Detect deps that have already been invoked

mg/deps.go

@@ -134,6 +134,13 @@ func checkFns(fns []interface{}) []Fn {
 			funcs[i] = fn
 			continue
 		}
+
+		// Check if the target provided is a not function so we can give a clear warning
+		t := reflect.TypeOf(f)
+		if t == nil || t.Kind() != reflect.Func {
+			panic(fmt.Errorf("non-function used as a target dependency: %T. The mg.Deps, mg.SerialDeps and mg.CtxDeps functions accept function names, such as mg.Deps(TargetA, TargetB)", f))
+		}
+
 		funcs[i] = F(f)
 	}
 	return funcs

mg/deps_internal_test.go

@@ -2,6 +2,7 @@ package mg
 
 import (
 	"bytes"
+	"fmt"
 	"log"
 	"os"
 	"strings"
@@ -33,3 +34,23 @@ func bar() {
 }
 
 func baz() {}
+
+func TestDepWasNotInvoked(t *testing.T) {
+	fn1 := func() error {
+		return nil
+	}
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Fatal("expected panic, but didn't get one")
+		}
+		gotErr := fmt.Sprint(err)
+		wantErr := "non-function used as a target dependency: <nil>. The mg.Deps, mg.SerialDeps and mg.CtxDeps functions accept function names, such as mg.Deps(TargetA, TargetB)"
+		if !strings.Contains(gotErr, wantErr) {
+			t.Fatalf(`expected to get "%s" but got "%s"`, wantErr, gotErr)
+		}
+	}()
+	func(fns ...interface{}) {
+		checkFns(fns)
+	}(fn1())
+}

mg/fn.go

@@ -100,8 +100,8 @@ func (f fn) Run(ctx context.Context) error {
 
 func checkF(target interface{}, args []interface{}) (hasContext, isNamespace bool, _ error) {
 	t := reflect.TypeOf(target)
-	if t.Kind() != reflect.Func {
-		return false, false, fmt.Errorf("non-function passed to mg.F: %T", target)
+	if t == nil || t.Kind() != reflect.Func {
+		return false, false, fmt.Errorf("non-function passed to mg.F: %T. The mg.F function accepts function names, such as mg.F(TargetA, \"arg1\", \"arg2\")", target)
 	}
 
 	if t.NumOut() > 1 {

mg/fn_test.go

@@ -114,6 +114,16 @@ func TestFuncCheck(t *testing.T) {
 	if err == nil {
 		t.Error("expected func(*int) error to be invalid")
 	}
+
+	defer func() {
+		if r := recover(); r !=nil {
+			t.Error("expected a nil function argument to be handled gracefully")
+		}
+	}()
+	_, _, err = checkF(nil, []interface{}{1,2})
+	if err == nil {
+		t.Error("expected a nil function argument to be invalid")
+	}
 }
 
 func TestF(t *testing.T) {