UserDataBorrowError when using `send` feature with Rust 1.93

[wez]

I noticed this when updating the toolchain; reproducing code below. The actual code uses a different struct and isn't simply an i64; this is just a minimal reproducing example.

With rustc 1.92, this prints "ok".

With rustc 1.93, this prints:

   BadArgument { to: Some("Delta.__add"), pos: 1, name: Some("self"),
                 cause: UserDataBorrowError }

Toggling off the send feature in Cargo.toml makes the error go away on
rustc 1.93 as well, suggesting the issue is in the Send-aware borrow
tracking path inside mlua.

The __add metamethod is registered with add_meta_method (which only
needs a shared borrow of this) and the operand is taken as
UserDataRef<Delta> (also a shared borrow). When both Lua operands refer
to the same underlying userdata (e.g. local x = d; return x + x), the
argument extraction reports a borrow conflict against the still-live
this borrow.

Adding two different userdata values of the same type works fine; only
same + same fails.

use mlua::{Lua, MetaMethod, UserData, UserDataMethods, UserDataRef};

#[derive(Clone, Copy)]
struct Delta(i64);

impl UserData for Delta {
    fn add_methods<M: UserDataMethods<Self>>(methods: &mut M) {
        methods.add_meta_method(MetaMethod::Add, |_, this, other: UserDataRef<Delta>| {
            Ok(Delta(this.0 + other.0))
        });
    }
}

fn main() -> mlua::Result<()> {
    let lua = Lua::new();
    let globals = lua.globals();
    globals.set("d1", Delta(1))?;
    globals.set("d2", Delta(2))?;

    // Two distinct userdata values: works on both 1.92 and 1.93.
    lua.load("local r = d1 + d2 assert(r) print('distinct ok')")
        .exec()?;

    // Same underlying userdata on both sides: works on 1.92, fails on 1.93
    // when the `send` feature is enabled.
    lua.load("local r = d1 + d1 print('same ok')").exec()?;

    Ok(())
}

[dependencies]
mlua = { version = "0.11", features = ["vendored", "lua54", "async", "send", "serialize"] }

[wez]

Poking at this a bit, it's due to the is_sync trick no longer working in rust 1.93. Cherry-picking and fixing up 63a255b resolves this, but it is a breaking change. I'm not in a rush to upgrade to 1.93 or beyond, so we need not go to any special lengths to fix this just for me.

[khvzak]

Unfortunately yes, it's a breaking change and a regression in the compiler (I probably should not have written the code that depends on compiler internals).

It's already included to v0.12.0-rc.1