fix(lua): allow nil values in serialized Lua arrays (#26329)

When we convert a Lua table to an Object, we consider the table a
"dictionary" if it contains only string keys, and an array if it
contains all numeric indices with no gaps. While rare, Lua tables can
have both strictly numeric indices and gaps (e.g. { [2] = 2 }). These
currently cannot be serialized because it is not considered an array.

However, we know the maximum index of the table and as long as all of
the keys in the table are numeric, it is still possible to serialize
this table as an array. The missing indices will have nil values.

1 files changed, 4 insertions, 0 deletions

diff --git a/test/functional/lua/api_spec.lua b/test/functional/lua/api_spec.lua
index d808693a9e..8c03eb60ec 100644
--- a/test/functional/lua/api_spec.lua
+++ b/test/functional/lua/api_spec.lua
@@ -245,4 +245,8 @@ describe('luaeval(vim.api.…)', function()
     eq('', funcs.luaeval('vim.api.nvim_replace_termcodes("", 0, 1.5, "test")'))
     eq('', funcs.luaeval('vim.api.nvim_replace_termcodes("", true, {}, {[vim.type_idx]=vim.types.array})'))
   end)
+
+  it('serializes sparse arrays in Lua', function()
+    eq({ [1] = vim.NIL, [2] = 2 }, exec_lua [[ return { [2] = 2 } ]])
+  end)
 end)