Use pattern_single instead of pattern in mbe pat

crates/mbe/src/tests.rs

@@ -1003,6 +1003,18 @@ fn test_underscore() {
     .assert_expand_items(r#"foo! { => }"#, r#"0"#);
 }
 
+#[test]
+fn test_vertical_bar_with_pat() {
+    parse_macro(
+        r#"
+            macro_rules! foo {
+                 (| $pat:pat | ) => { 0 }
+            }
+    "#,
+    )
+    .assert_expand_items(r#"foo! { | x | }"#, r#"0"#);
+}
+
 #[test]
 fn test_lifetime() {
     parse_macro(

crates/parser/src/grammar.rs

@@ -55,7 +55,7 @@ pub(crate) mod fragments {
     use super::*;
 
     pub(crate) use super::{
-        expressions::block_expr, paths::type_path as path, patterns::pattern, types::type_,
+        expressions::block_expr, paths::type_path as path, patterns::pattern_single, types::type_,
     };
 
     pub(crate) fn expr(p: &mut Parser) {

crates/parser/src/lib.rs

@@ -112,7 +112,7 @@ pub fn parse_fragment(
         FragmentKind::Path => grammar::fragments::path,
         FragmentKind::Expr => grammar::fragments::expr,
         FragmentKind::Type => grammar::fragments::type_,
-        FragmentKind::Pattern => grammar::fragments::pattern,
+        FragmentKind::Pattern => grammar::fragments::pattern_single,
         FragmentKind::Item => grammar::fragments::item,
         FragmentKind::Block => grammar::fragments::block_expr,
         FragmentKind::Visibility => grammar::fragments::opt_visibility,