Module syntax::ptrUnstable

The AST pointer

Provides P<T>, a frozen owned smart pointer, as a replacement for @T in the AST.

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 ExprAddrOf node taking a mutable borrow). The only reason @T in the AST hasn't caused issues is because of inefficient folding passes which would always deduplicate any such shared nodes. Even if the AST were to switch to an arena, this would still hold, i.e. it couldn't use &'a T, but rather a wrapper like P<'a, T>.

• 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 can 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.