chore: add seal performance workaround

Author: leodemoura

src/Lean/Meta/Tactic/Grind/Canon.lean

@@ -77,7 +77,15 @@ def canonElemCore (parent : Expr) (f : Expr) (i : Nat) (e : Expr) (useIsDefEqBou
   let eType ← inferType e
   let cs := s.argMap.find? key |>.getD []
   for (c, cType) in cs do
-    -- We first check the typesr
+    /-
+    We first check the types
+    The following checks are a performance bottleneck.
+    For example, in the test `grind_ite.lean`, there are many checks of the form:
+    ```
+    w_4 ∈ assign.insert v true → Prop =?= w_1 ∈ assign.insert v false → Prop
+    ```
+    where `grind` unfolds the definition of `DHashMap.insert` and `TreeMap.insert`.
+    -/
     if (← withDefault <| isDefEq eType cType) then
       if (← isDefEq e c) then
         -- We used to check `c.fvarsSubset e` because it is not

tests/lean/run/grind_ite.lean

@@ -126,6 +126,11 @@ we are allowed to increase the size of the branches by one, and still be smaller
   | var _ => 1
   | .ite i t e => 2 * normSize i + max (normSize t) (normSize e) + 1
 
+-- TODO: `grind` canonicalizer is spending a lot of time unfolding the following function.
+-- TODO: remove after the new module system will hide this declaration.
+seal Std.DHashMap.insert
+seal Std.TreeMap.insert
+
 def normalize (assign : Std.HashMap Nat Bool) : IfExpr → IfExpr
   | lit b => lit b
   | var v =>