit, a record pattern that accepts all values for all
its fields will be pretty-printed (in exhaustivity counter-examples
and in the debug output of the pattern-matching compiler0 as
a wildcard pattern _. This is correct for users, but it gives
inaccurate information in the debug output, as the two patterns are
distinct.

Instead I propose to show { _ } in this case, to mean a record whose
field values do not matter. This happens to not be valid OCaml syntax,
but it is perfectly understandable (it is the 0-ary version of the
{ <fields>; _ } syntax) and thus unlikely to confuse users.

The same code is used to print patterns in debug output, and to show exhaustiveness counter-examples to users. For debug output it is really necessary to differentiate _ from say { contents = _ } (which reads a mutable field) to understand what is going on. (I am not sure that such patterns are ever shown to users, as intuitively all-wildcard record patterns should not occur in exhaustiveness counter-examples.)

[trefis]

FTR: I wondered if the first commit might affect merlin in anyway (as the destruct feature relies on parmatch); after checking, I can confirm that it does not.

[Octachron]

I agree that this PR improves the debug log of the pattern matching compiler.
The updated record counter-example seems fine too.

[gasche]

Thanks!