Commits on Apr 14, 2022

1. tracing: fix the "log" feature making async block futures !Send (#2073 …

   )
   
   ## Motivation
   
   Currently, enabling the `log` feature can cause a compilation error when
   using `tracing` macros in async functions or blocks. This is because,
   when the `log` feature is enabled, the `tracing` macros will construct a
   `log::Record`, which is not `Send`, causing the async fn/block future to
   become `!Send`. This can break compilation when the future is `Send`
   without the `log` feature enabled. This is really not great, as it makes
   the feature no longer purely additive.
   
   ## Solution
   
   This branch fixes the issue by moving the `log::Record` construction
   behind a function call. Because the `log::Record` is now only
   constructed inside of a function, the `log::Record` is now never a local
   variable within the async block, so it no longer affects the future's
   auto traits.
   
   It's kind of a shame that the compiler can't otherwise determine that
   the `log::Record` can never be held across an await point, but sticking
   it in a separate function makes this obvious to the compiler. Also, as a
   side benefit, this may reduce the size of macro-generated code when the
   `log` feature is enabled a bit, which could improve performance
   marginally in some cases.
   
   I added two tests ensuring that `async fn` and `async` block futures are
   `Send` when containing `tracing` macro calls. Previously, on `master`,
   these failed when the `log` feature is enabled. After this change, the
   tests now pass. Thanks to @Noah-Kennedy for the original MCRE these
   tests were based on!
   
   Finally, while I was here, I took advantage of the opportunity to clean
   up the log integration code a bit. Now that the `log::Record`
   construction is behind a function call in `__macro_support`, the
   `LogValueSet` type, which is used to wrap a `ValueSet` and implement
   `fmt::Display` to add it to the textual message of `log::Record`, no
   longer needs to be exposed to the macros, so it can be made properly
   private, rather than `#[doc(hidden)] pub`. Also, I noticed that the
   `span` module implemented its own versions of `LogValueSet` (`FmtAttrs`
   and `FmtValues`), which were essentially duplicating the same behavior
   for logging span fields. I removed these and changed this code to use
   the `LogValueSet` type instead (which will format string `message`
   fields slightly nicer as well).
   
   Fixes #1793
   Fixes #1487
   
   Signed-off-by: Eliza Weisman <eliza@buoyant.io>

   

   hawkw committed Apr 14, 2022
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2. core: add Value impl for Box<T> where T: Value (#2071) …

   This commit adds a `Value` implementation for `Box<T> where T: Value`.
   This is *primarily* intended to make `Box<dyn Error + ...>` implement
   `Value`, building on the `Value` impls for `dyn Error + ...` added in
   #2066, but it may be useful for other boxed values as well.
   
   Refs: #1308

   

   hawkw committed Apr 14, 2022
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