[scythe]

Lua 5.2 introduced a new scoping rule (_ENV) that impacts the ability to implement efficient interpreters, which is why LuaJIT diverged from the main implementation. Insofar as Lua is notoriously the "fastest scripting language", Lua 5.1 is the "fastest Lua". I personally hope that the author eventually targets "LuaJIT flavored Lua", which incorporates some additional features from newer Lua without compromising the performance.

Edit: see replies

[naniwaduni]

The functional impact of _ENV in Lua 5.2 below the parser level is simply that globals/function environments no longer exist, becoming syntactical sugar for access to a closed-over table. There's a fairly direct (as in, you could write a compiler for it) source-to-source transformation on a 5.2 program (that doesn't use debug or string.dump) where you replace every "global" access with explicit access to _ENV, resulting in a 5.1 with the same semantics as the 5.2 source. This came along with ABI breakage (since PUC then removed all the interfaces that interact with function environments), but at the design level the change only relaxes constraints on interpreter implementation.

5.3 does change up the data model in ways that are incompatible with certain desirable interpreter features (you generally have to choose between 64-bit integers and NaN-boxing), but the incompatibility of 5.2 specifically with efficient interpreter design is at best based on a misunderstanding based on pre-release discussion on the mailing list.

(Real talk though, Lua 5.2 was being floated around the same time period that Mike Pall had committed to LuaJIT 2 as a full rewrite. You can read a bit in between the lines of just the RC dates.)

[wahern]

_ENV is the same thing as setfenv/getfenv in Lua 5.1, except its lexical. Unless the use of function environments also make LuaJIT slower (I doubt it as that would implicate _G and a host of other behaviors, all of which LuaJIT is renowned for making work performantly), I think the best way to interpret the original sentiment regarding _ENV is that it would be a PITA to support in a backward compatible manner. There have also been several patches over the years to add _ENV support to LuaJIT, and AFAICT they've all been relatively simple (but not necessarily simple for someone who is firmly of the opinion that Lua should have been frozen at 5.1).

[sigzero]

Interesting. Thanks for answering.