Borrow Checker

• Scalars, references, raw pointers, slices, ranges, and function values are copyable.
• Strings, collections, structs, enums, and aggregates containing owned values move by default.
• Moving a non-copy value makes the old binding unavailable.
• Reinitializing a binding clears the moved state.

• Binding one non-copy value to another can move it.
• Passing a value to a by-value parameter can move it.
• A self or mut self receiver consumes the receiver.
• A move closure owns captured non-copy values.

The frontend borrow checker treats a let-bound borrow as live until the end of its block. That can reject some safe programs, but it avoids accepting programs that could dangle or mutate through aliases.

• &mut x forbids other access to x while live.
• &x forbids writes to x while live.
• Field-granular places let &user.name avoid locking user.age.
• Returning a reference to a local is rejected.

Some borrow facts cannot be computed soundly from the surface tree alone. Loops, back-edges, and reborrow chains need explicit control-flow and place information, which is why the more precise pass belongs on MIR.

• NLL ends a borrow at last use instead of end of block.
• Reborrows track that a new reference reaches through an older one.
• The lexical pass remains the conservative guard until all relevant programs lower to MIR.