`manual_map` does not consider the borrowing relationship

Summary

In the example below, the suggestion given by clippy is wrong. Because of the conflicting borrows in the Scrutinee and Arm of the match expression.

Lint Name

manual_map

Reproducer

I tried this code (playground):

struct A(i32);

impl A {
    fn val(&self) -> Option<&i32> {
        Some(&self.0)
    }
    
    fn do_something(&mut self) -> &'static str {
        "result"
    }
}

let mut a = A(1);
let _ = match a.val() {
    None => None,
    Some(_) => Some(a.do_something()),
};

I saw this happen:

warning: manual implementation of `Option::map`
  --> src/main.rs:15:13
   |
15 |       let _ = match a.val() {
   |  _____________^
16 | |         None => None,
17 | |         Some(_) => Some(a.do_something()),
18 | |     };
   | |_____^ help: try this: `a.val().map(|_| a.do_something())`
   |
   = note: `#[warn(clippy::manual_map)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_map

Version

No response

Additional Labels

No response

Start with the manual_map lint and the Rust reproducer in this issue, then run Clippy against it. The fix is done when Clippy no longer suggests the invalid Option::map rewrite and the valid manual-map cases remain covered.