Detect deps that have already been invoked

mg/deps.go

@@ -130,6 +130,9 @@ 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

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 := "A dependency of the current target was defined improperly, with parenthesis"
+		if !strings.Contains(gotErr, wantErr) {
+			t.Fatalf(`expected to get "%s" but got "%s"`, wantErr, gotErr)
+		}
+	}()
+	func(fns ...interface{}) {
+		checkFns(fns)
+	}(fn1())
+}