Add a size assertion for NamedMatchVec.

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -103,6 +103,10 @@ struct MatcherTtFrame<'tt> {
 
 type NamedMatchVec = SmallVec<[NamedMatch; 4]>;
 
+// 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!(NamedMatchVec, 72);
+
 /// Represents a single "position" (aka "matcher position", aka "item"), as
 /// described in the module documentation.
 #[derive(Clone)]