You probably saw this one coming. It's a restructuring of the Lisp reader so that it no longer uses recursion for reading nested data structures.

The primary motivation is eliminating the limitation of the C stack (and potential overflow crash). As a happy side-effect, the change improves reader performance by a few percent, exact amount depending on what is being read. This translates into a small but measurable speed-up in loading packages (.el and .elc), and in byte-compilation. The performance increase is both from removal of recursion and closer attention to performance.

Care has been taken to not change the reader behaviour, although some error handling may differ in unimportant ways. Some obvious bugs found during the conversion have been fixed: for example, #_ followed by whitespace now represents the interned empty symbol; previously, it gave an unintended empty symbol. A non-breaking space after a single dot now results in the dot token instead of the dot as a symbol.