Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
511be304d7
This PR enables support for respecting user provided borrow annotations. This allows user to mark arguments of their definitions or local functions with `(x : @&Ty)` and have the borrow inference try its best to preserve this annotation, thus potentially reducing RC pressure. Note that in some cases this might not be possible. For example, the compiler prioritizes preserving tail calls over preserving borrow annotations. A precise reasoning of why the compiler chose to make its inference decisions can be obtained with `trace.Compiler.inferBorrow`. The implementation consists of two parts: 1. A propagator in ToLCNF. This is required because the elaborator does not place the borrow annotations in the function binders themselves but just in type annotations of let binders/global declarations while LCNF expects them in the lambda variable binders themselves. Thus ToLCNF now implements a (very weak but strong enough for this purpose) propagator of the borrow annotations of a type annotation into the variable binders of the term affected by the annotations 2. A weakening of the InferBorrow heuristic. It now has a set of "forced" and "non-forced" reasons to mark a variable as owned instead of borrowed. If a variable is user annotated as borrowed, it will only be marked as owned if the reason is a forced one, e.g. preservation of tail calls. |
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