cmd/gofmt: rewrite removes comments

[musiphil]

What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes (with 1.15.3)

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="$HOME/.cache/go-build"
GOENV="$HOME/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="$HOME/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="$HOME/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/google-golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build684341242=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ cat gofmt_ex.go
package tmp

var Entities = []*upb.Entity{
	// This is a comment.
	upb.Entity_builder{Name: proto.String("alpha")}.Build(),
	upb.Entity_builder{Name: proto.String("beta")}.Build(),
}

$ gofmt -d -r 'upb.Entity_builder{Name: proto.String(n)}.Build() -> Entity(n)' gofmt_ex.go

What did you expect to see?

The comment should stay intact.

diff -u gofmt_ex.go.orig gofmt_ex.go
--- gofmt_ex.go.orig
+++ gofmt_ex.go
@@ -1,7 +1,7 @@
 package tmp
 
 var Entities = []*upb.Entity{
 	// This is a comment.
-	upb.Entity_builder{Name: proto.String("alpha")}.Build(),
-	upb.Entity_builder{Name: proto.String("beta")}.Build(),
+	Entity("alpha"),
+	Entity("beta"),
 }

What did you see instead?

The comment is replaced by a blank line.

diff -u gofmt_ex.go.orig gofmt_ex.go
--- gofmt_ex.go.orig
+++ gofmt_ex.go
@@ -1,7 +1,7 @@
 package tmp
 
 var Entities = []*upb.Entity{
-	// This is a comment.
-	upb.Entity_builder{Name: proto.String("alpha")}.Build(),
-	upb.Entity_builder{Name: proto.String("beta")}.Build(),
+
+	Entity("alpha"),
+	Entity("beta"),
 }

[cagedmantis]

/cc @griesemer