`let_underscore_lock` false positive

Lint name: let_underscore_lock

I tried this code:

let _ = task_sender.lock().map(|mut s| s.take().map(|s| s.send(())));

The type of this is a Result as shown by this rustc warning if I remove the binding:

warning: unused `Result` that must be used
   --> bee-api/bee-rest-api/src/endpoints/routes/api/plugins/debug/white_flag.rs:137:17
    |
137 |                 task_sender.lock().map(|mut s| s.take().map(|s| s.send(())));
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: this `Result` may be an `Err` variant, which should be handled

warning: 1 warning emitted

With the binding I get the following clippy warning

error: non-binding let on a synchronization lock
   --> bee-api/bee-rest-api/src/endpoints/routes/api/plugins/debug/white_flag.rs:154:13
    |
154 |             let _ = sender.lock().map(|mut s| s.take().map(|s| s.send(())));
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_lock

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clippy 0.1.52 (9bc8c42b 2021-05-09)

rustc 1.52.1 (9bc8c42bb 2021-05-09)
binary: rustc
commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
commit-date: 2021-05-09
host: x86_64-apple-darwin
release: 1.52.1
LLVM version: 12.0.0

Start with the let_underscore_lock lint and reproduce the warning using the Rust snippet in the issue. Trace how this expression is classified, then verify that the reported case no longer produces a false positive while the unused Result warning remains applicable.