This pass is responsible for inferring borrow annotations to the parameters of functions and join points. When a parameter is marked as borrowed, the caller can be sure that the function will not decrement the reference count of the parameter. Thus, if the value is still used after the call, the caller does not need to inc it before calling in order to ensure that it stays alive.

The inference is done with a data flow analysis which initially assumes all arguments are passed as borrowed and subsequently refines this by marking parameters as owned as required and propagating the information throughout the program. The analysis has two reasons for marking a parameter as owned. Some parameters need to be owned for correctness, while others are heuristically marked as owned to reduce reference counting pressure inside of the function.

The correctness ones are the following:

• We preserve tail calls to recursive functions and join points by ensuring we never have to insert a dec after a tail call. This is done by marking parameters of tail-called functions/jps as owned if a respective argument at a call site is owned.
• We ensure that values which are subject to reset-reuse are owned so their reference count accurately reflects the real amount of references.

For performance we:

• propagate through annotations of functions that we call. That is if we call another function f which has an owned parameter we ensure that the argument we are passing is owned as well. In particular when f is partially applied we ensure that all arguments are owned.
• When passing a parameter into a constructor we ensure it is passed as owned so we do not have to inc before calling the constructor.

Furthermore, the performance related heuristics will be ignored if there is a user-defined borrow annotation. This allows the user to override the ABI of their function in exchange for potentially worse code in the function that is being analyzed. Doing so can benefit other functions that call the current one and thus reduce overall RC pressure.

def Lean.Compiler.LCNF.inferBorrow : Pass

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