Confused with lua.context closure lifetimes

[elipp]

Consider the following helper function:

fn set_global<'lua, T>(lua: &Lua, key: &str, value: T) -> LuaResult<()> 
where T: ToLua<'lua>
{
    lua.context(move |lua_ctx| {
        let globals = lua_ctx.globals();
        globals.set(key, value.to_lua(lua_ctx).unwrap())
    })
}

This results in the following:

error[E0521]: borrowed data escapes outside of closure_extensions                                       
   --> src/lib.rs:874:26
    |
869 | fn set_global<'lua, T>(lua: &Lua, key: &str, value: T) -> LuaResult<()> 
    |               ---- lifetime `'lua` defined here
...
872 |     lua.context(move |lua_ctx| {
    |                       -------
    |                       |
    |                       `lua_ctx` is a reference that is only valid in the closure body
    |                       has type `LuaContext<'1>`
873 |         let globals = lua_ctx.globals();
874 |         globals.set(key, value.to_lua(lua_ctx).unwrap())
    |                          ^^^^^^^^^^^^^^^^^^^^^
    |                          |
    |                          `lua_ctx` escapes the closure body here
    |                          argument requires that `'1` must outlive `'lua`
    |
    = note: requirement occurs because of the type `LuaContext<'_>`, which makes the generic argument `'_` invariant
    = note: the struct `LuaContext<'lua>` is invariant over the parameter `'lua`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

How would one create such a helper? I can work around it by making the function use a &Context<'lua> instead of &Lua, but for my particular setting, having to do lua.context(...) every time feels boilerplatey =)

[jugglerchris]

Hi,
The problem is that your lifetime 'lua is unrelated to the fresh lifetime created for the lua.context callback - and you can't mention that in set_global's signature.

What you need to do is require that T implements ToLua<'a> for any lifetime 'a, using a higher rank trait bound:

pub fn set_global<T>(lua: &Lua, key: &str, value: T) -> LuaResult<()> 
where T: for<'a> ToLua<'a>
{
// same body as before
}

The above compiles for me. Hope this helps!

[elipp]

Whoa! I've never seen that before.

Thank you for the fair answer 👍