How to use a dynamic span name with tracing?

I need to instrument spans with different names dynamically. How can I create a tracing span with dynamic naming?

use tracing; // 0.1.22

fn main() {
    let name: &'static str = "foo bar";
    let span = tracing::span!(tracing::Level::TRACE, name);
    let span_handle = span.enter();
    tracing::info!("Hello, world!");
    drop(span_handle);
}

error[E0435]: attempt to use a non-constant value in a constant
 --> src/main.rs:5:54
  |
5 |     let span = tracing::span!(tracing::Level::TRACE, name);
  |                                                      ^^^^ non-constant value

Playground

Instead of attempting to set the span's name dynamically, add a field:

let value = "forty-two";
let span = tracing::span!(tracing::Level::TRACE, "foo bar", value = value);

From the docs:

A span consists of fields, user-defined key-value pairs of arbitrary data that describe the context the span represents