cue/format: preserve comments associated with ellipsis

When simplification is enabled, the current implementation prints a
"dummy" ellipsis during formatting, instead of the original ellipsis.
This means that comments associated with the original ellipsis are not
printed in the formatted CUE.
To fix this, we associate the original comments with the dummy ellipsis
so they won't get lost.

Fixes #3198.

Signed-off-by: Noam Dolovich <noam.tzvi.dolovich@gmail.com>
Change-Id: Ibebbd97ac768de7a2c0d9855c9ec6c7a497435c8
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1195884
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/format/node.go

@@ -104,7 +104,7 @@ func hasDocComments(d ast.Decl) bool {
 func (f *formatter) walkDeclList(list []ast.Decl) {
 	f.before(nil)
 	d := 0
-	hasEllipsis := false
+	var ellipsis ast.Decl
 	for i, x := range list {
 		if i > 0 {
 			f.print(declcomma)
@@ -128,7 +128,7 @@ func (f *formatter) walkDeclList(list []ast.Decl) {
 			}
 		}
 		if f.printer.cfg.simplify && internal.IsEllipsis(x) {
-			hasEllipsis = true
+			ellipsis = x
 			continue
 		}
 		f.decl(x)
@@ -156,8 +156,11 @@ func (f *formatter) walkDeclList(list []ast.Decl) {
 		}
 		f.print(f.current.parentSep)
 	}
-	if hasEllipsis {
-		f.decl(&ast.Ellipsis{})
+	if ellipsis != nil {
+		// ensure that comments associated with the original ellipsis are preserved
+		n := &ast.Ellipsis{}
+		ast.SetComments(n, ast.Comments(ellipsis))
+		f.decl(n)
 		f.print(f.current.parentSep)
 	}
 	f.after(nil)

cue/format/testdata/comments.golden

@@ -148,7 +148,11 @@ foo: [
 {
 
 	foo: {
-		...
+		// some
+		... // comment
 	}
-	...
+	// comments
+	... // surrounding
+
+	// an ellipsis
 }