avoid panic when counting zero-sized outputs in count() (#1618)

* avoid panic when counting zero-sized outputs in count()

* run cargo fmt

src/multi/mod.rs

@@ -573,7 +573,8 @@ where
 {
   move |i: I| {
     let mut input = i.clone();
-    let max_initial_capacity = MAX_INITIAL_CAPACITY_BYTES / crate::lib::std::mem::size_of::<O>();
+    let max_initial_capacity =
+      MAX_INITIAL_CAPACITY_BYTES / crate::lib::std::mem::size_of::<O>().max(1);
     let mut res = crate::lib::std::vec::Vec::with_capacity(count.min(max_initial_capacity));
 
     for _ in 0..count {

tests/issues.rs

@@ -229,3 +229,14 @@ fn issue_1459_clamp_capacity() {
   let mut parser = count::<_, _, (), _>(char('a'), usize::MAX);
   assert_eq!(parser("a"), Err(nom::Err::Error(())));
 }
+
+#[test]
+fn issue_1617_count_parser_returning_zero_size() {
+  use nom::{bytes::complete::tag, combinator::map, error::Error, multi::count};
+
+  // previously, `count()` panicked if the parser had type `O = ()`
+  let parser = map(tag::<_, _, Error<&str>>("abc"), |_| ());
+  // shouldn't panic
+  let result = count(parser, 3)("abcabcabcdef").expect("parsing should succeed");
+  assert_eq!(result, ("def", vec![(), (), ()]));
+}