go/printer: never print a newline before the returned results

Otherwise, if one ends up with a "return result" where the two nodes are
in separate lines, the printer would incorrectly print a naked return:

	return
		result

The fix is simple - by not telling exprList what the previous position
is, it never adds a leading linebreak. This is the same mechanism used
for identifier lists and values, so it seems appropriate.

All other exprList calls that can produce a leading linebreak don't seem
buggy, because closing tokens such as parentheses and colons are needed
to finish the statement.

Verified that the test failed before the patch as well:

	--- FAIL: TestIssue32854 (0.00s)
	    printer_test.go:806: got "return\n\tcall()", want "return call()"

Finally, verified that 'gofmt -l -w src misc' doesn't make any new
changes, just in case we introduced any regression.

Fixes #32854.

Change-Id: I3384fbd711de06e742407df874c9ad85626d5d6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/184121
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/printer/nodes.go

@@ -1237,10 +1237,12 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
 			// lead to more nicely formatted code in general.
 			if p.indentList(s.Results) {
 				p.print(indent)
-				p.exprList(s.Pos(), s.Results, 1, noIndent, token.NoPos, false)
+				// Use NoPos so that a newline never goes before
+				// the results (see issue #32854).
+				p.exprList(token.NoPos, s.Results, 1, noIndent, token.NoPos, false)
 				p.print(unindent)
 			} else {
-				p.exprList(s.Pos(), s.Results, 1, 0, token.NoPos, false)
+				p.exprList(token.NoPos, s.Results, 1, 0, token.NoPos, false)
 			}
 		}
 

src/go/printer/printer_test.go

@@ -775,3 +775,34 @@ func TestParenthesizedDecl(t *testing.T) {
 		t.Errorf("got %q, want %q", noparen, original)
 	}
 }
+
+// Verify that we don't print a newline between "return" and its results, as
+// that would incorrectly cause a naked return.
+func TestIssue32854(t *testing.T) {
+	src := `package foo
+
+func f() {
+        return Composite{
+                call(),
+        }
+}`
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Replace the result with call(), which is on the next line.
+	fd := file.Decls[0].(*ast.FuncDecl)
+	ret := fd.Body.List[0].(*ast.ReturnStmt)
+	ret.Results[0] = ret.Results[0].(*ast.CompositeLit).Elts[0]
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, ret); err != nil {
+		t.Fatal(err)
+	}
+	want := "return call()"
+	if got := buf.String(); got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}