Wrong suggestion of `const fn` if type implements Drop

[jfrimmel]

Hello together,

The following code snippet (in src/lib.rs):

pub struct A;
impl A {
    #![warn(clippy::missing_const_for_fn)]
    pub fn a(self) -> B {
        B
    }
}
impl Drop for A {
    fn drop(&mut self) {}
}

pub struct B;

leads to this suggestion:

warning: this could be a const_fn
 --> src/lib.rs:4:5
  |
4 | /     pub fn a(self) -> B {
5 | |         B
6 | |     }
  | |_____^
  |
note: lint level defined here
 --> src/lib.rs:3:13
  |
3 |     #![warn(clippy::missing_const_for_fn)]
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

but the suggested fix (i.e. add the const qualifier) is wrong. After the fix the following error occurs:

error[E0493]: destructors cannot be evaluated at compile-time
 --> src/lib.rs:4:20
  |
4 |     pub const fn a(self) -> B {
  |                    ^^^^ constant functions cannot evaluate destructors

error: aborting due to previous error

Meta

$ cargo clippy -V
clippy 0.0.212 (e3cb40e 2019-06-25)

$ rustc --version
rustc 1.37.0 (eae3437df 2019-08-13)