Work around pointer aliasing issue in Vec::extend_from_slice, extend_with_element
Due to missing noalias annotations for &mut T in general (issue #31681), in larger programs extend_from_slice and extend_with_element may both compile very poorly. What is observed is that the .set_len() calls are not lifted out of the loop, even for `Vec<u8>`. Use a local length variable for the Vec length instead, and use a scope guard to write this value back to self.len when the scope ends or on panic. Then the alias analysis is easy. This affects extend_from_slice, extend_with_element, the vec![x; n] macro, Write impls for Vec<u8>, BufWriter, etc (but may / may not have triggered since inlining can be enough for the compiler to get it right).
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