Fix nil checks and improve error message

The new mg.F function is the root of the nil-pointer error that can happen
when someone accidentally calls mg.Deps(TargetA()). I have fixed the check
for whether a function was passed so that it won't panic.

I then added a similar check, is the target a function, to
Deps/SerialDeps/CxtSerialDeps functions so that we can give a better
error message when it's passed a non-function. The error message
otherwise will reference mg.F which I don't think will help point people
to the real problem, which is how they called mg.Deps/SerialDeps.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

mg/deps.go

@@ -130,13 +130,17 @@ func runDeps(ctx context.Context, fns []Fn) {
 func checkFns(fns []interface{}) []Fn {
 	funcs := make([]Fn, len(fns))
 	for i, f := range fns {
-		if f == nil {
-			panic("A dependency of the current target was defined improperly, with parenthesis. Dependencies should be defined as mg.Deps(MyDep), not mg.Deps(MyDep())")
-		}
 		if fn, ok := f.(Fn); ok {
 			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

@@ -45,7 +45,7 @@ func TestDepWasNotInvoked(t *testing.T) {
 			t.Fatal("expected panic, but didn't get one")
 		}
 		gotErr := fmt.Sprint(err)
-		wantErr := "A dependency of the current target was defined improperly, with parenthesis"
+		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)
 		}

mg/fn.go

@@ -100,7 +100,7 @@ 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 {
+	if t == nil || t.Kind() != reflect.Func {
 		return false, false, fmt.Errorf("non-function passed to mg.F: %T", target)
 	}