lib.functionArgs: fix correctness for nested functors

Restore the correct behaviour prior to commit bb71feaec1
("lib.functionArgs: save a function call on every lookup")
while preserving its performance gain with best efforts.

lib/tests/misc.nix

@@ -57,6 +57,7 @@ let
     foldlAttrs
     foldr
     functionArgs
+    genAttrs
     generators
     genList
     getExe
@@ -930,6 +931,54 @@ runTests {
     expected = "1.2.3";
   };
 
+  testFunctionArgsNestedFunctors =
+    let
+      faOrig = {
+        x = false;
+      };
+      faAlt = {
+        x = false;
+        y = true;
+      };
+      f0 = { x, ... }: x;
+    in
+    {
+      expr = {
+        a = functionArgs {
+          __functor = self: {
+            __functor = self: f0;
+            __functionArgs = faAlt;
+          };
+        };
+        b = functionArgs { __functor = self: { __functor = self: f0; }; };
+        c = functionArgs {
+          __functor = {
+            __functor = self: outer: f0;
+          };
+        };
+        d = functionArgs {
+          __functor = {
+            __functor = self: outer: {
+              __functor = self: f0;
+              __functionArgs = faAlt;
+            };
+          };
+        };
+        e = functionArgs {
+          __functor = {
+            __functor = self: outer: { __functor = self: f0; };
+          };
+        };
+      };
+      expected = {
+        a = faAlt;
+        b = faOrig;
+        c = faOrig;
+        d = faAlt;
+        e = faOrig;
+      };
+    };
+
   testIsFunctionStr = {
     expr = isFunction "a";
     expected = false;

lib/trivial.nix

@@ -1104,9 +1104,22 @@ in
   */
   functionArgs =
     let
-      functionArgs = builtins.functionArgs;
+      functionArgsLambda = builtins.functionArgs;
     in
-    f: if f ? __functor then f.__functionArgs or (functionArgs (f.__functor f)) else functionArgs f;
+    f:
+    if f ? __functor then
+      f.__functionArgs or (
+        let
+          fInner = f.__functor f;
+        in
+        # Peel one recursion to avoid calling functionArgs
+        if fInner ? __functor then
+          fInner.__functionArgs or (functionArgs fInner)
+        else
+          functionArgsLambda fInner
+      )
+    else
+      functionArgsLambda f;
 
   /**
     Check whether something is a function or something