Fix#4033 search_is_some
Fixes#4033.
Suggest `any(|x| ..)` instead of `any(|&x| ..)` for `find(|&x| ..).is_some()` (Lint [search_is_some](https://rust-lang.github.io/rust-clippy/master/index.html#search_is_some))
FnDecl of `find`:
```rust
fn find<P>(&mut self, mut p: P) -> Option<Self::Item> where
P: FnMut(&Self::Item) -> bool
```
FnDecl of `any`:
```rust
fn any<F>(&mut self, mut f: F) -> bool where
F: FnMut(Self::Item) -> bool
```
If match on `|&_|` in closure of `find`, only use `|_|` in the suggestion.
PS. It's the first time that I have used the `hir` API, please correct me if there is any mistake 😺
useless_let_if_seq handle interior mutability
fixes#3043
This passes all tests, including a new one specifically dealing with a type with interior mutability. The main thing I'm unsure of is whether the span I used in the call to `is_freeze` is the most appropriate span to use, or if it matters.
Do not trigger redundant_closure for non-function types
fixes#3898
Added a check for the entity being called in the closure body to be a FnDef. This way lint does not trigger for ADTs (Box) but I'm not sure if it's correct and not too restrictive.
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changelog: Fix false positive in `redundant_closure` pertaining to non-function types
