Incorrect suggestion to use copied() method on an iterator of mutable references

[DomWilliams0]

When deferencing an iterator of references to a Copy with .map(|a_ref| *a_ref) clippy suggests using .copied(), which works for immutable references but doesn't for mutables, as shown in the example below.

$ cat src/main.rs
fn main() {
    let mut three = 3;
    let three_ref = &mut three;
    let v: Vec<&mut i32> = vec![three_ref];

    let _iter: Vec<i32> = v.into_iter().map(|i| *i).collect();
}

$ cargo clippy
    Checking clippy v0.1.0 (/tmp/clippy)
warning: You are using an explicit closure for copying elements
 --> src/main.rs:6:26
  |
6 |     let _iter: Vec<i32> = v.into_iter().map(|i| *i).collect();
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^ help: Consider calling the dedicated `copied` method: `v.into_iter().copied()`
  |
  = note: `#[warn(clippy::map_clone)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_clone

$ cat src/main.rs
...
    //let _iter: Vec<i32> = v.into_iter().map(|i| *i).collect();
    let _iter: Vec<i32> = v.into_iter().copied().collect();

$ cargo clippy   
    Checking clippy v0.1.0 (/tmp/clippy)
error[E0271]: type mismatch resolving `<std::vec::IntoIter<&mut i32> as std::iter::Iterator>::Item == &_`
 --> src/main.rs:7:41
  |
7 |     let _iter: Vec<i32> = v.into_iter().copied().collect();
  |                                         ^^^^^^ types differ in mutability
  |
  = note:   expected type `&mut i32`
          found reference `&_`

error[E0599]: no method named `collect` found for struct `std::iter::Copied<std::vec::IntoIter<&mut i32>>` in the current scope
    --> src/main.rs:7:50
     |
7    |       let _iter: Vec<i32> = v.into_iter().copied().collect();
     |                                                    ^^^^^^^ method not found in `std::iter::Copied<std::vec::IntoIter<&mut i32>>`
     |
     = note: the method `collect` exists but the following trait bounds were not satisfied:
             `<std::vec::IntoIter<&mut i32> as std::iter::Iterator>::Item = &_`
             which is required by `std::iter::Copied<std::vec::IntoIter<&mut i32>>: std::iter::Iterator`
             `std::iter::Copied<std::vec::IntoIter<&mut i32>>: std::iter::Iterator`
             which is required by `&mut std::iter::Copied<std::vec::IntoIter<&mut i32>>: std::iter::Iterator`

Clippy version:

$ cargo clippy -V
clippy 0.0.212 (204bb9b 2020-03-17)