Fix fail to parse :: for meta in mbe

Author: edwin0cheng

crates/mbe/src/tests.rs

@@ -954,7 +954,8 @@ fn test_meta() {
     .assert_expand_items(
         r#"foo! { cfg(target_os = "windows") }"#,
         r#"# [cfg (target_os = "windows")] fn bar () {}"#,
-    );
+    )
+    .assert_expand_items(r#"foo! { hello::world }"#, r#"# [hello :: world] fn bar () {}"#);
 }
 
 #[test]

crates/parser/src/grammar.rs

@@ -95,7 +95,7 @@ pub(crate) mod fragments {
                 // https://doc.rust-lang.org/reference/paths.html#simple-paths
                 // The start of an meta must be a simple path
                 match p.current() {
-                    IDENT | T![::] | T![super] | T![self] | T![crate] => p.bump_any(),
+                    IDENT | T![super] | T![self] | T![crate] => p.bump_any(),
                     T![=] => {
                         p.bump_any();
                         match p.current() {
@@ -105,6 +105,7 @@ pub(crate) mod fragments {
                         }
                         break;
                     }
+                    _ if p.at(T![::]) => p.bump(T![::]),
                     _ => break,
                 }
             }