parse configured EnvFilter once

This allows the error handling to be done upfront instead of for each
use. In particular, the `toml` error now points to the span of text in
the config file where the misconfigured EnvFilter value is. This is much
better than the previous error that did not indicate what was actually
causing it to happen.

src/config/env_filter_clone.rs

@@ -0,0 +1,44 @@
+//! A workaround for [`EnvFilter`] not directly implementing [`Clone`]
+//!
+//! This will be unnecessary after [tokio-rs/tracing#2956][0] is merged.
+//!
+//! [0]: https://github.com/tokio-rs/tracing/pull/2956
+#![warn(missing_docs, clippy::missing_docs_in_private_items)]
+
+use std::str::FromStr;
+
+use serde::{de, Deserialize, Deserializer};
+use tracing_subscriber::EnvFilter;
+
+/// A workaround for [`EnvFilter`] not directly implementing [`Clone`]
+///
+/// Use [`FromStr`] or [`Deserialize`] to construct this type, then [`From`] or
+/// [`Into`] to convert it into an [`EnvFilter`] when needed.
+#[derive(Debug)]
+pub(crate) struct EnvFilterClone(String);
+
+impl FromStr for EnvFilterClone {
+    type Err = <EnvFilter as FromStr>::Err;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        EnvFilter::from_str(s)?;
+        Ok(Self(s.to_owned()))
+    }
+}
+
+impl From<&EnvFilterClone> for EnvFilter {
+    fn from(other: &EnvFilterClone) -> Self {
+        EnvFilter::from_str(&other.0)
+            .expect("env filter syntax should have been validated already")
+    }
+}
+
+impl<'de> Deserialize<'de> for EnvFilterClone {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        Self::from_str(&s).map_err(de::Error::custom)
+    }
+}