Put HashMap's hasher in an Result and Default::default() it to Err(Default::default)

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HashMap implements Clone, but cloning HashMap::default() should NOT produce the same hasher state. Instead, there should be a distinction between a default HashMap and an empty/cleared HashMap. Both are empty, but the former also has the hasher set to Err(Default::default).

This is important because one could easily do:

let v: Vec<HashSet<&str>> = vec![];
// some stuff here
if v.len() < my.len() {
    v.resize(my.len(), HashSet::default());
}

and accidentally introduce a DoS exploit to their code. (this would be partially solved by resize_default, but I don't think it's a proper solution.)

Start with the HashMap/HashSet example and the linked comment archive. Determine whether the distinction between default and cleared maps has consensus and what RFC or implementation scope is required; done means an agreed, reviewable resolution.