This implements the first half of #3302: It improves the extensible
`apply_rfl` tactic (the one that looks at `refl` attributes, part of
the `rfl` macro) to
* Check itself and ahead of time that the lhs and rhs are defEq, and
give
a nice consistent error message when they don't (instead of just passing
on
the less helpful error message from `apply Foo.refl`), and using the
machinery that `apply` uses to elaborate expressions to highlight diffs
in implicit arguments.
* Also handle `Eq` and `HEq` (built in) and `Iff` (using the attribute)
Care is taken that, as before, the current transparency setting affects
comparing the lhs and rhs, but not the reduction of the relation
So before we had
```lean
opaque P : Nat → Nat → Prop
@[refl] axiom P.refl (n : Nat) : P n n
/--
error: tactic 'apply' failed, failed to unify
P ?n ?n
with
P 42 23
⊢ P 42 23
-/
#guard_msgs in
example : P 42 23 := by apply_rfl
opaque withImplicitNat {n : Nat} : Nat
/--
error: tactic 'apply' failed, failed to unify
P ?n ?n
with
P withImplicitNat withImplicitNat
⊢ P withImplicitNat withImplicitNat
-/
#guard_msgs in
example : P (@withImplicitNat 42) (@withImplicitNat 23) := by apply_rfl
```
and with this PR the messages we get are
```
error: tactic 'apply_rfl' failed, The lhs
42
is not definitionally equal to rhs
23
⊢ P 42 23
```
resp.
```
error: tactic 'apply_rfl' failed, The lhs
@withImplicitNat 42
is not definitionally equal to rhs
@withImplicitNat 23
⊢ P withImplicitNat withImplicitNat
```
A test file checks the various failure modes and error messages.
I believe this `apply_rfl` can serve as the only implementation of
`rfl`, which would then complete #3302, and actually expose these
improved
error messages to the user. But as that seems to require a
non-trivial bootstrapping dance, it’ll be separate.
fc963ffceb