From 3bdd5ec5a9a553b4592eca0e69afa95d1f30e1da Mon Sep 17 00:00:00 2001
From: Griffin Smith <root@gws.fyi>
Date: Wed, 28 Sep 2022 16:53:07 -0400
Subject: [PATCH] feat(comb): Allow arrays as input to `alt`

Now that min-const-generics is stabilized, it's possible
to (backwards-compatibly) write a version of `alt` that uses a const
array as input instead of being limited to only tuples - this allows us
to lift the 21-element limitation on tuples passed to `alt`.

Upstream: rust-bakery/nom#1556
---
 src/combinator/branch.rs | 20 ++++++++++++++++++++
 src/combinator/tests.rs  | 23 +++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/src/combinator/branch.rs b/src/combinator/branch.rs
index 2a712ec1..e0df48f3 100644
--- a/src/combinator/branch.rs
+++ b/src/combinator/branch.rs
@@ -112,6 +112,26 @@ pub fn permutation<I: Stream, O, E: ParserError<I>, List: Permutation<I, O, E>>(
     trace("permutation", move |i: &mut I| l.permutation(i))
 }
 
+impl<const N: usize, I: Stream, O, E: ParserError<I>, P: Parser<I, O, E>> Alt<I, O, E> for [P; N] {
+    fn choice(&mut self, input: &mut I) -> PResult<O, E> {
+        let mut error = None;
+
+        let start = input.checkpoint();
+        for branch in self {
+            input.reset(start.clone());
+            match branch.parse_next(input) {
+                Err(ErrMode::Backtrack(e)) => error = Some(e),
+                res => return res,
+            }
+        }
+
+        match error {
+            Some(e) => Err(ErrMode::Backtrack(e.append(input, ErrorKind::Alt))),
+            None => Err(ErrMode::assert(input, "`alt` needs at least one parser")),
+        }
+    }
+}
+
 macro_rules! alt_trait(
   ($first:ident $second:ident $($id: ident)+) => (
     alt_trait!(__impl $first $second; $($id)+);
diff --git a/src/combinator/tests.rs b/src/combinator/tests.rs
index 62dc420e..9d2b49d8 100644
--- a/src/combinator/tests.rs
+++ b/src/combinator/tests.rs
@@ -13,6 +13,7 @@ use crate::stream::Stream;
 use crate::token::take;
 use crate::unpeek;
 use crate::IResult;
+use crate::PResult;
 use crate::Parser;
 use crate::Partial;
 
@@ -648,6 +649,28 @@ fn alt_incomplete() {
     );
 }
 
+#[test]
+fn alt_array() {
+    fn alt1<'i>(i: &mut &'i [u8]) -> PResult<&'i [u8]> {
+        alt(["a", "bc", "def"]).parse_next(i)
+    }
+
+    let i = &b"a"[..];
+    assert_eq!(alt1.parse_peek(i), Ok((&b""[..], (&b"a"[..]))));
+
+    let i = &b"bc"[..];
+    assert_eq!(alt1.parse_peek(i), Ok((&b""[..], (&b"bc"[..]))));
+
+    let i = &b"defg"[..];
+    assert_eq!(alt1.parse_peek(i), Ok((&b"g"[..], (&b"def"[..]))));
+
+    let i = &b"z"[..];
+    assert_eq!(
+        alt1.parse_peek(i),
+        Err(ErrMode::Backtrack(error_position!(&i, ErrorKind::Tag)))
+    );
+}
+
 #[test]
 fn permutation_test() {
     #[allow(clippy::type_complexity)]