debug_assert!(_old.is_none())
}
+ /// Installs the remapping `remap` in scope while `f` is being executed.
+ /// This causes references to the `LocalDefId` keys to be changed to
+ /// refer to the values instead.
+ ///
+ /// The remapping is used when one piece of AST expands to multiple
+ /// pieces of HIR. For example, the function `fn foo<'a>(...) -> impl Debug + 'a`,
+ /// expands to both a function definition (`foo`) and a TAIT for the return value,
+ /// both of which have a lifetime parameter `'a`. The remapping allows us to
+ /// rewrite the `'a` in the return value to refer to the
+ /// `'a` declared on the TAIT, instead of the function.
fn with_remapping<R>(
&mut self,
remap: FxHashMap<LocalDefId, LocalDefId>,