Type Definition rustc_resolve::macros::MacroRulesScopeRef

pub(crate) type MacroRulesScopeRef<'a> = PtrKey<'a, Cell<MacroRulesScope<'a>>>;

macro_rules! scopes are always kept by reference and inside a cell. The reason is that we update scopes with value MacroRulesScope::Invocation(invoc_id) in-place after invoc_id gets expanded. This helps to avoid uncontrollable growth of macro_rules! scope chains, which usually grow lineraly with the number of macro invocations in a module (including derives) and hurt performance.