Add a static size assertion for `MatcherPos`.

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -207,6 +207,10 @@ struct MatcherPos<'root, 'tt> {
     stack: SmallVec<[MatcherTtFrame<'tt>; 1]>,
 }
 
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(MatcherPos<'_, '_>, 192);
+
 impl<'root, 'tt> MatcherPos<'root, 'tt> {
     /// Adds `m` as a named match for the `idx`-th metavar.
     fn push_match(&mut self, idx: usize, m: NamedMatch) {