The Valen programming language, an experimental project aiming to provide memory-safe mutable aliasing, is implementing a borrow checker that operates by walking through functions backwards. While most compilers and borrow checkers (such as those in Rust or Vale) perform a forward pass to determine region inference or safety, Valen's architecture processes the function from the final statement back to the beginning. This reversal is designed to address specific technical hurdles in memory safety, particularly regarding the identification of the last point of use for a variable and the complexities of loop-based invalidation.
The mechanism relies on a multi-phase compilation process. After parsing the source into an Abstract Syntax Tree (AST) and performing static typing, the compiler performs 'static grouping.' This phase determines exactly what part of a variable or data structure a reference points to, effectively embedding 'path' information into the variable's type. In the final safety-checking phase, the compiler performs the backward scan. It maintains a 'pending' list of variables that are used later in the function. When the scan encounters a 'USE' event, the variable is added to the pending list. When it encounters a 'MUTATE' event, the compiler checks to ensure that no variables in the pending list are invalidated by that mutation. If a mutation affects a pending variable, the compiler flags a use-after-free error. This allows the compiler to identify conflicts immediately because it already knows which variables must remain valid until the end of the function.
One of the primary motivations for this reverse approach is the handling of loops and the avoidance of complex 'fetch and navigate' operations. In a forward scan, identifying the last use of a variable can be difficult if that use is nested within a loop or requires looking ahead to see if a variable is ever 'revived' by a subsequent assignment. A backward scan naturally identifies the most recent use first, simplifying the bookkeeping required to determine if a variable is still valid. However, this approach faces its own set of challenges, such as maintaining performance when scanning code multiple times for different proof goals. By moving away from the constraint-solving models used in languages like Rust, Valen seeks a more syntax-directed algorithm that simplifies the rules of memory safety while maintaining compatibility with the Rust ecosystem.