diff --git a/serde_derive/src/internals/attr.rs b/serde_derive/src/internals/attr.rs
index cea79663..a7757ec4 100644
--- a/serde_derive/src/internals/attr.rs
+++ b/serde_derive/src/internals/attr.rs
@@ -339,13 +339,7 @@ impl Container {
                                 rename_all_ser_rule.set(&m.path, rename_rule);
                                 rename_all_de_rule.set(&m.path, rename_rule);
                             }
-                            Err(_err) => cx.error_spanned_by(
-                                s,
-                                format!(
-                                    "unknown rename rule for #[serde(rename_all = {:?})]",
-                                    s.value(),
-                                ),
-                            ),
+                            Err(err) => cx.error_spanned_by(s, err),
                         }
                     }
                 }
@@ -356,25 +350,13 @@ impl Container {
                         if let Some(ser) = ser {
                             match RenameRule::from_str(&ser.value()) {
                                 Ok(rename_rule) => rename_all_ser_rule.set(&m.path, rename_rule),
-                                Err(_err) => cx.error_spanned_by(
-                                    ser,
-                                    format!(
-                                        "unknown rename rule for #[serde(rename_all = {:?})]",
-                                        ser.value(),
-                                    ),
-                                ),
+                                Err(err) => cx.error_spanned_by(ser, err),
                             }
                         }
                         if let Some(de) = de {
                             match RenameRule::from_str(&de.value()) {
                                 Ok(rename_rule) => rename_all_de_rule.set(&m.path, rename_rule),
-                                Err(_err) => cx.error_spanned_by(
-                                    de,
-                                    format!(
-                                        "unknown rename rule for #[serde(rename_all = {:?})]",
-                                        de.value(),
-                                    ),
-                                ),
+                                Err(err) => cx.error_spanned_by(de, err),
                             }
                         }
                     }
@@ -930,13 +912,7 @@ impl Variant {
                                 rename_all_ser_rule.set(&m.path, rename_rule);
                                 rename_all_de_rule.set(&m.path, rename_rule);
                             }
-                            Err(_err) => cx.error_spanned_by(
-                                s,
-                                format!(
-                                    "unknown rename rule for #[serde(rename_all = {:?})]",
-                                    s.value()
-                                ),
-                            ),
+                            Err(err) => cx.error_spanned_by(s, err),
                         }
                     }
                 }
@@ -947,25 +923,13 @@ impl Variant {
                         if let Some(ser) = ser {
                             match RenameRule::from_str(&ser.value()) {
                                 Ok(rename_rule) => rename_all_ser_rule.set(&m.path, rename_rule),
-                                Err(_err) => cx.error_spanned_by(
-                                    ser,
-                                    format!(
-                                        "unknown rename rule for #[serde(rename_all = {:?})]",
-                                        ser.value(),
-                                    ),
-                                ),
+                                Err(err) => cx.error_spanned_by(ser, err),
                             }
                         }
                         if let Some(de) = de {
                             match RenameRule::from_str(&de.value()) {
                                 Ok(rename_rule) => rename_all_de_rule.set(&m.path, rename_rule),
-                                Err(_err) => cx.error_spanned_by(
-                                    de,
-                                    format!(
-                                        "unknown rename rule for #[serde(rename_all = {:?})]",
-                                        de.value(),
-                                    ),
-                                ),
+                                Err(err) => cx.error_spanned_by(de, err),
                             }
                         }
                     }
diff --git a/serde_derive/src/internals/case.rs b/serde_derive/src/internals/case.rs
index d9634e73..9ed8cd47 100644
--- a/serde_derive/src/internals/case.rs
+++ b/serde_derive/src/internals/case.rs
@@ -5,6 +5,8 @@
 #[allow(deprecated, unused_imports)]
 use std::ascii::AsciiExt;
 
+use std::fmt::{self, Display};
+
 use self::RenameRule::*;
 
 /// The different possible ways to change case of fields in a struct, or variants in an enum.
@@ -108,6 +110,16 @@ pub struct ParseError<'a> {
     unknown: &'a str,
 }
 
+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,
+        )
+    }
+}
+
 #[test]
 fn rename_variants() {
     for &(original, lower, upper, camel, snake, screaming, kebab, screaming_kebab) in &[