implementation specific names are un-hygenically leaked into user code in the event! macro (& friends)

[iitalics]

Bug Report

Version

$ cargo tree | grep tracing
├── tracing v0.1.36
│ ├── tracing-attributes v0.1.22 (proc-macro)
│ └── tracing-core v0.1.29
└── tracing-subscriber v0.3.15
├── tracing-core v0.1.29 ()
└── tracing-log v0.1.3
└── tracing-core v0.1.29 ()

Description

within the event! macro, some identifiers such as display, are un-hygienically leaked into the user's format arguments, which causes them to shadow local variables.

example:

use tracing::event;

fn main() {
    let display = "foo";
    let s = event!(tracing::Level::INFO, "length: {}", display.len());
}

i expect this to log length: 3, but instead i get this error:

error[E0599]: no method named `len` found for fn item `fn(_) -> DisplayValue<_> {display::<_>}` in the current scope
 --> src/main.rs:5:64
  |
5 |     let s = event!(tracing::Level::INFO, "length: {}", display.len());
  |                                                        ------- ^^^ method not found in `fn(_) -> DisplayValue<_> {display::<_>}`
  |                                                        |
  |                                                        this is a function, perhaps you wish to call it

to be honest i thought that rust's hygene in macro_rules! would have prevented this class of error but apparently it is still a problem

[hawkw]

This is, unfortunately, a known issue (see #805 and, more recently, #2278), which we can't fix without a breaking change. We "fixed" it two years ago in #806, but unfortunately had to revert that change (2feb61d) when it turned out existing code was actually relying on those names being imported (#820).

Sorry! I hate this too, but here we are. :(

[yshui]

I hit this problem too. Can we fix this with a non-backward compatible version number bump?

[davidbarsky]

this can be fixed in tracing 0.2, yes.