go/ast: CommentMap mishandles loose, trailing file comments

[mvdan]

go version devel +812b34efae Mon Sep 4 00:07:33 2017 +0000 linux/amd64

Construct a comment map for:

package p

func f() {
        return
}

// loose trailing comment

The loose trailing comment is associated in the CommentMap with the
return statement inside f. It should probably be associated with f
instead, and the NewCommentMap docs updated with new rules.

Live proof: https://play.golang.org/p/XCktcSxYl8

[mvdan]

CC @griesemer

[mvdan]

I also see that there even is a test that includes this:

"50: *ast.Ident":      "the very last comment\n",

In that case, the trailing comment in the file is attached to an ast.Ident. I don't see a TODO nor any open issue about this, so I'm wondering if this is actually on purpose.

Another option would be to attach it to the whole ast.File. Either sounds better than the current behavior. The docs don't seem to cover this case.

[mvdan]

Closed by mistake in one of @josharian's private repos, I assume.

[mvdan]

A couple of other weird cases:

https://play.golang.org/p/d46URuVRTJ
https://play.golang.org/p/d79_UMvkzS

[josharian]

Closed by mistake in one of @josharian's private repos, I assume.

Indeed. FWIW, here's the diff I used, which fixed it the way I wanted it fixed for my private purposes. I haven't thought about whether it is an appropriate fix for the go/ast package.

diff --git a/astutil/commentmap.go b/astutil/commentmap.go
index c6f0d62..7c8170f 100644
--- a/astutil/commentmap.go
+++ b/astutil/commentmap.go
@@ -206,7 +206,8 @@ func NewCommentMap(fset *token.FileSet, node ast.Node, comments []*ast.CommentGr
 			switch {
 			case pg != nil &&
 				(pgend.Line == r.pos.Line ||
-					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line):
+					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line ||
+					q == nil):
 				// 1) comment starts on same line as previous node group ends, or
 				// 2) comment starts on the line immediately after the
 				//    previous node group and there is an empty line before
diff --git a/astutil/commentmap_test.go b/astutil/commentmap_test.go
index 3df0e65..a645032 100644
--- a/astutil/commentmap_test.go
+++ b/astutil/commentmap_test.go
@@ -92,9 +92,9 @@ var res = map[string]string{
 	"31: *ast.ExprStmt":   " associated with s1\nalso associated with s1\n",
 	"37: *ast.ExprStmt":   "associated with s2\nalso associated with s2\nline comment for s2\n",
 	"45: *ast.FuncDecl":   "associated with f2\nf2\n",
+	"48: *ast.FuncDecl":   "the very last comment\n",
 	"49: *ast.AssignStmt": "addition\n",
 	"49: *ast.BasicLit":   " 1\n",
-	"50: *ast.Ident":      "the very last comment\n",
 }