Update Dig error types & reporting, more easily distinguish Dig error…

…s vs. non-Dig errors (#360)

This update:
- Introduces the dig.Error interface, which is implemented by all errors returned by Dig, allowing users to use errors.Is and errors.As with Dig errors. 
- Replaces the concept of errors causing other errors (i.e. the causer interface) with Go's error wrapping conventions, so errors caused by other errors can now be unwrapped to their causing error with errors.Unwrap. 
- Updates the behavior of dig.RootCause to return the first non-dig.Error in the chain of errors passed to it, if there is one. Otherwise, it will return the first error that does not wrap another error (the bottom of the chain).
- Adds tests testing the new behavior of dig.RootCause in error_test.go

cycle_error.go

@@ -22,7 +22,9 @@ package dig
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
+	"io"
 
 	"go.uber.org/dig/internal/digreflect"
 )
@@ -37,6 +39,8 @@ type errCycleDetected struct {
 	scope *Scope
 }
 
+var _ digError = errCycleDetected{}
+
 func (e errCycleDetected) Error() string {
 	// We get something like,
 	//
@@ -60,9 +64,16 @@ func (e errCycleDetected) Error() string {
 	return b.String()
 }
 
+func (e errCycleDetected) writeMessage(w io.Writer, v string) {
+	fmt.Fprint(w, e.Error())
+}
+
+func (e errCycleDetected) Format(w fmt.State, c rune) {
+	formatError(e, w, c)
+}
+
 // IsCycleDetected returns a boolean as to whether the provided error indicates
 // a cycle was detected in the container graph.
 func IsCycleDetected(err error) bool {
-	_, ok := RootCause(err).(errCycleDetected)
-	return ok
+	return errors.As(err, &errCycleDetected{})
 }

decorate.go

@@ -21,7 +21,6 @@
 package dig
 
 import (
-	"errors"
 	"fmt"
 	"reflect"
 
@@ -224,10 +223,8 @@ func (s *Scope) Decorate(decorator interface{}, opts ...DecorateOption) error {
 	}
 	for _, k := range keys {
 		if _, ok := s.decorators[k]; ok {
-			return fmt.Errorf("cannot decorate using function %v: %s already decorated",
-				dn.dtype,
-				k,
-			)
+			return newErrInvalidInput(
+				fmt.Sprintf("cannot decorate using function %v: %s already decorated", dn.dtype, k), nil)
 		}
 		s.decorators[k] = dn
 	}
@@ -275,7 +272,7 @@ func findResultKeys(r resultList) ([]key, error) {
 			keys = append(keys, key{t: innerResult.Type, name: innerResult.Name})
 		case resultGrouped:
 			if innerResult.Type.Kind() != reflect.Slice {
-				return nil, errors.New("decorating a value group requires decorating the entire value group, not a single value")
+				return nil, newErrInvalidInput("decorating a value group requires decorating the entire value group, not a single value", nil)
 			}
 			keys = append(keys, key{t: innerResult.Type.Elem(), group: innerResult.Group})
 		case resultObject:

dig_test.go

@@ -1525,7 +1525,7 @@ func TestProvideConstructorErrors(t *testing.T) {
 			{
 				desc:        "dig.Out",
 				constructor: func(out) io.Writer { return nil },
-				msg:         "dig_test.out embeds a dig.Out",
+				msg:         `dig_test.out embeds a dig.Out`,
 			},
 			{
 				desc:        "*dig.Out",
@@ -1547,8 +1547,7 @@ func TestProvideConstructorErrors(t *testing.T) {
 					`cannot provide function "go.uber.org/dig_test".TestProvideConstructorErrors\S+`,
 					`dig_test.go:\d+`, // file:line
 					`bad argument 1:`,
-					`cannot depend on result objects:`,
-					tt.msg)
+					`cannot depend on result objects: `+tt.msg)
 			})
 		}
 	})
@@ -1572,8 +1571,7 @@ func TestProvideConstructorErrors(t *testing.T) {
 			`cannot provide function "go.uber.org/dig_test".TestProvideConstructorErrors\S+`,
 			`dig_test.go:\d+`, // file:line
 			`bad result 1:`,
-			"cannot specify a name for result objects:",
-			"dig_test.out embeds dig.Out",
+			"cannot specify a name for result objects: dig_test.out embeds dig.Out",
 		)
 	})
 
@@ -1601,8 +1599,7 @@ func TestProvideConstructorErrors(t *testing.T) {
 			`cannot provide function "go.uber.org/dig_test".TestProvideConstructorErrors\S+`,
 			`dig_test.go:\d+`, // file:line
 			`bad field "Result1" of dig_test.Result2:`,
-			"cannot specify a name for result objects:",
-			"dig_test.Result1 embeds dig.Out",
+			"cannot specify a name for result objects: dig_test.Result1 embeds dig.Out",
 		)
 	})
 }
@@ -1909,8 +1906,7 @@ func TestCantProvideParameterObjects(t *testing.T) {
 			`cannot provide function "go.uber.org/dig_test".TestCantProvideParameterObjects\S+`,
 			`dig_test.go:\d+`, // file:line
 			"bad result 1:",
-			`cannot provide parameter objects:`,
-			`dig_test.Args embeds a dig.In`,
+			"cannot provide parameter objects: dig_test.Args embeds a dig.In",
 		)
 	})
 
@@ -1926,8 +1922,7 @@ func TestCantProvideParameterObjects(t *testing.T) {
 			`cannot provide function "go.uber.org/dig_test".TestCantProvideParameterObjects\S+`,
 			`dig_test.go:\d+`, // file:line
 			"bad result 1:",
-			`cannot provide parameter objects:`,
-			`\*dig_test.Args embeds a dig.In`,
+			`cannot provide parameter objects: \*dig_test.Args embeds a dig.In`,
 		)
 	})
 }
@@ -2203,6 +2198,18 @@ func testProvideCycleFails(t *testing.T, dryRun bool) {
 	})
 }
 
+func TestProvideErrNonCycle(t *testing.T) {
+	c := digtest.New(t)
+	type A struct{}
+	type B struct{}
+	newA := func() *A { return &A{} }
+
+	c.RequireProvide(newA)
+	err := c.Invoke(func(*B) {})
+	require.Error(t, err)
+	assert.False(t, dig.IsCycleDetected(err))
+}
+
 func TestIncompleteGraphIsOkay(t *testing.T) {
 	t.Parallel()
 
@@ -2407,8 +2414,7 @@ func testProvideFailures(t *testing.T, dryRun bool) {
 			`cannot provide function "go.uber.org/dig_test".testProvideFailures\S+`,
 			`dig_test.go:\d+`, // file:line
 			"bad result 1:",
-			`cannot return a pointer to a result object, use a value instead:`,
-			`\*dig_test.out is a pointer to a struct that embeds dig.Out`,
+			`cannot return a pointer to a result object, use a value instead: \*dig_test.out is a pointer to a struct that embeds dig.Out`,
 		)
 	})
 
@@ -2427,8 +2433,7 @@ func testProvideFailures(t *testing.T, dryRun bool) {
 			`cannot provide function "go.uber.org/dig_test".testProvideFailures\S+`,
 			`dig_test.go:\d+`, // file:line
 			"bad result 1:",
-			`cannot build a result object by embedding \*dig.Out, embed dig.Out instead:`,
-			`dig_test.out embeds \*dig.Out`,
+			`cannot build a result object by embedding \*dig.Out, embed dig.Out instead: dig_test.out embeds \*dig.Out`,
 		)
 	})
 
@@ -2905,8 +2910,7 @@ func testInvokeFailures(t *testing.T, dryRun bool) {
 		require.Error(t, err)
 		dig.AssertErrorMatches(t, err,
 			"bad argument 1:",
-			"cannot depend on a pointer to a parameter object, use a value instead:",
-			`\*dig_test.in is a pointer to a struct that embeds dig.In`,
+			`cannot depend on a pointer to a parameter object, use a value instead: \*dig_test.in is a pointer to a struct that embeds dig.In`,
 		)
 	})
 
@@ -2925,8 +2929,7 @@ func testInvokeFailures(t *testing.T, dryRun bool) {
 		require.Error(t, err)
 		dig.AssertErrorMatches(t, err,
 			"bad argument 1:",
-			"cannot depend on result objects:",
-			"dig_test.in embeds a dig.Out",
+			"cannot depend on result objects: dig_test.in embeds a dig.Out",
 		)
 	})
 
@@ -2942,8 +2945,7 @@ func testInvokeFailures(t *testing.T, dryRun bool) {
 		require.Error(t, err)
 		dig.AssertErrorMatches(t, err,
 			"bad argument 1:",
-			`cannot build a parameter object by embedding \*dig.In, embed dig.In instead:`,
-			`dig_test.in embeds \*dig.In`,
+			`cannot build a parameter object by embedding \*dig.In, embed dig.In instead: dig_test.in embeds \*dig.In`,
 		)
 	})