Fix Rust Beta warning about deprecated AtomicBool method.

[rfk]

When compiling on Rust Beta, we're generating a warning about:

warning: use of deprecated associated function
std::sync::atomic::AtomicBool::compare_and_swap:
Use compare_exchange or compare_exchange_weak instead

This commit does as advised in the hope of eliminating the warning,
which is currently failing some scheduled CI jobs.

[codecov-io]

Codecov Report

Merging #3804 (880221f) into main (1d739d7) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #3804   +/-   ##
=======================================
  Coverage   18.67%   18.67%           
=======================================
  Files         229      229           
  Lines       27290    27289    -1     
=======================================
  Hits         5096     5096           
+ Misses      22194    22193    -1     

Impacted Files	Coverage Δ
components/places/src/api/places_api.rs	0.00% <0.00%> (ø)
components/viaduct/src/backend/ffi.rs	0.00% <0.00%> (ø)

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[mhammond]

This looks correct to me, but I admit to not understanding why compare_exchange is documented as returning current but also saying it only succeeds when current == the current value - surely the return value can be implied in all cases?

I'm also mildly surprised https://doc.rust-lang.org/std/sync/atomic/struct.AtomicBool.html doesn't note that compare_and_swap is deprecated!

Regardless, neither of those points are strictly relevant to this patch.

[rfk]

I admit to not understanding why compare_exchange is documented as returning current but also saying it only succeeds
when current == the current value - surely the return value can be implied in all cases?

Yeah, I struggle a bit here wondering if I was missing something, but found rust-lang/rust#31767 (comment) which suggests there isn't anything deep going on, it's just returning the info because it happens to be available.