Better Debug for String

src/string.rs

@@ -2,7 +2,7 @@ use std::borrow::{Borrow, Cow};
 use std::hash::{Hash, Hasher};
 use std::os::raw::c_void;
 use std::string::String as StdString;
-use std::{slice, str};
+use std::{fmt, slice, str};
 
 #[cfg(feature = "serialize")]
 use {
@@ -17,7 +17,7 @@ use crate::types::LuaRef;
 /// Handle to an internal Lua string.
 ///
 /// Unlike Rust strings, Lua strings may not be valid UTF-8.
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 pub struct String<'lua>(pub(crate) LuaRef<'lua>);
 
 impl<'lua> String<'lua> {
@@ -124,6 +124,35 @@ impl<'lua> String<'lua> {
     }
 }
 
+impl<'lua> fmt::Debug for String<'lua> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let bytes = self.as_bytes();
+        // Check if the string is valid utf8
+        if let Ok(s) = str::from_utf8(bytes) {
+            return s.fmt(f);
+        }
+
+        // Format as bytes
+        write!(f, "b\"")?;
+        for &b in bytes {
+            // https://doc.rust-lang.org/reference/tokens.html#byte-escapes
+            match b {
+                b'\n' => write!(f, "\\n")?,
+                b'\r' => write!(f, "\\r")?,
+                b'\t' => write!(f, "\\t")?,
+                b'\\' | b'"' => write!(f, "\\{}", b as char)?,
+                b'\0' => write!(f, "\\0")?,
+                // ASCII printable
+                0x20..=0x7e => write!(f, "{}", b as char)?,
+                _ => write!(f, "\\x{b:02x}")?,
+            }
+        }
+        write!(f, "\"")?;
+
+        Ok(())
+    }
+}
+
 impl<'lua> AsRef<[u8]> for String<'lua> {
     fn as_ref(&self) -> &[u8] {
         self.as_bytes()

tests/string.rs

@@ -83,3 +83,18 @@ fn test_string_hash() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_string_debug() -> Result<()> {
+    let lua = Lua::new();
+
+    // Valid utf8
+    let s = lua.create_string("hello")?;
+    assert_eq!(format!("{s:?}"), r#""hello""#);
+
+    // Invalid utf8
+    let s = lua.create_string(b"hello\0world\r\n\t\xF0\x90\x80")?;
+    assert_eq!(format!("{s:?}"), r#"b"hello\0world\r\n\t\xf0\x90\x80""#);
+
+    Ok(())
+}