tracing macros awaiting Future is not Send with feature 'log'

[b-zee]

Bug Report

Version

$ cargo tree | grep tracing
└── tracing v0.1.26
    ├── tracing-attributes v0.1.15 (proc-macro)
    └── tracing-core v0.1.18

Description

Consider the following example, where a debug event is created from a Future that is awaited.

// src/main.rs
fn main() {
    tokio::spawn(async {
        tracing::debug!("{:?}", async { 5 }.await);
    });
}

This works like it should, except with the log feature.

# Cargo.toml
tracing = { version = "0.1.26", features = [ "log" ] }

Result:

error: future cannot be sent between threads safely
   --> src/main.rs:2:5
    |
2   |     tokio::spawn(async {
    |     ^^^^^^^^^^^^ future created by async block is not `Send`
    | 
   ::: ~/.local/share/cargo/registry/src/g.yxqyang.asia-1ecc6299db9ec823/tokio-1.9.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ---- required by this bound in `tokio::spawn`
    |
    = help: within `[ArgumentV1<'_>]`, the trait `Sync` is not implemented for `core::fmt::Opaque`
note: future is not `Send` as this value is used across an await
   --> src/main.rs:3:33
    |
3   |         tracing::debug!("{:?}", async { 5 }.await);
    |         ------------------------^^^^^^^^^^^^^^^^^--
    |         |                       |
    |         |                       await occurs here, with `log::Record::builder()` maybe used later
    |         `log::Record::builder()` is later dropped here
    |         has type `RecordBuilder<'_>` which is not `Send`
help: consider moving this into a `let` binding to create a shorter lived borrow
   --> src/main.rs:3:9
    |
3   |         tracing::debug!("{:?}", async { 5 }.await);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the macro `$crate::__tracing_log` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: could not compile `project`

[hawkw]

As a workaround, consider something like:

let x = async { 5 }.await;
tracing::debug!("{:?}", x);

which should work regardless of whether or not the log feature is enabled.

I'm not sure if this is something we can easily fix in tracing (it may require changing something in the log crate), but I'll take a look.

[jplatte]

Just stumbled upon this (see reference above), rustc was super unhelpful in pinning down what caused the issue (I even thought I had a fix temporarily when I tried to fix this in an entirely different way). Would be very nice if this was just fixed.