utils: add helper for adding unbounded slices to tracing spans

src/utils.rs

@@ -1,7 +1,7 @@
 pub(crate) mod error;
 
 use std::{
-    cmp,
+    cmp, fmt,
     str::FromStr,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -170,3 +170,42 @@ pub(crate) fn deserialize_from_str<
     }
     deserializer.deserialize_str(Visitor(std::marker::PhantomData))
 }
+
+/// Debug-formats the given slice, but only up to the first `max_len` elements.
+/// Any further elements are replaced by an ellipsis.
+///
+/// See also [`debug_slice_truncated()`],
+pub(crate) struct TruncatedDebugSlice<'a, T> {
+    inner: &'a [T],
+    max_len: usize,
+}
+
+impl<T: fmt::Debug> fmt::Debug for TruncatedDebugSlice<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.inner.len() <= self.max_len {
+            write!(f, "{:?}", self.inner)
+        } else {
+            f.debug_list()
+                .entries(&self.inner[..self.max_len])
+                .entry(&"...")
+                .finish()
+        }
+    }
+}
+
+/// See [`TruncatedDebugSlice`]. Useful for `#[instrument]`:
+///
+/// ```ignore
+/// #[tracing::instrument(fields(
+///     foos = debug_slice_truncated(foos, N)
+/// ))]
+/// ```
+pub(crate) fn debug_slice_truncated<T: fmt::Debug>(
+    slice: &[T],
+    max_len: usize,
+) -> tracing::field::DebugValue<TruncatedDebugSlice<'_, T>> {
+    tracing::field::debug(TruncatedDebugSlice {
+        inner: slice,
+        max_len,
+    })
+}