[pfdietz]

Or I believe if it can determine the closure has dynamic extent, for example if it's passed to a standard function for which it knows the closure will not escape.

[BoingBoomTschak]

Indeed, it's newish (from this summer https://github.com/sbcl/sbcl/releases/tag/sbcl-2.5.6) and mainly thanks to C. Zhang's work.

https://github.com/sbcl/sbcl/commit/021414445224e692ebb9c48c... https://github.com/sbcl/sbcl/commit/2c56c5901df1a5ea98ae19e9...