This PR adds a linter (`linter.unusedSimpArgs`) that complains when a
simp argument (`simp [foo]`) is unused. It should do the right thing if
the `simp` invocation is run multiple times, e.g. inside `all_goals`. It
does not trigger when the `simp` call is inside a macro. The linter
message contains a clickable hint to remove the simp argument.
I chose to display a separate warning for each unused argument. This
means that the user has to click multiple times to remove all of them
(and wait for re-elaboration in between). But this just means multiple
endorphine kicks, and the main benefit over a single warning that would
have to span the whole argument list is that already the squigglies tell
the users about unused arguments.
This closes#4483.
Making Init and Std clean wrt to this linter revealed close to 1000
unused simp args, a pleasant experience for anyone enjoying tidying
things: #8905
after this change, `simp` will be able to discharge side-goals that,
after simplification, are of the form `∀ …, a = b` with `a =?= b`.
Usually these side-goals are solved by simplification using `eq_self`,
but that does not work when there are metavariables involved.
This enables us to have rewrite rules like
```
theorem List.foldl_subtype (p : α → Prop) (l : List (Subtype p)) (f : β → Subtype p → β)
(g : β → α → β) (b : β)
(hf : ∀ b x h, f b ⟨x, h⟩ = g b x) :
l.foldl f b = (l.map (·.val)).foldl g b := by
```
where the parameter `g` does not appear on the lhs, but can be solved
for using the `hf` equation. See `tests/lean/run/simpHigherOrder.lean`
for more examples.
The motivating use-case is that `simp` should be able to clean up the
usual
```
l.attach.map (fun <x, _> => x)
```
idiom often seen in well-founded recursive functions with nested
recursion.
Care needs to be taken with adding such rules to the default simp set if
the lhs is very general, and thus causes them to be tried everywhere.
Performance impact of just this PR (no additional simp rules) on mathlib
is unsuspicious:
http://speed.lean-fro.org/mathlib4/compare/b5bc44c7-e53c-4b6c-9184-bbfea54c4f80/to/ae1d769b-2ff2-4894-940c-042d5a698353
I tried a few alternatives, e.g. letting `simp` apply `eq_self` without
bumping the mvar depth, or just solve equalities directly, but that
broke too much things, and adding code to the default discharger seemed
simpler.
