Auto merge of #83858 - joshtriplett:unsafe-cell-always-inline, r=Mark-Simulacrum
Auto merge of #83858 - joshtriplett:unsafe-cell-always-inline, r=Mark-Simulacrum

Use `#[inline(always)]` on trivial UnsafeCell methods

UnsafeCell is the standard building block for shared mutable data
structures. UnsafeCell should add zero overhead compared to using raw
pointers directly.

Some reports suggest that debug builds, or even builds at opt-level 1,
may not always be inlining its methods. Mark the methods as
`#[inline(always)]`, since once inlined the methods should result in no
actual code other than field accesses.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust