diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index cd9a05bb29d..96b0bf1f5ca 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2790,6 +2790,52 @@ static RegisterPrimOp primop_functionArgs({
     .fun = prim_functionArgs,
 });
 
+static void prim_functionInfo(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0], pos);
+    if (args[0]->isPrimOpApp() || args[0]->isPrimOp()) {
+        v.mkAttrs(&state.emptyBindings);
+        return;
+    }
+    if (!args[0]->isLambda())
+        state.debugThrowLastTrace(TypeError({
+            .msg = hintfmt("'functionArgs' requires a function"),
+            .errPos = state.positions[pos]
+        }));
+
+    if (!args[0]->lambda.fun->hasFormals()) {
+        v.mkAttrs(&state.emptyBindings);
+        return;
+    }
+
+    auto formals = state.buildBindings(args[0]->lambda.fun->formals->formals.size());
+    for (auto & i : args[0]->lambda.fun->formals->formals)
+        // !!! should optimise booleans (allocate only once)
+        formals.alloc(i.name, i.pos).mkBool(i.def);
+    auto attrs = state.buildBindings(3);
+    if(args[0]->lambda.fun->arg) attrs.alloc("arg").mkString(state.symbols[args[0]->lambda.fun->arg]);
+    attrs.alloc("ellipsis").mkBool(args[0]->lambda.fun->formals->ellipsis);
+    attrs.alloc("formals").mkAttrs(formals);
+    v.mkAttrs(attrs);
+}
+
+static RegisterPrimOp primop_functionInfo({
+    .name = "__functionInfo",
+    .args = {"f"},
+    .doc = R"(
+      Return information about the function including the following:
+
+        * arg - The name of the argument, this value is present for functions
+                that don't use attrset matching(i.e. `x: true`) and functions
+                that use an @-pattern(i.e. `{...}@argName`). Otherwise this
+                attr is absent.
+        * ellipsis - Set to true for functions of the form `{ a, b, ... }`.
+        * formals - Returns formal arguments, identical to
+          `builtins.functionArgs`.
+    )",
+    .fun = prim_functionInfo,
+});
+
 /*  */
 static void prim_mapAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
diff --git a/src/libexpr/tests/primops.cc b/src/libexpr/tests/primops.cc
index ce3b5d11fae..eac9ac81383 100644
--- a/src/libexpr/tests/primops.cc
+++ b/src/libexpr/tests/primops.cc
@@ -232,6 +232,55 @@ namespace nix {
         ASSERT_THAT(*y->value, IsTrue());
     }
 
+    TEST_F(PrimOpTest, functionInfoWithEllipsisAndArg) {
+        auto v = eval("builtins.functionInfo ({ x, y ? 123, ...}@inputs: 1)");
+        ASSERT_THAT(v, IsAttrsOfSize(3));
+
+        auto arg = v.attrs->find(createSymbol("arg"));
+        ASSERT_NE(arg, nullptr);
+        ASSERT_THAT(*arg->value, IsStringEq("inputs"));
+
+        auto ellipsis = v.attrs->find(createSymbol("ellipsis"));
+        ASSERT_NE(ellipsis, nullptr);
+        ASSERT_THAT(*ellipsis->value, IsTrue());
+
+        auto formals = v.attrs->find(createSymbol("formals"));
+        ASSERT_NE(formals, nullptr);
+        ASSERT_THAT(*formals->value, IsAttrsOfSize(2));
+
+        auto x = formals->value->attrs->find(createSymbol("x"));
+        ASSERT_NE(x, nullptr);
+        ASSERT_THAT(*x->value, IsFalse());
+
+        auto y = formals->value->attrs->find(createSymbol("y"));
+        ASSERT_NE(y, nullptr);
+        ASSERT_THAT(*y->value, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, functionInfoNoEllipsisNoArg) {
+        auto v = eval("builtins.functionInfo ({ x, y ? 123}: 1)");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+
+        auto arg = v.attrs->find(createSymbol("arg"));
+        ASSERT_EQ(arg, v.attrs->end());
+
+        auto ellipsis = v.attrs->find(createSymbol("ellipsis"));
+        ASSERT_NE(ellipsis, nullptr);
+        ASSERT_THAT(*ellipsis->value, IsFalse());
+
+        auto formals = v.attrs->find(createSymbol("formals"));
+        ASSERT_NE(formals, nullptr);
+        ASSERT_THAT(*formals->value, IsAttrsOfSize(2));
+
+        auto x = formals->value->attrs->find(createSymbol("x"));
+        ASSERT_NE(x, nullptr);
+        ASSERT_THAT(*x->value, IsFalse());
+
+        auto y = formals->value->attrs->find(createSymbol("y"));
+        ASSERT_NE(y, nullptr);
+        ASSERT_THAT(*y->value, IsTrue());
+    }
+
     TEST_F(PrimOpTest, mapAttrs) {
         auto v = eval("builtins.mapAttrs (name: value: value * 10) { a = 1; b = 2; }");
         ASSERT_THAT(v, IsAttrsOfSize(2));