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core: add
#[must_use]
attributes to iterator adaptor structs.
It can be a little unintuitive that something like `v.iter().map(|x| println!("{}", x));` does nothing: the majority of the iterator adaptors are lazy and do not execute anything until something calls `next`, e.g. a `for` loop, `collect`, `fold`, etc. The majority of such errors can be seen by someone writing something like the above, i.e. just calling an iterator adaptor and doing nothing with it (and doing this is certainly useless), so we can co-opt the `must_use` lint, using the message functionality to give a hint to the reason why. Fixes rust-lang#14666.
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r+