Add #[must_use] to conversions that move self

[jkugelman]

Everything here got the same message. Is the wording okay?

#[must_use = "`self` will be dropped if the result is not used"]

I want to draw attention to these methods in particular:

alloc::sync::Arc<MaybeUninit<T>>     unsafe fn assume_init(self) -> Arc<T>;
alloc::sync::Arc<[MaybeUninit<T>]>   unsafe fn assume_init(self) -> Arc<[T]>;
core::pin::Pin<&'a mut T>            const fn into_ref(self) -> Pin<&'a T>;
core::pin::Pin<&'a mut T>            const fn get_mut(self) -> &'a mut T;
core::pin::Pin<&'a mut T>            const unsafe fn get_unchecked_mut(self) -> &'a mut T;
core::pin::Pin<&'a mut T>            unsafe fn map_unchecked_mut(self, func: F) -> Pin<&'a mut U>;
core::pin::Pin<&'a mut Pin<P>>       fn as_deref_mut(self) -> Pin<&'a mut P::Target>;

Parent issue: #89692

r? @joshtriplett

[joshtriplett]

@bors r+

[bors]

📌 Commit b115781 has been approved by joshtriplett

[bors]

⌛ Testing commit b115781 with merge 86d6d2b...

[bors]

☀️ Test successful - checks-actions
Approved by: joshtriplett
Pushing 86d6d2b to master...

[rust-timer]

Finished benchmarking commit (86d6d2b): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression