Add str_truncated() to debug suite.

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

src/core/utils/debug.rs

@@ -9,17 +9,13 @@ pub struct TruncatedSlice<'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()
-		}
-	}
+/// Debug-formats the given str, but only up to the first `max_len` elements.
+/// Any further elements are replaced by an ellipsis.
+///
+/// See also [`str_truncated()`],
+pub struct TruncatedStr<'a> {
+	inner: &'a str,
+	max_len: usize,
 }
 
 /// See [`TruncatedSlice`]. Useful for `#[instrument]`:
@@ -36,3 +32,48 @@ pub fn slice_truncated<T: fmt::Debug>(
 ) -> tracing::field::DebugValue<TruncatedSlice<'_, T>> {
 	tracing::field::debug(TruncatedSlice { inner: slice, max_len })
 }
+
+/// See [`TruncatedStr`]. Useful for `#[instrument]`:
+///
+/// ```
+/// use tuwunel_core::utils::debug::str_truncated;
+///
+/// #[tracing::instrument(fields(foos = str_truncated(foos, 42)))]
+/// fn bar(foos: &str) {}
+/// ```
+#[must_use]
+pub fn str_truncated(s: &str, max_len: usize) -> tracing::field::DebugValue<TruncatedStr<'_>> {
+	tracing::field::debug(TruncatedStr { inner: s, max_len })
+}
+
+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()
+		}
+	}
+}
+
+impl fmt::Debug for TruncatedStr<'_> {
+	#[allow(clippy::string_slice)]
+	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+		if self.inner.len() <= self.max_len {
+			write!(f, "{:?}", self.inner)
+		} else {
+			let len = self
+				.inner
+				.char_indices()
+				.skip_while(|(i, _)| *i < self.max_len)
+				.map(|(i, _)| i)
+				.next()
+				.expect("At least one char_indice >= len for str");
+
+			write!(f, "{:?}...", &self.inner[..len])
+		}
+	}
+}