cmd/goimports: work around lack of runtime/trace in gccgo

Moves runtime/trace support (including its command line flag) behind
a "gc" build tag to allow goimports to build under gccgo, which does
not support runtime/trace.

Updates golang/go#15544.

Change-Id: I017a44089c0a4f3d3ba98815d57a141e25b3fe56
Reviewed-on: https://go-review.googlesource.com/26998
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/goimports/goimports.go

@@ -19,7 +19,6 @@ import (
 	"path/filepath"
 	"runtime"
 	"runtime/pprof"
-	"runtime/trace"
 	"strings"
 
 	"golang.org/x/tools/imports"
@@ -36,7 +35,6 @@ var (
 	cpuProfile     = flag.String("cpuprofile", "", "CPU profile output")
 	memProfile     = flag.String("memprofile", "", "memory profile output")
 	memProfileRate = flag.Int("memrate", 0, "if > 0, sets runtime.MemProfileRate")
-	traceProfile   = flag.String("trace", "", "trace profile output")
 
 	options = &imports.Options{
 		TabWidth:  8,
@@ -227,12 +225,10 @@ func gofmtMain() {
 		defer flush()
 		defer pprof.StopCPUProfile()
 	}
-	if *traceProfile != "" {
-		bw, flush := bufferedFileWriter(*traceProfile)
-		trace.Start(bw)
-		defer flush()
-		defer trace.Stop()
-	}
+	// doTrace is a conditionally compiled wrapper around runtime/trace. It is
+	// used to allow goimports to compile under gccgo, which does not support
+	// runtime/trace. See https://golang.org/issue/15544.
+	defer doTrace()()
 	if *memProfileRate > 0 {
 		runtime.MemProfileRate = *memProfileRate
 		bw, flush := bufferedFileWriter(*memProfile)

cmd/goimports/goimports_gc.go

@@ -0,0 +1,26 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build gc
+
+package main
+
+import (
+	"flag"
+	"runtime/trace"
+)
+
+var traceProfile = flag.String("trace", "", "trace profile output")
+
+func doTrace() func() {
+	if *traceProfile != "" {
+		bw, flush := bufferedFileWriter(*traceProfile)
+		trace.Start(bw)
+		return func() {
+			flush()
+			trace.Stop()
+		}
+	}
+	return func() {}
+}

cmd/goimports/goimports_not_gc.go

@@ -0,0 +1,11 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !gc
+
+package main
+
+func doTrace() func() {
+	return func() {}
+}