Provide list of recognized rename rules on parse error

serde_derive/src/internals/case.rs

@@ -120,11 +120,18 @@ pub struct ParseError<'a> {
 
 impl<'a> Display for ParseError<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(
-            f,
-            "unknown rename rule for #[serde(rename_all = {:?})]",
-            self.unknown,
-        )
+        f.write_str("unknown rename rule `rename_all = \"")?;
+        self.unknown.escape_debug().fmt(f)?;
+        f.write_str("\"`, expected one of ")?;
+        for (i, (name, _rule)) in RENAME_RULES.iter().enumerate() {
+            if i > 0 {
+                f.write_str(", ")?;
+            }
+            f.write_str("\"")?;
+            name.escape_debug().fmt(f)?;
+            f.write_str("\"")?;
+        }
+        Ok(())
     }
 }