Any upstreaming was unlikely to ever be acceptable to the PUC-Rio folks.
PUC-Rio Lua uses C (with a tiny bit of C++ for stack unwinding on errors if folks don't want to use `longjmp()`,) Luau is strictly C++. Luau rewrites quite a lot of the core structures, and they aren't compatible. Further, Luau is a fork of PUC-Rio Lua _5.1_ (the current is 5.4 I think?) and intentionally picks and chooses features from later Lua versions. Even things like improvements to the core VM interpreter loop wouldn't be upstreamable, because Lua 5.2+ has a fundamentally different model to allow for yielding in metamethods. PUC-Rio would not accept changes other than fixes for critical bugs in Lua 5.1.
I like both PUC-Rio Lua and Luau, but a hard fork with no upstreaming was the only option here. The architectural differences of the VMs are so large now that it would amount to PUC-Rio adopting Luau. Supposing there is some tiny bit that PUC-Rio is interested in, they can cherry-pick it out of Luau, since they use the same license.
PUC-Rio Lua uses C (with a tiny bit of C++ for stack unwinding on errors if folks don't want to use `longjmp()`,) Luau is strictly C++. Luau rewrites quite a lot of the core structures, and they aren't compatible. Further, Luau is a fork of PUC-Rio Lua _5.1_ (the current is 5.4 I think?) and intentionally picks and chooses features from later Lua versions. Even things like improvements to the core VM interpreter loop wouldn't be upstreamable, because Lua 5.2+ has a fundamentally different model to allow for yielding in metamethods. PUC-Rio would not accept changes other than fixes for critical bugs in Lua 5.1.
I like both PUC-Rio Lua and Luau, but a hard fork with no upstreaming was the only option here. The architectural differences of the VMs are so large now that it would amount to PUC-Rio adopting Luau. Supposing there is some tiny bit that PUC-Rio is interested in, they can cherry-pick it out of Luau, since they use the same license.