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> As another commenter wrote "how do you allocate memory without an allocator?"

You call these things something other than an "allocating" and "allocators". Seriously, few people would consider adding a value to a hashmap an intentionally allocational activity, but it is. Same with adding an element to a vector, or any of the dependent actions on it.

Seriously

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For "adding an element to a vector" it's actually not necessarily what you meant here and in some contexts it makes sense to be explicit about whether to allocate.

Rust's Vec::push may grow the Vec, so it has amortized O(1) and might allocate.

However Vec::push_within_capacity never grows the Vec, it is unamortized O(1) and never allocates. If our value wouldn't fit we get the value back instead.

Yep exactly, at it's simplest all an allocator is doing is keeping track of which memory areas are owned and their addresses. In a sense even the stack pointer is a kind of allocator (you can assign the memory pointed to by the stack pointer and then increment it for the next use)

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