Chapter 12: Strengthen Rust Type Boundaries

Work in progress. This course has not yet received the author’s final audit. Chapters, exercises, and commands may still change before the course is marked complete.

The engine’s runtime behavior is complete. This chapter makes its Rust ownership and sharing claims executable without changing expression results.

Prerequisites: Chapter 11, Any, trait objects, threads, and lifetime variance.

By the end of this chapter, you will:

• return opaque borrowed array iterators;
• recover concrete expressions through checked Any downcasts; and
• prove Send + Sync, captured-state, and lifetime-shortening boundaries.

cargo x copy-test --chapter 12
cargo test -p type-exercise-starter chapter_12 --locked

The first run should fail on one or more iterator, trait-object, thread, or lifetime guarantees.

Checkpoint 1: keep iterator storage private

• Target: type-exercise-starter/src/array.rs::Array::iter and type-exercise-starter/src/array/iterator.rs::ArrayIterator.
• Change: return impl Iterator<Item = Option<Self::RefItem<'_>>> while preserving borrowed strings and nullable rows.
• Preserve: callers do not name the concrete iterator type.
• Run: the Chapter 12 focused test and doctests.
• Passing means: integer and string iteration keep their Chapter 1 ownership behavior.

Checkpoint 2: recover a concrete expression safely

• Target: type-exercise-starter/src/expression.rs::Expression: Any + Send + Sync.
• Change: upcast a trait object to dyn Any and use downcast_ref for checked recovery.
• Preserve: mismatches return None; never cast raw pointers.
• Run: the focused test.
• Passing means: erased objects can be inspected without weakening object safety.

Checkpoint 3: prove sharing and variance

• Target: type-exercise-starter/src/binder.rs::FunctionRegistry::{register, register_unary, register_binary, register_ternary} and type-exercise-starter/src/column.rs::ColumnViewImpl<'a>.
• Change: require captured factories to be Send + Sync + 'static and demonstrate shortening a valid column borrow.
• Preserve: a borrow cannot be lengthened or escape its backing array.
• Run: focused, cumulative, and compile-fail doctests.
• Passing means: expressions and registries can be shared across worker threads while views remain tied to their data.

Required and extension work

Opaque iteration, checked recovery, thread-safety, and covariance are required. Unsafe downcasts, custom executors, and arbitrary lifetime conversion helpers are not.

cargo test -p type-exercise-starter chapter_12 --locked
cargo test -p type-exercise-starter --doc --locked
cargo test -p type-exercise-starter --lib --locked

Next: Chapter 13 adds a batch async boundary.