Fix sync lock let binding

[cyqsimon]

Current master is rejected by rust 1.68.0:

    Checking https v1.12.5 (/foo/bar/http)
error: non-binding let on a synchronization lock
   --> src/main.rs:185:9
    |
185 |     let _ = end_handler.wait(mx.lock().unwrap()).unwrap();
    |         ^   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this binding will immediately drop the value assigned to it
    |         |
    |         this lock is not assigned to a binding and is immediately dropped
    |
    = note: `#[deny(let_underscore_lock)]` on by default
help: consider binding to an unused variable to avoid immediately dropping the value
    |
185 |     let _unused = end_handler.wait(mx.lock().unwrap()).unwrap();
    |         ~~~~~~~
help: consider immediately dropping the value
    |
185 |     drop(end_handler.wait(mx.lock().unwrap()).unwrap());
    |     ~~~~~                                             +

error: could not compile `https` due to previous error

This is a simple patch for it.

[nabijaczleweli]

Duplicate of #151.

[nabijaczleweli]

Still, why the hell is this a New Lint that refuses old, fully correct code?

[cyqsimon]

Ah okay sorry. I only checked latest tag not master develop branch.

Still, why the hell is this a New Lint that refuses old, fully correct code?

I guess this lint makes more sense in the more general case when the internal type isn't (). Although I do agree that they should have made it a warning instead of a hard error.

[nabijaczleweli]

Fix released in v1.12.6.