imports: add Options.FormatOnly

Fixes golang/go#14500

Change-Id: Ied9d772e5f606ce6716193faa2c1a285f0ab00b9
Reviewed-on: https://go-review.googlesource.com/21532
Reviewed-by: Robert Griesemer <gri@golang.org>

imports/fix_test.go

@@ -19,8 +19,9 @@ import (
 var only = flag.String("only", "", "If non-empty, the fix test to run")
 
 var tests = []struct {
-	name    string
+	name       string
-	in, out string
+	formatOnly bool
+	in, out    string
 }{
 	// Adding an import to an existing parenthesized import
 	{
@@ -748,6 +749,31 @@ func main() { fmt.Println() }
 import "fmt"
 
 func main() { fmt.Println() }
+`,
+	},
+
+	// FormatOnly
+	{
+		name:       "format only",
+		formatOnly: true,
+		in: `package main
+
+import (
+"fmt"
+"golang.org/x/foo"
+)
+
+func main() {}
+`,
+		out: `package main
+
+import (
+	"fmt"
+
+	"golang.org/x/foo"
+)
+
+func main() {}
 `,
 	},
 }
@@ -782,6 +808,7 @@ func TestFixImports(t *testing.T) {
 	}
 
 	for _, tt := range tests {
+		options.FormatOnly = tt.formatOnly
 		if *only != "" && tt.name != *only {
 			continue
 		}

imports/imports.go

@@ -31,6 +31,8 @@ type Options struct {
 	Comments  bool // Print comments (true if nil *Options provided)
 	TabIndent bool // Use tabs for indent (true if nil *Options provided)
 	TabWidth  int  // Tab width (8 if nil *Options provided)
+
+	FormatOnly bool // Disable the insertion and deletion of imports
 }
 
 // Process formats and adjusts imports for the provided file.
@@ -50,9 +52,11 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 		return nil, err
 	}
 
-	_, err = fixImports(fileSet, file, filename)
+	if !opt.FormatOnly {
-	if err != nil {
+		_, err = fixImports(fileSet, file, filename)
-		return nil, err
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	sortImports(fileSet, file)