Improve panic message on Punctuated index out of bounds

Author: dtolnay

src/punctuated.rs

@@ -1072,7 +1072,7 @@ impl<T, P> Index<usize> for Punctuated<T, P> {
     type Output = T;
 
     fn index(&self, index: usize) -> &Self::Output {
-        if index == self.len() - 1 {
+        if index.checked_add(1) == Some(self.len()) {
             match &self.last {
                 Some(t) => t,
                 None => &self.inner[index].0,
@@ -1085,7 +1085,7 @@ impl<T, P> Index<usize> for Punctuated<T, P> {
 
 impl<T, P> IndexMut<usize> for Punctuated<T, P> {
     fn index_mut(&mut self, index: usize) -> &mut Self::Output {
-        if index == self.len() - 1 {
+        if index.checked_add(1) == Some(self.len()) {
             match &mut self.last {
                 Some(t) => t,
                 None => &mut self.inner[index].0,

tests/test_punctuated.rs

@@ -85,7 +85,7 @@ fn may_dangle() {
 }
 
 #[test]
-#[should_panic = "attempt to subtract with overflow"]
+#[should_panic = "index out of bounds: the len is 0 but the index is 0"]
 fn index_out_of_bounds() {
     let p = Punctuated::<syn::Ident, Token![,]>::new();
     let _ = p[0].clone();