Add #[must_use] to alloc constructors #89726
                
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Added
#[must_use]. to the various forms ofnew,pin, andwith_capacityin thealloccrate. No extra explanations given as I couldn't think of anything useful to add.I figure this deserves extra scrutiny compared to the other PRs I've done so far. In particular:
pin/pin_inmethods I touched. Are there legitimate use cases for pinning and not using the result? Pinning's a difficult concept I'm not very comfortable with.Box's constructors. Do people ever create boxes just for the side effects... allocating or zeroing out memory?Parent issue: #89692
r? @joshtriplett