Index Selection and Lowering

Indexing has one semantic selection boundary in typecheck/indexing.rs. The selector receives the target type, index type, requested readonly or readwrite access, lexical generic requirements, and resolved coercion surface. It returns one direct array/slice/str projection, one lexical requirement, or one accessible receiver coercion to a built-in projection. AST shape and source spelling are not semantic fallbacks.

TypecheckIndexPlan is the immutable handoff to later stages. It records source spans, target, index, and element types, access capability, projection kind, requirement identity, and the optional conversion plan. Fact collection records readonly plans for value access and readwrite plans for assignment targets and &+ borrows. Generic specialization substitutes the plan and re-runs the same type-level selector against the concrete resolver; an unresolved requirement cannot reach native lowering.

Ordinary conversion facts contain the selected receiver coercion for nongeneric indexing. Generic lowering obtains the specialized conversion from the index plan. Slice lowering then invokes the coercion body exactly once and reuses the existing checked slice projection. Vec<T> therefore does not own a parallel indexing implementation or bounds-check path.

Aggregate element replacement uses a separate drop-aware staging service. It evaluates and materializes the selected index before the replacement, builds the replacement in an unowned temporary slot, copies the old element through the checked slice projection, destroys that old value, and transfers the replacement bytes into the element. An out-of-bounds index traps before destruction or overwrite. Copy aggregates retain their cheaper direct-copy path; move-only elements do not acquire a second lexical owner in either staging slot.

Integer binary lowering snapshots each deferred memory projection before lowering the next operand. Indexed values can require scratch registers for checked address calculation, so leaving such an operand deferred would let materializing the other operand overwrite its value and would violate left-to-right evaluation. Constants and stable local locations remain direct values and do not consume an unnecessary temporary.

The ownership model continues to describe an indexed borrow as a projection of the original container place. A coercion does not create independent storage: a borrow of values[index] retains the values loan, and a readwrite projection requires a writable source place.