macros use fields more than once

[Skepfyr]

Bug Report

Some tracing macros can use the fields provided more than once causing confusing and non-local error messages.

Version

Discovered on 0.1.29, from a code read also looks to be present on master.

Crates

tracing

Description

#[derive(Debug)]
struct Foo;

#[derive(Debug)]
struct Bar(Foo);

pub fn foo(foo: Foo) {
    tracing::error!("{:?}", Bar(foo))
}

when the above code is compiled with the log feature enabled, you get the following compiler error:

error[E0382]: use of moved value: `foo`
 --> src/lib.rs:8:33
  |
7 | pub fn foo(foo: Foo) {
  |            --- move occurs because `foo` has type `Foo`, which does not implement the `Copy` trait
8 |     tracing::error!("{:?}", Bar(foo))
  |                                 ^^^
  |                                 |
  |                                 value moved here
  |                                 value used here after move

For more information about this error, try `rustc --explain E0382`.

(You can also extend this to weird and wonderful effects, such as a counter that depends on the feature flags and log level.)

This error is confusing by itself, but is made worse by the fact that it can be triggered indirectly; this (A) compiles fine without the log feature enabled, but if a crate (B) depends on A and (even indirectly) enables the log feature then A will fail to compile as part of B, but will compile fine by itself.

From a quick code read I think there may be other macros affected by this, but I haven't investigated any further.

[hawkw]

Yeah, this has --- unfortunately --- been a known issue for quite a while now (see #196). I'd really, really like to fix this, but I wasn't able to find a good solution last time I spent time looking into this.

If you're interested in helping out, a fresh pair of eyes could be useful! I'm happy to answer any questions if you do take a look at it.

[Skepfyr]

Staring at this I agree it's a pain. My only thought at the moment would be to use the ValueSet to produce the log. I think ValueSet has all the information needed to produce the formatted log string, the only change would be to always construct it and pass it to log rather than just when that trace level is enabled.