golang/internal/highlight: check idx < len before indexing

Change-Id: I2d36c2b64e3af00861ce9913f733247877b2239e
GitHub-Last-Rev: f6b4312
GitHub-Pull-Request: #556
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642756
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>

Author: xzbdmw

gopls/internal/golang/highlight.go

@@ -166,9 +166,9 @@ func formatStringAndIndex(info *types.Info, call *ast.CallExpr) (*ast.BasicLit,
 		return nil, -1
 	}
 	idx := sig.Params().Len() - 2
-	if idx < 0 {
-		// Skip checking variadic functions without
-		// fixed arguments.
+	if !(0 <= idx && idx < len(call.Args)) {
+		// Skip checking functions without a format string parameter, or
+		// missing the corresponding format argument.
 		return nil, -1
 	}
 	// We only care about literal format strings, so fmt.Sprint("a"+"b%s", "bar") won't be highlighted.

gopls/internal/test/marker/testdata/highlight/highlight_printf.txt

@@ -53,3 +53,10 @@ func Indexed() {
 func MultipleIndexed() {
 	fmt.Printf("%[1]d %[1].2d", 3) //@hiloc(m1, "%[1]d", write),hiloc(m2, "3", read),hiloc(m3, "%[1].2d", write),highlightall(m1, m2, m3)
 }
+
+// This test checks that gopls doesn't crash (index out of bounds)
+// while haven't fill the last non-variadic argument.
+func NoEffectOnUnfinishedArg() {
+	var s string //@hiloc(var, "s", write)
+	fmt.Fprintf(s) //@hiloc(firstArg, "s", read),highlightall(var, firstArg)
+}