feat: trace non-easy whnf invocations (#3774)

src/Lean/Meta/WHNF.lean

@@ -936,21 +936,22 @@ private def cache (useCache : Bool) (e r : Expr) : MetaM Expr := do
 @[export lean_whnf]
 partial def whnfImp (e : Expr) : MetaM Expr :=
   withIncRecDepth <| whnfEasyCases e fun e => do
-    checkSystem "whnf"
     let useCache ← useWHNFCache e
     match (← cached? useCache e) with
     | some e' => pure e'
     | none    =>
-      let e' ← whnfCore e
-      match (← reduceNat? e') with
-      | some v => cache useCache e v
-      | none   =>
-        match (← reduceNative? e') with
+      withTraceNode `Meta.whnf (fun _ => return m!"Non-easy whnf: {e}") do
+        checkSystem "whnf"
+        let e' ← whnfCore e
+        match (← reduceNat? e') with
         | some v => cache useCache e v
         | none   =>
-          match (← unfoldDefinition? e') with
-          | some e'' => cache useCache e (← whnfImp e'')
-          | none => cache useCache e e'
+          match (← reduceNative? e') with
+          | some v => cache useCache e v
+          | none   =>
+            match (← unfoldDefinition? e') with
+            | some e'' => cache useCache e (← whnfImp e'')
+            | none => cache useCache e e'
 
 /-- If `e` is a projection function that satisfies `p`, then reduce it -/
 def reduceProjOf? (e : Expr) (p : Name → Bool) : MetaM (Option Expr) := do