[dbernheisel]

Summary

• Stop the eager allocation of an _ENV upvalue at the entry of each nested function in Lua.Compiler.Scope.
• A closure now captures _ENV only at its first reference to a global. The existing upvalue chain (resolve_env_ref/1 and ensure_upvalue/4) does this.
• Upvalue numbers now match PUC-Lua. A closure over local c has c in slot 1. A closure that reads c and then print has c in slot 1 and _ENV in slot 2.
• debug.setupvalue(f, 1, v) now changes c. Before, it changed _ENV.
• Update the tests that pinned the old numbering. A real lua binary gave the new expected values.
• Add tests for first-use order, for _ENV forwarding through a parent that does not use globals, for an _ENV parameter, and for debug.setupvalue in a later evaluation.
• Update two comments that described the eager capture.

Evidence

iex

# main (3f2aead)
iex(1)> {_, lua} = Lua.eval!(Lua.new(sandboxed: []), "local c = 1 function getc() return c end")
iex(2)> {name, lua} = Lua.eval!(lua, "return debug.setupvalue(getc, 1, 99)")
{["_ENV"], #Lua<>}
iex(3)> Lua.eval!(lua, "return getc()") |> elem(0)
[1]

# this branch
iex(1)> {_, lua} = Lua.eval!(Lua.new(sandboxed: []), "local c = 1 function getc() return c end")
iex(2)> {name, lua} = Lua.eval!(lua, "return debug.setupvalue(getc, 1, 99)")
{["c"], #Lua<>}
iex(3)> Lua.eval!(lua, "return getc()") |> elem(0) |> hd()
99

PUC-Lua 5.5.1 gives the same result for the same source: debug.setupvalue(getc, 1, 99) returns c, and getc() returns 99.