### What it does
Checks for public `impl` or `fn` missing generalization
over different hashers and implicitly defaulting to the default hashing
algorithm (`SipHash`).

### Why is this bad?
`HashMap` or `HashSet` with custom hashers cannot be
used with them.

### Known problems
Suggestions for replacing constructors can contain
false-positives. Also applying suggestions can require modification of other
pieces of code, possibly including external crates.

### Example
```
impl<K: Hash + Eq, V> Serialize for HashMap<K, V> { }

pub fn foo(map: &mut HashMap<i32, i32>) { }
```
could be rewritten as
```
impl<K: Hash + Eq, V, S: BuildHasher> Serialize for HashMap<K, V, S> { }

pub fn foo<S: BuildHasher>(map: &mut HashMap<i32, i32, S>) { }
```