Julia 1.13 skips system-image objects during GC marking and replaces the hash function with RapidhashNano

Sources: release highlights, HN discussion
Summary: Objects in the system image and package images are loaded permanently marked, so a full collection scales with the heap the program created rather than with the amount of code loaded. The release reports a full collection in a bare session dropping from 35 ms to 2 ms, and from 187 ms to 68 ms with GLMakie loaded. The hash function is replaced with RapidhashNano.
Why it matters: Two changes need action from package authors rather than users: the default hash seed changed, so a custom hash method must accept the seed as an argument and never default it, and Pkg.test no longer passes --check-bounds=yes, so test processes inherit the bounds-checking mode from the parent session.