From 49f6bd2bdefe91c0df15798c315d4af47f9f35b3 Mon Sep 17 00:00:00 2001
From: silvanshade <silvanshade@users.noreply.github.com>
Date: Tue, 28 May 2024 09:02:34 -0600
Subject: [PATCH] Refactor `dispatch!`; fixes #344

---
 src/combinator/branch.rs |  2 +
 src/macros/dispatch.rs   | 82 ++++++++++++++++++++++++++++++++++++----
 2 files changed, 76 insertions(+), 8 deletions(-)

diff --git a/src/combinator/branch.rs b/src/combinator/branch.rs
index 8913dcb9..ec0b6f7d 100644
--- a/src/combinator/branch.rs
+++ b/src/combinator/branch.rs
@@ -3,6 +3,8 @@ use crate::error::{ErrMode, ErrorKind, ParserError};
 use crate::stream::Stream;
 use crate::*;
 
+#[doc(hidden)]
+pub use crate::__dispatch_cases;
 #[doc(inline)]
 pub use crate::dispatch;
 
diff --git a/src/macros/dispatch.rs b/src/macros/dispatch.rs
index b763d4ba..3fe2846f 100644
--- a/src/macros/dispatch.rs
+++ b/src/macros/dispatch.rs
@@ -37,19 +37,85 @@
 ///
 /// assert_eq!(escaped.parse_peek("\\nHello"), Ok(("Hello", '\n')));
 /// ```
+#[macro_export]
+#[doc(hidden)] // forced to be visible in intended location
+macro_rules! __dispatch_cases {
+    ([ ] [ $i:tt $initial:tt $($acc:tt)* ]) => {
+        match $initial {
+            $($acc)*
+        }
+    };
+    ([ $pat:pat $(if $pred:expr)? => $expr:expr $(,)? ] [ $i:tt $($acc:tt)* ]) => {
+        $crate::combinator::__dispatch_cases!([ ] [ $i $($acc)* $pat $(if $pred)? => $expr.parse_next($i), ])
+    };
+    ([ $pat:pat $(if $pred:expr)? => $expr:expr, $($rest:tt)+ ] [ $i:tt $($acc:tt)* ]) => {
+        $crate::combinator::__dispatch_cases!([ $($rest)+ ] [ $i $($acc)* $pat $(if $pred)? => $expr.parse_next($i), ])
+    };
+    // NOTE: Must be the last rule otherwise the `{ ... },` (trailing comma) will fail to parse.
+    ([ $pat:pat $(if $pred:expr)? => { $($body:tt)* } $($rest:tt)+ ] [ $i:tt $($acc:tt)* ]) => {
+        $crate::combinator::__dispatch_cases!([ $($rest)+ ] [ $i $($acc)* $pat $(if $pred)? => { $($body)* }.parse_next($i), ])
+    };
+}
+
 #[macro_export]
 #[doc(hidden)] // forced to be visible in intended location
 macro_rules! dispatch {
-    ($match_parser: expr; $( $pat:pat $(if $pred:expr)? => $expr: expr ),+ $(,)? ) => {
-        $crate::combinator::trace("dispatch", move |i: &mut _|
-        {
+    ($match_parser:expr; $($cases:tt)+) => {
+        $crate::combinator::trace("dispatch", move |i: &mut _| {
             use $crate::Parser;
             let initial = $match_parser.parse_next(i)?;
-            match initial {
-                $(
-                    $pat $(if $pred)? => $expr.parse_next(i),
-                )*
-            }
+            $crate::combinator::__dispatch_cases!([ $($cases)+ ] [ i initial ])
         })
+    };
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn parse_block_with_trailing_comma() {
+        use crate::combinator::{dispatch, empty, fail, preceded};
+        use crate::prelude::*;
+        use crate::token::any;
+
+        #[allow(unused)]
+        fn escaped(input: &mut &str) -> PResult<char> {
+            preceded('\\', escape_seq_char).parse_next(input)
+        }
+
+        #[allow(unused)]
+        fn escape_seq_char(input: &mut &str) -> PResult<char> {
+            dispatch! {any;
+                'b' => {
+                    empty.value('\u{8}')
+                },
+                'f' => empty.value('\u{c}'),
+                _ => fail::<_, char, _>,
+            }
+            .parse_next(input)
+        }
+    }
+
+    #[test]
+    fn parse_block_sans_trailing_comma() {
+        use crate::combinator::{dispatch, empty, fail, preceded};
+        use crate::prelude::*;
+        use crate::token::any;
+
+        #[allow(unused)]
+        fn escaped(input: &mut &str) -> PResult<char> {
+            preceded('\\', escape_seq_char).parse_next(input)
+        }
+
+        #[allow(unused)]
+        fn escape_seq_char(input: &mut &str) -> PResult<char> {
+            dispatch! {any;
+                'b' => {
+                    empty.value('\u{8}')
+                }
+                'f' => empty.value('\u{c}'),
+                _ => fail::<_, char, _>,
+            }
+            .parse_next(input)
+        }
     }
 }