gofix: do not convert url in field names

There's some ambiguity in the U{url: url} case as it could be
both a map or a struct literal, but given context it's more
likely a struct, so U{url: url_} rather than U{url_: url_}.
At least that was the case for me.

R=golang-dev, rsc, adg
CC=golang-dev
https://golang.org/cl/4972052

src/cmd/gofix/url.go

@@ -46,7 +46,12 @@ func url(f *ast.File) bool {
 	fixed := false
 
 	// Update URL code.
+	var skip interface{}
 	urlWalk := func(n interface{}) {
+		if n == skip {
+			skip = nil
+			return
+		}
 		// Is it an identifier?
 		if ident, ok := n.(*ast.Ident); ok && ident.Name == "url" {
 			ident.Name = "url_"
@@ -57,6 +62,12 @@ func url(f *ast.File) bool {
 			fixed = urlDoFields(fn.Params) || fixed
 			fixed = urlDoFields(fn.Results) || fixed
 		}
+		// U{url: ...} is likely a struct field.
+		if kv, ok := n.(*ast.KeyValueExpr); ok {
+			if ident, ok := kv.Key.(*ast.Ident); ok && ident.Name == "url" {
+				skip = ident
+			}
+		}
 	}
 
 	// Fix up URL code and add import, at most once.
@@ -64,7 +75,7 @@ func url(f *ast.File) bool {
 		if fixed {
 			return
 		}
-		walk(f, urlWalk)
+		walkBeforeAfter(f, urlWalk, nop)
 		addImport(f, "url")
 		fixed = true
 	}

src/cmd/gofix/url_test.go

@@ -80,10 +80,15 @@ import (
 	"http"
 )
 
+type U struct{ url int }
+type M map[int]int
+
 func f() {
 	http.ParseURL(a)
 	var url = 23
 	url, x := 45, y
+	_ = U{url: url}
+	_ = M{url + 1: url}
 }
 
 func g(url string) string {
@@ -98,10 +103,15 @@ func h() (url string) {
 
 import "url"
 
+type U struct{ url int }
+type M map[int]int
+
 func f() {
 	url.Parse(a)
 	var url_ = 23
 	url_, x := 45, y
+	_ = U{url: url_}
+	_ = M{url_ + 1: url_}
 }
 
 func g(url_ string) string {