Improve error message on wrong type of mg.Deps argument (#240) (#241)

This change adds a location of mg.Deps invocation, shrinking the search space
from "all your build scripts" to "this particular mg.Deps invocation"

mage/main_test.go

@@ -1261,6 +1261,25 @@ func TestAliasToImport(t *testing.T) {
 
 }
 
+var wrongDepRx = regexp.MustCompile("^Error: Invalid type for dependent function.*@ main.FooBar .*magefile.go")
+
+func TestWrongDependency(t *testing.T) {
+	stderr := &bytes.Buffer{}
+	inv := Invocation{
+		Dir:    "./testdata/wrong_dep",
+		Stderr: stderr,
+		Stdout: ioutil.Discard,
+	}
+	code := Invoke(inv)
+	if code != 1 {
+		t.Fatalf("expected 1, but got %v", code)
+	}
+	actual := stderr.String()
+	if !wrongDepRx.MatchString(actual) {
+		t.Fatalf("expected matching %q, but got %q", wrongDepRx, actual)
+	}
+}
+
 /// This code liberally borrowed from https://github.com/rsc/goversion/blob/master/version/exe.go
 
 type exeType int

mage/testdata/wrong_dep/magefile.go

@@ -0,0 +1,16 @@
+//+build mage
+
+package main
+
+import (
+	"github.com/magefile/mage/mg"
+)
+
+var Default = FooBar
+
+func WrongSignature(i int) {
+}
+
+func FooBar() {
+	mg.Deps(WrongSignature)
+}

mg/deps.go

@@ -214,6 +214,21 @@ func (o *onceFun) run() error {
 	return o.err
 }
 
+// Returns a location of mg.Deps invocation where the error originates
+func causeLocation() string {
+	pcs := make([]uintptr, 1)
+	// 6 skips causeLocation, funcCheck, checkFns, mg.CtxDeps, mg.Deps in stacktrace
+	if runtime.Callers(6, pcs) != 1 {
+		return "<unknown>"
+	}
+	frames := runtime.CallersFrames(pcs)
+	frame, _ := frames.Next()
+	if frame.Function == "" && frame.File == "" && frame.Line == 0 {
+		return "<unknown>"
+	}
+	return fmt.Sprintf("%s %s:%d", frame.Function, frame.File, frame.Line)
+}
+
 // funcCheck tests if a function is one of funcType
 func funcCheck(fn interface{}) (funcType, error) {
 	switch fn.(type) {
@@ -227,7 +242,7 @@ func funcCheck(fn interface{}) (funcType, error) {
 		return contextErrorType, nil
 	}
 
-	err := fmt.Errorf("Invalid type for dependent function: %T. Dependencies must be func(), func() error, func(context.Context), func(context.Context) error, or the same method on an mg.Namespace.", fn)
+	err := fmt.Errorf("Invalid type for dependent function: %T. Dependencies must be func(), func() error, func(context.Context), func(context.Context) error, or the same method on an mg.Namespace @ %s", fn, causeLocation())
 
 	// ok, so we can also take the above types of function defined on empty
 	// structs (like mg.Namespace). When you pass a method of a type, it gets