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`File.join` is a hotspot for common libraries such as Zeitwerk and Bootsnap. It has a fairly flexible signature, but 99% of the time it's called with just two (or a small number of) UTF-8 strings. If we optimistically optimize for that use case we can cut down a large number of type and encoding checks, significantly speeding up the method. The one remaining expensive check we could try to optimize is `str_null_check`. Given it's common to use the same base string for joining, we could memoize it. Also we could precompute it for literal strings. ``` compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71eaf) +PRISM [arm64-darwin25] built-ruby: ruby 4.1.0dev (2026-01-18T12:10:38Z spedup-file-join 069bab58d4) +PRISM [arm64-darwin25] warming up.... | |compare-ruby|built-ruby| |:-------------|-----------:|---------:| |two_strings | 2.475M| 9.444M| | | -| 3.82x| |many_strings | 551.975k| 2.346M| | | -| 4.25x| |array | 514.946k| 522.034k| | | -| 1.01x| |mixed | 621.236k| 633.189k| | | -| 1.02x| ```