False positive in needless_option_as_deref if Option cannot be moved

[Blaidd-Drwg]

Lint name:
needless_option_as_deref

I tried this code:

fn main() {
    let mut i = 0;
    let mut opt_vec = vec![Some(&mut i)];
    opt_vec[0].as_deref_mut().unwrap();
}

Clippy complains:

warning: derefed type is same as origin
 --> src/main.rs:4:5
  |
4 |     opt_vec[0].as_deref_mut().unwrap();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `opt_vec[0]`
  |
  = note: `#[warn(clippy::needless_option_as_deref)]` on by default

However, when applying the suggestion, the build fails:

error[E0507]: cannot move out of index of `Vec<Option<&mut i32>>`
 --> src/main.rs:4:5
  |
4 |     opt_vec[0].unwrap();
  |     ^^^^^^^^^^ move occurs because value has type `Option<&mut i32>`, which does not implement the `Copy` trait
  |
help: consider borrowing the `Option`'s content
  |
4 |     opt_vec[0].as_ref().unwrap();
  |               +++++++++

Meta

Rust version (rustc -Vv):

rustc 1.58.0-nightly (bd41e09da 2021-10-18)
binary: rustc
commit-hash: bd41e09da334697c0f993b36685cb599061d9faa
commit-date: 2021-10-18
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

@rustbot label +I-suggestion-causes-error

[WaffleLapkin]

Also note that adding as_ref/as_mut is not correct in this case either, since it changes the output type to a double ref (see playground)

[H2CO3]

@WaffleLapkin this URLO case hits the "as_mut() is not enough" path as well, the fix is to .map(BorrowMut::borrow_mut).