Reasons for !Send.

[navneetankur]

mlua::Lua is !Send by default. What is the reason for it except for existence of Owned types like OwnedTable?

Internally I see it uses Rc. But it should be fine if nothing Rc-like can be taken out of mlua::Lua correct?
And what is difference between with and without [send] feature. Other than Rc/Arc and Owned types?

Basically If I wrap mlua::Lua in Exclusive, can I forcefully mark it as Send through newtype?

[khvzak]

mlua::Lua is !Send by default. What is the reason for it except for existence of Owned types like OwnedTable?
And what is difference between with and without [send] feature. Other than Rc/Arc and Owned types?

send feature flag adds Send requirements for Rust callbacks and userdata types stored in Lua via MaybeSend trait.
Given that Lua itself is single threaded, not having Send requirement by default makes a lot of sense.

Internally I see it uses Rc. But it should be fine if nothing Rc-like can be taken out of mlua::Lua correct?

Could you point please where Rc (not Arc) is used?

[navneetankur]

send feature flag adds Send requirements for Rust callbacks and userdata types stored in Lua via MaybeSend trait.

Okay, thanks. I was wondering if it does something else other than marking it Send and disabling owned type returning fn.

Given that Lua itself is single threaded, not having Send requirement by default makes a lot of sense.

I don't understand this statement completely. From purely theoretical perspective single threaded means it should not be Sync. It can still be send like RefCell if it does not make a shallow clone which can be left behind in the current thread, if sent to another thread (the way Rc does). Of course there can be other non-general reason for something to not be Send like per thread platform specific setup before running or thread local storage or something.

Could you point please where Rc (not Arc) is used?

When I first asked this question a think I saw some logic which uses Rc if #[cfg(not(feature = "send"))]. Which would make sense, but made me wonder if !Send only due to use of Rc.
TypeId::of::<Rc<RefCell>>().