Avoid hash_slice in VecDeque's Hash implementation

[LunaBorowska]

Fixes #80303.

[rust-highfive]

r? @sfackler

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[clarfonthey]

Is there a reason why hash_slice hashes the length and not just the items? Maybe that could be changed instead.

[sfackler]

@bors r+

Is there a reason why hash_slice hashes the length and not just the items? Maybe that could be changed instead.

It prevents hashes of values like these from colliding:

#[derive(Hash)]
struct Foo {
    a: Vec<u8>,
    b: Vec<u8>,
}

let thing1 = Foo {
    a: vec![1, 2],
    b: vec![3, 4],
};

let thing2 = Foo {
    a: vec![1],
    b: vec![2, 3, 4],
};

[bors]

📌 Commit ae3a515 has been approved by sfackler