Adds the ability to show a diff when `guard_msgs` fails, using the
histogram diff algorithm pioneered in jgit. This algorithm tends to
produce more user-friendly diffs, but it can be quadratic in the worst
case. Empirically, the quadratic case of this implementation doesn't
seem to be slow enough to matter for messages smaller than hundreds of
megabytes, but if it's ever a problem, we can mitigate it the same way
jgit does by falling back to Myers diff.
See lean/run/guard_msgs.lean in the tests directory for some examples of
its output.
This is a quite substantial tactic.
It also includes the infamour `NatCast` typeclass (which I've equipped
with a module-doc). I wasn't at all sure where that should live, so it
is currently randomly in `Lean/Elan/Tactic/NatCast.lean`: presumably if
we're doing this it will go somewhere in `Init`.
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Co-authored-by: Leonardo de Moura <leomoura@amazon.com>
This upstreams NatCast and IntCast alone independent of norm_cast in
#3322.
This will allow more efficiently upstreaming parts of Std.Data.Int
relevant for omega.
---------
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
For many data structures having an ordering is necessary. This one adds the `Ord` type class and a deriving handler for it. The ordering is based on order of constructors followed by lexicographical ordering within a constructor.
