Is there any way to have dynamic fields?

[CleanCut]

Is there any way to have dynamic fields? Something like:

let cloud_account_id = "cloud.account.id";
tracing::info!(cloud_account_id = 234, "created the cloud account id");

and have it output something with the cloud.account.id field and value 234?

Even if it weren't technically "dynamic", but at least in some sort of constant, that'd be fine for me:

const CLOUD_ACCOUNT_ID: 'static str = "cloud.account.id";
tracing::info!(CLOUD_ACCOUNT_ID = 234, "created the cloud account id");

I tried making my own type and implementing AsField, which looked like it should have worked...

impl AsField for MyType {
    // ...
}

fn main() {
    let cloud_account_id = MyType::new("cloud.account.id");
    tracing::info!(cloud_account_id = 234, "created the cloud account id");
}

but AsField is a sealed trait 😭 , so I can't implement it outside tracing itself AFAICT.

---

If the answer is "No, there's no way to do that." Then my followup questions would be

• Is there any straightforward way to add such a feature?
  • Maybe something like this? tracing::info!($cloud_account_id = 234, "created the cloud account id");
• Or some workaround that I could do myself to kinda-sorta pretend that feature exists via my own macro(s) or something?

[bryangarza]

I believe that what you are describing (with the variable) is not possible, because the field names have to be static data (to minimize the number of allocations). Using a constant could potentially work if somewhere were to update the tracing macros to support it.

Q: Or some workaround that I could do myself to kinda-sorta pretend that feature exists via my own macro(s) or something?
A: Maybe if you have a macro that expands to the field name at compile-time tracing::info!(cloud_account_id!() = 234, "created the cloud account id");

[CleanCut]

I tried the workaround you suggested, but it looks like macros such as info! won't accept that format (either the ! or the () or both, I'm guessing?)

error: format argument must be a string literal
  --> src/main.rs:31:20
   |
31 |     tracing::info!(cloud_account_id!() = 123, "created the cloud account");
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: you might be missing a string literal to format with
   |
31 |     tracing::info!("{} {}", cloud_account_id!() = 123, "created the cloud account");
   |                    ++++++++

[bryangarza]

Ah, okay. Yeah then seems it's not possible without modifying the macros to accept this sort of thing.

[CleanCut]

It looks like modifying the macros may work! I've got a basic POC working already in #2254