Borrow checking

Borrow checking lives under check/borrow_check/. Its top-level judgment is borrow_check, which checks that the loans issued while executing a basic block are respected.

borrow_check

At the top level, borrow checking works over:

• a TypeckEnv
• a set of assumptions
• a FlowState
• a Block

borrow_check delegates to borrow_check_block, which walks the block and updates the flow state statement by statement.

FlowState

FlowState is the flow-sensitive state threaded through borrow checking. It contains:

• scopes
• current
• breaks
• continues

Its current field is a PointFlowState, which tracks:

• loans_live
• outlives

Loans

A Loan represents a borrow produced by an expression like &'a place. It records:

• the lifetime of the borrow
• the borrowed place
• the borrow kind

Borrow checking adds loans to the current flow state and checks later accesses against the live loans.

Liveness

The borrow checker computes liveness with LivePlaces and the LiveBefore trait.

This is used to determine which places must be valid before a statement or expression, including at break and continue targets.

Outlives

Borrow checking also accumulates pending outlives constraints, represented as PendingOutlives { a, b }.

The outlives.rs code verifies these constraints:

• existential variables succeed trivially
• universal variables must be justified by the assumptions