tracing 0.1.38

This tracing release changes the Drop implementation for Instrumented
Futures so that the attached Span is entered when dropping the Future. This
means that events emitted by the Future's Drop implementation will now be
recorded within its Span. It also adds #[inline] hints to methods called in
the event! macro's expansion, for an improvement in both binary size and
performance.

Additionally, this release updates the tracing-attributes dependency to
v0.1.24, which updates the syn dependency to v2.x.x.
tracing-attributes v0.1.24 also includes improvements to the #[instrument]
macro; see the tracing-attributes 0.1.24 release notes for
details.

Added

• Instrumented futures will now enter the attached Span in their Drop
  implementation, allowing events emitted when dropping the future to occur
  within the span (#2562)
• #[inline] attributes for methods called by the event! macros, making
  generated code smaller (#2555)
• attributes: level argument to #[instrument(err)] and
  #[instrument(ret)] to override the level of
  the generated return value event (#2335)
• attributes: Improved compiler error message when #[instrument] is added to a const fn
  (#2418)

Changed

• tracing-attributes: updated to 0.1.24
• Removed unneeded cfg-if dependency (#2553)
• attributes: Updated syn dependency to 2.0 (#2516)

Fixed

• attributes: Fix clippy::unreachable warnings in #[instrument]-generated code (#2356)
• attributes: Removed unused "visit" feature flag from syn dependency (#2530)

Documented

• attributes: Documented default level for #[instrument(err)] (#2433)
• attributes: Improved documentation for levels in #[instrument] (#2350)

Thanks to @nitnelave, @jsgf, @Abhicodes-crypto, @LukeMathWalker, @andrewpollack,
@quad, @klensy, @davidpdrsn, @dbidwell94, @ldm0, @NobodyXu, @ilsv, and @daxpedda
for contributing to this release!