go/printer: printed output cannot be parsed if comments are retained

[adonovan]

This program:
  http://play.golang.org/p/f3VrIs2Z70
parses a syntactically well-formed Go program, inserts an additional statement, then
prints it out again.  The result cannot be parsed because a newline was inserted in an
inappropriate place to accommodate a comment.  Here's the output:

package P

func f() {
    print("12"
    /*hi*/)
    g(0, 1)
}

Note that the length of the literal "12" is critical: if a shorter expression
is used, the /*hi*/ comment, which is retains its association with its original byte
offset, will not be inserted at that point.

[griesemer]

Comment 1:

The current go/printer has its limitations, and one of them is that it relies crucially
on correct position information in the input AST. Arguably the input to the printer is
incorrect.

Status changed to Thinking.

[rsc]

Comment 2:

Labels changed: added go1.3maybe.

[rsc]

Comment 3:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 4:

Labels changed: added repo-main.