go/analysis/internal/checker: format files modified by -fix

When running a checker in -fix mode, try to format the file before
writing it.

Change-Id: I760f851f0ccd4a68c97949b21dabae39cb4ffaeb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/209861
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: matloob

go/analysis/internal/checker/checker.go

@@ -13,6 +13,8 @@ import (
 	"encoding/gob"
 	"flag"
 	"fmt"
+	"go/format"
+	"go/parser"
 	"go/token"
 	"go/types"
 	"io/ioutil"
@@ -341,6 +343,7 @@ func applyFixes(roots []*action) {
 
 	visitAll(roots)
 
+	fset := token.NewFileSet() // Shared by parse calls below
 	// Now we've got a set of valid edits for each file. Get the new file contents.
 	for f, tree := range editsForFile {
 		contents, err := ioutil.ReadFile(f.Name())
@@ -375,6 +378,15 @@ func applyFixes(roots []*action) {
 			out.Write(contents[cur:])
 		}
 
+		// Try to format the file.
+		ff, err := parser.ParseFile(fset, f.Name(), out.Bytes(), parser.ParseComments)
+		if err == nil {
+			var buf bytes.Buffer
+			if err = format.Node(&buf, fset, ff); err == nil {
+				out = buf
+			}
+		}
+
 		ioutil.WriteFile(f.Name(), out.Bytes(), 0644)
 	}
 }