suggestions for `or_fun_call` can cause a borrowck error:

[jyn514]

error[E0500]: closure requires unique access to `this` but it is already borrowed
    --> src/simple_api/mod.rs:1150:55
     |
1147 |         unsafe fn get_last_buffer(this: &mut Key) -> &mut Vec<u8> {
     |                                         - let's call the lifetime of this reference `'1`
...
1150 |             this.subscripts.last_mut().unwrap_or_else(|| this.variable.as_mut_vec())
     |             ------------------------------------------^^----------------------------
     |             |                                         |  |
     |             |                                         |  second borrow occurs due to use of `this` in closure
     |             |                                         closure construction occurs here
     |             borrow occurs here
     |             returning this value requires that `this.subscripts` is borrowed for `'1`

Originally posted by @jyn514 in #6675 (comment)

[camsteffen]

Actually we should suggest using match instead (maybe not machine-applicable).