split / cleanup core utils.

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/debug.rs

@@ -0,0 +1,40 @@
+use std::fmt;
+
+/// Debug-formats the given slice, but only up to the first `max_len` elements.
+/// Any further elements are replaced by an ellipsis.
+///
+/// See also [`slice_truncated()`],
+pub struct TruncatedSlice<'a, T> {
+	inner: &'a [T],
+	max_len: usize,
+}
+
+impl<T: fmt::Debug> fmt::Debug for TruncatedSlice<'_, 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 [`TruncatedSlice`]. Useful for `#[instrument]`:
+///
+/// ```
+/// use conduit_core::utils::debug::slice_truncated;
+///
+/// #[tracing::instrument(fields(foos = slice_truncated(foos, 42)))]
+/// fn bar(foos: &[&str]);
+/// ```
+pub fn slice_truncated<T: fmt::Debug>(
+	slice: &[T], max_len: usize,
+) -> tracing::field::DebugValue<TruncatedSlice<'_, T>> {
+	tracing::field::debug(TruncatedSlice {
+		inner: slice,
+		max_len,
+	})
+}