Module rustc_ast::ptr

Expand description

The AST pointer.

Provides P<T>, a frozen owned smart pointer.

Motivations and benefits

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::AddrOf node taking a mutable borrow).
Immutability: P<T> disallows mutating its inner T, unlike Box<T> (unless it contains an Unsafe interior, but that may be denied later). This mainly prevents mistakes, but also enforces a kind of “purity”.
Efficiency: folding can reuse allocation space for P<T> and 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).
Maintainability: P<T> provides a fixed interface - Deref, and_then and 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.