Module rustc_ast::ptr

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.

Structs

• P
  An owned smart pointer.

Functions

• P
  Construct a P<T> from a T value.