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

go1.21 (discovered on tip)

Does this issue reproduce with the latest release?

Yes

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

Whatever play.go.dev is using.

What did you do?

package main

import (
	"fmt"
	"go/format"

	"github.com/google/go-cmp/cmp"
)

var in = []byte(`package main
		//some comment
		var x string`)

func main() {
	first_format, err := format.Source(in)
	if err != nil {
		fmt.Println(err)
		return
	}
	second_format, err := format.Source(first_format)
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println(cmp.Diff(string(first_format), string(second_format)))
}

https://go.dev/play/p/b4foaIeTooZ

What did you expect to see?

I expected no diff between formatting the file once and formatting the input twice.

What did you see instead?

The first formatting removed the identation but does not add a space between // and some comment.
The second formatting adds a space between // and some comment.
As far as I can tell go/format.Source only adds spaces between // and the comment text if these comment is not indented.