Allow leading pipe in `matches!()` patterns.

This is allowed in `match` statement, and stated in https://internals.rust-lang.org/t/leading-pipe-in-core-matches/14699/2 that it should be allowed in these macros too.

library/core/src/macros/mod.rs

@@ -141,7 +141,7 @@ macro_rules! assert_ne {
 #[allow_internal_unstable(core_panic)]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro assert_matches {
-    ($left:expr, $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => ({
+    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => ({
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -153,7 +153,7 @@ pub macro assert_matches {
             }
         }
     }),
-    ($left:expr, $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => ({
+    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => ({
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -321,7 +321,7 @@ pub macro debug_assert_matches($($arg:tt)*) {
 #[macro_export]
 #[stable(feature = "matches_macro", since = "1.42.0")]
 macro_rules! matches {
-    ($expression:expr, $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
+    ($expression:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
         match $expression {
             $( $pattern )|+ $( if $guard )? => true,
             _ => false

library/core/tests/macros.rs

@@ -12,3 +12,9 @@ fn assert_escape() {
 fn assert_ne_trailing_comma() {
     assert_ne!(1, 2,);
 }
+
+#[rustfmt::skip]
+#[test]
+fn matches_leading_pipe() {
+    matches!(1, | 1 | 2 | 3);
+}