The AST pointer.
P<T>, a frozen owned smart pointer.
Identity: sharing AST nodes is problematic for the various analysis passes (e.g., one may be able to bypass the borrow checker with a shared
ExprKind::AddrOfnode taking a mutable borrow).
P<T>disallows mutating its inner
Box<T>(unless it contains an
Unsafeinterior, but that may be denied later). This mainly prevents mistakes, but can also enforces a kind of "purity".
Efficiency: folding can reuse allocation space for
Vec<T>, the latter even when the input and output types differ (as it would be the case with arenas or a GADT AST using type parameters to toggle features).
P<T>provides a fixed interface -
map- which can remain fully functional even if the implementation changes (using a special thread-local heap, for example). Moreover, a switch to, e.g.,
P<'a, T>would be easy and mostly automated.
An owned smart pointer.