FP skip_while_next: requires different mutabiltiy

        fn case_ignoreable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
            use core::unicode::derived_property::{Case_Ignorable, Cased};
            match iter.skip_while(|&c| Case_Ignorable(c)).next() {
                Some(c) => Cased(c),
                None => false,
            }
        }

This code causes clippy to warn:

warning: called `skip_while(p).next()` on an `Iterator`
   --> src/liballoc/str.rs:398:19
    |
398 |             match iter.skip_while(|&c| Case_Ignorable(c)).next() {
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(clippy::skip_while_next)]` on by default
    = help: this is more succinctly expressed by calling `.find(!p)` instead
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next

However, using iter.find(|&c| !Case_Ignorable(c)) does not work because it requires different mutability and a change in function signature:

        fn case_ignoreable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
            use core::unicode::derived_property::{Case_Ignorable, Cased};
            match iter.find(|&c| !Case_Ignorable(c)) {
                Some(c) => Cased(c),
                None => false,
            }
        }

    Checking alloc v0.0.0 (/home/matthias/vcs/github/rust/src/liballoc)
error[E0596]: cannot borrow `iter` as mutable, as it is not declared as mutable
   --> src/liballoc/str.rs:398:19
    |
396 |         fn case_ignoreable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
    |                                                                 ---- help: consider changing this to be mutable: `mut iter`
397 |             use core::unicode::derived_property::{Case_Ignorable, Cased};
398 |             match iter.find(|&c| !Case_Ignorable(c)) {
    |                   ^^^^ cannot borrow as mutable

Start with the example in src/liballoc/str.rs and reproduce the skip_while_next warning using the shown function signatures. Read the skip_while_next lint implementation and its tests, if present, to understand how the suggested find call handles iterator mutability. Done means the lint no longer gives an unusable suggestion for this case.