gopls/internal/golang: don't lose ... when split/joining variadics

We wrap the last argument f(x...) in an explicit Ellipsis
to record that it was a variadic call, and add the "..."
back with corresponding logic when creating the edits.

Fixes golang/go#70519

Change-Id: I1fdfa5f3ccb000c9622f856ed7703b31d7911620
Reviewed-on: https://go-review.googlesource.com/c/tools/+/631335
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

gopls/internal/golang/fix.go

@@ -35,7 +35,8 @@ import (
 // The supplied token positions (start, end) must belong to
 // pkg.FileSet(), and the returned positions
 // (SuggestedFix.TextEdits[*].{Pos,End}) must belong to the returned
-// FileSet.
+// FileSet, which is not necessarily the same.
+// (See [insertDeclsAfter] for explanation.)
 //
 // A fixer may return (nil, nil) if no fix is available.
 type fixer func(ctx context.Context, s *cache.Snapshot, pkg *cache.Package, pgf *parsego.File, start, end token.Pos) (*token.FileSet, *analysis.SuggestedFix, error)

gopls/internal/golang/lines.go

@@ -151,6 +151,15 @@ func processLines(fset *token.FileSet, items []ast.Node, comments []*ast.Comment
 		}
 
 		edits = append(edits, analysis.TextEdit{Pos: pos, End: end, NewText: []byte(sep + indent)})
+
+		// Print the Ellipsis if we synthesized one earlier.
+		if is[*ast.Ellipsis](nodes[i]) {
+			edits = append(edits, analysis.TextEdit{
+				Pos:     nodes[i].End(),
+				End:     nodes[i].End(),
+				NewText: []byte("..."),
+			})
+		}
 	}
 
 	return &analysis.SuggestedFix{TextEdits: edits}
@@ -205,6 +214,18 @@ func findSplitJoinTarget(fset *token.FileSet, file *ast.File, src []byte, start,
 		for _, arg := range node.Args {
 			items = append(items, arg)
 		}
+
+		// Preserve "..." by wrapping the last
+		// argument in an Ellipsis node
+		// with the same Pos/End as the argument.
+		// See corresponding logic in processLines.
+		if node.Ellipsis.IsValid() {
+			last := &items[len(items)-1]
+			*last = &ast.Ellipsis{
+				Ellipsis: (*last).Pos(),      // determines Ellipsis.Pos()
+				Elt:      (*last).(ast.Expr), // determines Ellipsis.End()
+			}
+		}
 	case *ast.CompositeLit:
 		for _, arg := range node.Elts {
 			items = append(items, arg)

gopls/internal/test/marker/testdata/codeaction/splitlines-variadic.txt

@@ -0,0 +1,55 @@
+This is a regression test for #70519, in which the ellipsis
+of a variadic call would go missing after split/join lines.
+
+-- go.mod --
+module example.com
+go 1.18
+
+-- a/a.go --
+package a
+
+var a, b, c []any
+func f(any, any, ...any)
+
+func _() {
+	f(a, b, c...) //@codeaction("a", "refactor.rewrite.splitLines", result=split)
+
+	f(
+		a,
+		b,
+		c..., /*@codeaction("c", "refactor.rewrite.joinLines", result=joined)*/
+	)
+}
+
+-- @split/a/a.go --
+package a
+
+var a, b, c []any
+func f(any, any, ...any)
+
+func _() {
+	f(
+		a,
+		b,
+		c...,
+	) //@codeaction("a", "refactor.rewrite.splitLines", result=split)
+
+	f(
+		a,
+		b,
+		c..., /*@codeaction("c", "refactor.rewrite.joinLines", result=joined)*/
+	)
+}
+
+-- @joined/a/a.go --
+package a
+
+var a, b, c []any
+func f(any, any, ...any)
+
+func _() {
+	f(a, b, c...) //@codeaction("a", "refactor.rewrite.splitLines", result=split)
+
+	f(a, b, c..., /*@codeaction("c", "refactor.rewrite.joinLines", result=joined)*/)
+}
+