[makspll]

Lua values are read straight off the stack: LuaScriptValue and MultiLuaScriptValue implement mlua's from_stack hooks, so strings are read in place and tables are walked with the raw API (lua_next, lua_rawiter on Luau) without creating a Lua reference per string or table. Functions and userdata convert as before. After #717, half of a table's conversion was mlua creating those references.

lua_next raises an error when the table changed under it, which a __gc finalizer can do: converting an entry may allocate (a number key turned into a string) and so run a collection. Tables are therefore iterated inside a protected call, one per table (as Table::for_each does), so the error becomes a conversion error instead of jumping over the Rust frames of the conversion:

• Lua 5.2+: a plain lua_pcall of iterate_table, pushing a C function without upvalues allocates nothing there.
• Lua 5.1 / LuaJIT: pushing the function allocates and can itself raise, so mlua's Lua::exec_raw makes the call.
• Luau needs none: lua_rawiter raises no errors.

Same results on every variant: a test converts the same values both ways, and a table changed by a finalizer during its conversion is an error, on lua54, luau, lua51 and luajit.

Prebuilt binaries, 3 alternating rounds, two tables per call:

bench	main	this PR	with exec_raw on every variant	unprotected
function/call table args Lua	2.10 µs	1.38 µs (−34%)	1.59 µs	1.43 µs
function/call 4 args Lua	1.26 µs	0.96 µs (−24%)	1.09 µs	0.93 µs

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/languages/bevy_mod_scripting_lua/src/bindings/script_value.rs	23% smaller

[makspll]

/bench

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Bencher Report

Project	Bevy Scripting
Branch	agent/lua-stack-conversion
Testbed	linux-gha

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