go/printer: AST rewriting and then formatting results in mangled docs

[mdempsky]

In golang.org/cl/12373, I wrote a cmd/fix rule to rewrite "types.Typ[x]" into "x.Basic()". However, the naive approach causes

package foo

func f() {
    _ = types.Typ[types.Int]
}

// dummy godoc
func g() {
}

to get formatted as

package foo

func f() {
    _ = types.Int.Basic(

    // dummy godoc
    )
}

func g() {
}

I.e., the godoc for g is moved up into the arguments section for the .Basic() invocation. See http://play.golang.org/p/dwq4E8dsMW for a working example. Edit: See http://play.golang.org/p/CP8ylmNjmQ for an updated working example using ast.CommentMap.

As a workaround, I found setting Rparen seems to prevent the issue. I wouldn't expect that to be necessary though.

[griesemer]

This is a well-known problem: comments are placed based on their original source code positions and thus don't "flow" with the nodes they seem to be attached to (they are not attached). You can use an ast.CommentSet to work around this issue.

Unfortunately this is "working as intended" - and also one of the major reasons why the go/ast could use a major overhaul.

Closing this.

[mdempsky]

@griesemer I tried following the ast.CommentMap example in the obvious way, but go/printer still mangles the godoc comment: http://play.golang.org/p/CP8ylmNjmQ

Am I misunderstanding/misapplying how to use it here?

[griesemer]

@mdempsky Need to investigate.