cmd/gofmt: reformats block comments

[gopherbot]

by borman@google.com:

What steps will reproduce the problem?

Use /* */ for block comments.  Example:

func f() {
        /*
         * don't do this right now
        fmt.Printf(`a
multi-line
string`)
        */
        return
}

Gofmt alters the content of the comment (which includes the /* and */).  The first time
gofmt is called it produces:

func f() {
        /*
                 * don't do this right now
                fmt.Printf(`a
        multi-line
        string`)
        */
        return
}

and if called again it produces:

func f() {
        /*
                         * don't do this right now
                        fmt.Printf(`a
                multi-line
                string`)
        */
        return
}

gofmt should not have altered the comment in the first place.  The /* was indented
correctly up front.

[gopherbot]

Comment 1 by borman@google.com:

Another indentation fail with /* */:
Start with:
/*
        This is a comment
        It is indented with 8 spaces.
        And has several lines
 */
Gofmt changes this to:
/*
   This is a comment
   It is indented with 8 spaces.
   And has several lines
*/
It has changed my 8 spaces into 3.

[griesemer]

Comment 2:

Owner changed to @griesemer.

[gopherbot]

Comment 3 by hamo.by:

The first issue you reported seems a expected output for issue #1835.

[griesemer]

Comment 4:

There is a tension between gofmt leaving comments alone and "adjusting" them to match
surrounding code. There is a know bug where formatting of /* comments is not idempotent
(as witnessed in the first example above), and where formatting doesn't change anymore
after two applications of gofmt (as is also the case in the first example above).
Being completely idempotent is a very hard problem (short of simply running gofmt twice
internally).
Independent of idempotency, the question is what gofmt should be doing with /* comments.
You are suggesting that it should leave it alone if the first line is indented
correctly. Perhaps that is the right answer. But I suspect there are counter examples.
Leaving alone for now.

Status changed to Thinking.

[rsc]

Comment 5:

Labels changed: added priority-later, go1.2maybe, removed priority-triage.

[rsc]

Comment 6:

Labels changed: added feature.

[robpike]

Comment 7:

Leaving alone for 1.2.

Labels changed: added go1.3maybe, removed go1.2maybe.

[robpike]

Comment 8:

Labels changed: removed go1.3maybe.

[josharian]

Comment 9:

Another use for leaving /* comments */ completely untouched is for example output with
whitespace: https://golang.org/issue/6416

[rsc]

Comment 10:

Labels changed: added go1.3maybe.

[rsc]

Comment 11:

Labels changed: removed feature.

[rsc]

Comment 13:

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

[rsc]

Comment 14:

Labels changed: added repo-main.

[dmitshur]

Comment 15:

I ran into this issue recently, with an even simpler reproduce case:
http://play.golang.org/p/Pwn4cOHtMN

[griesemer]

Comment 16:

Cool! Thanks.

[dmitshur]

I've done some investigating here.

I ran into this issue recently, with an even simpler reproduce case:

http://play.golang.org/p/Pwn4cOHtMN

That particular issue is due to an unhandled edge case in stripCommonPrefix of go/printer.

It happens when a comment block consists of a first line with /*, 1 or more blank lines, last line with */. The internal prefix variable ends up never being set to a correct value before being used.

Handling it correctly will fix that particular sample I provided (http://play.golang.org/p/Pwn4cOHtMN), but it seems like it's not the same issue as what's described in the original post.

Should I break http://play.golang.org/p/Pwn4cOHtMN out into a separate issue then? Or just consider this current issue to be it?

[dmitshur]

Okay, I've created failing test cases in shurcooL/play@bfc69a3 and a simple fix for the issue in shurcooL/play@f416cec. I didn't try to refactor it in any way.

I can submit a CL for review as soon as I'm sure if I should say "fixes #5128" or create a separate issue...

[dmitshur]

After re-reading the original issue and the responses it received, I feel that I should move http://play.golang.org/p/Pwn4cOHtMN into a separate, much smaller issue. That way I can proceed to make a CL that completely fixes and closes it. So, I've created #9751.

It has a simple and direct fix. I've made a CL for it.

[c0b]

I have an Example test case:

func ExampleDecode() {
	call-my-test-case it print some output

	// Output:
	/*
here is the output in a block comment
   in multi lines with different indentation
	*/
}

The test case is passing, it's no problem

but when I run go fmt, it constantly change the block comment to this style

	/*
	here is the output in a block comment
	   in multi lines with different indentation
	*/

and causing test case to FAIL;

wonder is it the same case here, is there any way to let go fmt not touching my block comment?

[griesemer]

@c0b There's no way to let go fmt know that it should leave comments alone. In your case, try the use of // comments for the Example output instead (e.g., like in https://golang.org/src/text/scanner/example_test.go).

A future gofmt (version) probably should not touch comments at all, but that has other issues.

[vearutop]

It is a real pain to work with multi-line // output: in example tests. I'm trying to assert YAML value.

• /* ... */ gets broken by gofmt.
• // per-line is unmanageable, I use Goland to comment multiple lines with hotkey, but it does not add space between // and line content.

Currently I have to paste expected content, add // with hotkey and then replace // to // to make things work.

I would appreciate that gofmt does not touch comments following // output:.

[c0b]

There's no way to let go fmt know that it should leave comments alone

how about to change go fmt to recognize the block comment followed by keywords like Output: this convention is in-use by go test; should be considered a builtin feature of go?

there might be other keywords should be recognized as well, if go fmt doesn't want to recognize all of them, then may create a special comment marker like gofmt: DO NOT TOUCH in the first line of the block comment?

A future gofmt (version) probably should not touch comments at all, but that has other issues.

@griesemer are you aware of any ongoing effort of this future gofmt? can connect related PR to here?

[komuw]

I just spent 1 hour trying to get gofmt to leave alone the // output: of a test example I was working on(#41980), to no avail; until I searched and came across this issue.

I feel like gofmt re-formatting block comments in the specific case of // output: of a test example should be a separate issue that should be decided on independent of this one. But maybe this is just my frustration showing.

[komuw]

It seems impossible, at least for me, to get the following reproducer test to pass; if you run gofmt before running go test. Because gofmt will always reformat the // output comment. And hence this sounds like a bug to me rather than a feature request.

package main

import (
	"fmt"
)

func Hello() {
	fmt.Printf("Hello \n\nWorld")
}

// 1. https://golang.org/pkg/testing/#hdr-Examples
// 2. https://blog.golang.org/examples
func ExampleHello() {
	Hello()

	// Output:
	// Hello 
	//
	// World
}