perf: rewrite UnusedVariables lint (#3186)

This is a rewrite of the `UnusedVariables` lint to inline and simplify
many of the dependent functions to try to improve the performance of
this lint, which quite often shows up in perf reports.

* The mvar assignment scanning is one of the most expensive parts of the
process, so we do two things to improve this:
  * Lazily perform the scan only if we need it
* Use an object-pointer hashmap to ensure that we don't have quadratic
behavior when there are many mvar assignments with slight differences.
* The dependency on `Lean.Server` is removed, meaning we don't need to
do the LSP conversion stuff anymore. The main logic of reference finding
is inlined.
* We take `fvarAliases` into account, and union together fvars which are
aliases of a base fvar. (It would be great if we had `UnionFind` here.)

More docs will be added once we confirm an actual perf improvement.

---------

Co-authored-by: Sebastian Ullrich <[email protected]>

src/Init/ByCases.lean

@@ -21,9 +21,9 @@ macro_rules
 
 /-! ## if-then-else -/
 
-@[simp] theorem if_true {h : Decidable True} (t e : α) : ite True t e = t := if_pos trivial
+@[simp] theorem if_true {_ : Decidable True} (t e : α) : ite True t e = t := if_pos trivial
 
-@[simp] theorem if_false {h : Decidable False} (t e : α) : ite False t e = e := if_neg id
+@[simp] theorem if_false {_ : Decidable False} (t e : α) : ite False t e = e := if_neg id
 
 theorem ite_id [Decidable c] {α} (t : α) : (if c then t else t) = t := by split <;> rfl
 

src/Init/Control/ExceptCps.lean

@@ -18,6 +18,7 @@ namespace ExceptCpsT
 def run {ε α : Type u} [Monad m] (x : ExceptCpsT ε m α) : m (Except ε α) :=
   x _ (fun a => pure (Except.ok a)) (fun e => pure (Except.error e))
 
+set_option linter.unusedVariables false in  -- `s` unused
 @[always_inline, inline]
 def runK {ε α : Type u} (x : ExceptCpsT ε m α) (s : ε) (ok : α → m β) (error : ε → m β) : m β :=
   x _ ok error

src/Init/Data/Ord.lean

@@ -119,6 +119,7 @@ class Ord (α : Type u) where
 
 export Ord (compare)
 
+set_option linter.unusedVariables false in  -- allow specifying `ord` explicitly
 /--
 Compare `x` and `y` by comparing `f x` and `f y`.
 -/
@@ -215,7 +216,7 @@ protected def opposite (ord : Ord α) : Ord α where
 /--
 `ord.on f` compares `x` and `y` by comparing `f x` and `f y` according to `ord`.
 -/
-protected def on (ord : Ord β) (f : α → β) : Ord α where
+protected def on (_ : Ord β) (f : α → β) : Ord α where
   compare := compareOn f
 
 /--

src/Lean/Compiler/LCNF/JoinPoints.lean

@@ -346,7 +346,7 @@ We call this whenever we enter a new local function. It clears both the
 current join point and the list of candidates since we can't lift join
 points outside of functions as explained in `mergeJpContextIfNecessary`.
 -/
-def withNewFunScope (decl : FunDecl) (x : ExtendM α): ExtendM α := do
+def withNewFunScope (x : ExtendM α): ExtendM α := do
   withReader (fun ctx => { ctx with currentJp? := none, candidates := {} }) do
     withNewScope do
       x
@@ -412,7 +412,7 @@ where
       withNewCandidate decl.fvarId do
         return Code.updateFun! code decl (← go k)
     | .fun decl k =>
-      let decl ← withNewFunScope decl do
+      let decl ← withNewFunScope do
         decl.updateValue (← go decl.value)
       withNewCandidate decl.fvarId do
         return Code.updateFun! code decl (← go k)

src/Lean/Elab/Term.lean

@@ -665,7 +665,7 @@ def mkTypeMismatchError (header? : Option String) (e : Expr) (eType : Expr) (exp
   return m!"{header}{← mkHasTypeButIsExpectedMsg eType expectedType}"
 
 def throwTypeMismatchError (header? : Option String) (expectedType : Expr) (eType : Expr) (e : Expr)
-    (f? : Option Expr := none) (extraMsg? : Option MessageData := none) : TermElabM α := do
+    (f? : Option Expr := none) (_extraMsg? : Option MessageData := none) : TermElabM α := do
   /-
     We ignore `extraMsg?` for now. In all our tests, it contained no useful information. It was
     always of the form: