Print char 0 in error messages as '\0' rather than '\u{0}'

src/error.rs

@@ -37,15 +37,26 @@ impl Display for Error {
             ErrorKind::UnexpectedChar(pos, ch) => {
                 write!(
                     formatter,
-                    "unexpected character {:?} while parsing {}",
-                    ch, pos,
+                    "unexpected character {} while parsing {}",
+                    QuotedChar(*ch),
+                    pos,
                 )
             }
             ErrorKind::UnexpectedCharAfter(pos, ch) => {
-                write!(formatter, "unexpected character {:?} after {}", ch, pos)
+                write!(
+                    formatter,
+                    "unexpected character {} after {}",
+                    QuotedChar(*ch),
+                    pos,
+                )
             }
             ErrorKind::ExpectedCommaFound(pos, ch) => {
-                write!(formatter, "expected comma after {}, found {:?}", pos, ch)
+                write!(
+                    formatter,
+                    "expected comma after {}, found {}",
+                    pos,
+                    QuotedChar(*ch),
+                )
             }
             ErrorKind::LeadingZero(pos) => {
                 write!(formatter, "invalid leading zero in {}", pos)
@@ -96,3 +107,18 @@ impl Debug for Error {
         Ok(())
     }
 }
+
+struct QuotedChar(char);
+
+impl Display for QuotedChar {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        // Standard library versions prior to https://github.com/rust-lang/rust/pull/95345
+        // print character 0 as '\u{0}'. We prefer '\0' to keep error messages
+        // the same across all supported Rust versions.
+        if self.0 == '\0' {
+            formatter.write_str("'\\0'")
+        } else {
+            write!(formatter, "{:?}", self.0)
+        }
+    }
+}

tests/test_version_req.rs

@@ -336,7 +336,7 @@ pub fn test_parse_errors() {
     let err = req_err("\0");
     assert_to_string(
         err,
-        "unexpected character '\\u{0}' while parsing major version number",
+        "unexpected character '\\0' while parsing major version number",
     );
 
     let err = req_err(">= >= 0.0.2");