Make provide's errors more expressive for fx.Annotate-ed functions (#844

)

* Make provide's errors more expressive for fx.Annotate-ed functions

Because of the use of reflection in fx.Annotate, the error
messages for provide tend to refer to the annotated function as
`"reflect".makeFuncStub` which is not helpful for debugging.

This change updates error messages that previously looked like:
```
Failed: cannot provide function "reflect".makeFuncStub (/usr/local/go/src/reflect/asm_arm64.s:12):
cannot provide *fx_test.Logger from [0].Field0: already provided by "reflect".makeFuncStub
(/usr/local/go/src/reflect/asm_arm64.s:12)
```

To looking like this:
```
Failed: cannot provide function "reflect".makeFuncStub (/usr/local/go/src/reflect/asm_amd64.s:12):
cannot provide *fx_test.Logger from [0].Field0: already provided by "go.uber.org/fx_test".TestAnnotate.func1
(/gocode/src/github.com/uber-go/fx/annotated_test.go:515)
```

WIP: Planning on making a future fix that gives a more
readable name to the first function which still is referred to
as "reflect".makeFuncStub after this change.

* Switch to storing ptr value instead of ProvideOption

Co-authored-by: Moises Vega <moises.vega@gmail.com>

* Switch to storing ptr value instead of ProvideOption

Co-authored-by: Moises Vega <moises.vega@gmail.com>

* Switch to storing ptr value instead of ProvideOption

Co-authored-by: Moises Vega <moises.vega@gmail.com>

* Update annotated_test.go test naming

Co-authored-by: Moises Vega <moises.vega@gmail.com>

* Fix lint issue

Co-authored-by: Moises Vega <moises.vega@gmail.com>
Co-authored-by: Sung Yoon Whang <sungyoon@uber.com>

annotated.go

@@ -264,6 +264,7 @@ type annotated struct {
 	ParamTags  []string
 	ResultTags []string
 	As         [][]reflect.Type
+	FuncPtr    uintptr
 }
 
 func (ann annotated) String() string {
@@ -302,6 +303,7 @@ func (ann *annotated) Build() (interface{}, error) {
 
 	newFnType := reflect.FuncOf(paramTypes, resultTypes, false)
 	origFn := reflect.ValueOf(ann.Target)
+	ann.FuncPtr = origFn.Pointer()
 	newFn := reflect.MakeFunc(newFnType, func(args []reflect.Value) []reflect.Value {
 		args = remapParams(args)
 		var results []reflect.Value

annotated_test.go

@@ -787,6 +787,22 @@ func TestAnnotate(t *testing.T) {
 		defer app.RequireStart().RequireStop()
 	})
 
+	t.Run("provide an already provided function using Annotate", func(t *testing.T) {
+		t.Parallel()
+
+		app := NewForTest(t,
+			fx.Provide(fx.Annotate(newA, fx.ResultTags(`name:"a"`))),
+			fx.Provide(fx.Annotate(newA, fx.ResultTags(`name:"a"`))),
+			fx.Invoke(
+				fx.Annotate(newB, fx.ParamTags(`name:"a"`)),
+			),
+		)
+		err := app.Err()
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "already provided")
+		assert.Contains(t, err.Error(), "\"go.uber.org/fx_test\".TestAnnotate.func1")
+	})
+
 	t.Run("specify more ParamTags than Params", func(t *testing.T) {
 		t.Parallel()
 

provide.go

@@ -109,6 +109,7 @@ func runProvide(c container, p provide, opts ...dig.ProvideOption) error {
 			return fmt.Errorf("fx.Provide(%v) from:\n%+vFailed: %v", constructor, p.Stack, err)
 		}
 
+		opts = append(opts, dig.LocationForPC(constructor.FuncPtr))
 		if err := c.Provide(ctor, opts...); err != nil {
 			return fmt.Errorf("fx.Provide(%v) from:\n%+vFailed: %v", constructor, p.Stack, err)
 		}