Add #[non_exhaustive] to error enums and restore FrameTooLarge (#172)

Mark CapsuleError, ConnectError, and SettingsError as #[non_exhaustive]
so adding/removing variants is no longer a semver-breaking change.
Restore the FrameTooLarge variants that were removed in the patch release.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: kixelated

web-transport-proto/src/capsule.rs

@@ -204,6 +204,7 @@ fn is_grease(val: u64) -> Option<u64> {
 }
 
 #[derive(Debug, Clone, thiserror::Error)]
+#[non_exhaustive]
 pub enum CapsuleError {
     #[error("unexpected end of buffer")]
     UnexpectedEnd,

web-transport-proto/src/connect.rs

@@ -10,6 +10,7 @@ use thiserror::Error;
 
 // Errors that can occur during the connect request.
 #[derive(Error, Debug, Clone)]
+#[non_exhaustive]
 pub enum ConnectError {
     #[error("unexpected end of input")]
     UnexpectedEnd,
@@ -53,6 +54,9 @@ pub enum ConnectError {
     #[error("structured field error: {0}")]
     StructuredFieldError(Arc<sfv::Error>),
 
+    #[error("frame too large")]
+    FrameTooLarge,
+
     #[error("non-200 status: {0:?}")]
     ErrorStatus(http::StatusCode),
 
@@ -385,7 +389,7 @@ async fn read_headers_frame<S: AsyncRead + Unpin>(stream: &mut S) -> Result<Vec<
 
         let size = size.into_inner();
         if size > MAX_FRAME_SIZE {
-            return Err(std::io::Error::other("frame too large"))?;
+            return Err(ConnectError::FrameTooLarge);
         }
 
         let mut payload = stream.take(size);
@@ -551,8 +555,8 @@ mod tests {
         let mut cursor = Cursor::new(wire);
         let err = ConnectRequest::read(&mut cursor).await.unwrap_err();
         assert!(
-            matches!(err, ConnectError::Io(_)),
-            "expected Io, got {err:?}"
+            matches!(err, ConnectError::FrameTooLarge),
+            "expected FrameTooLarge, got {err:?}"
         );
     }
 
@@ -621,8 +625,8 @@ mod tests {
         let mut cursor = Cursor::new(wire);
         let err = ConnectResponse::read(&mut cursor).await.unwrap_err();
         assert!(
-            matches!(err, ConnectError::Io(_)),
-            "expected Io, got {err:?}"
+            matches!(err, ConnectError::FrameTooLarge),
+            "expected FrameTooLarge, got {err:?}"
         );
     }
 

web-transport-proto/src/settings.rs

@@ -86,6 +86,7 @@ settings! {
 }
 
 #[derive(Error, Debug, Clone)]
+#[non_exhaustive]
 pub enum SettingsError {
     #[error("unexpected end of input")]
     UnexpectedEnd,
@@ -99,6 +100,9 @@ pub enum SettingsError {
     #[error("invalid size")]
     InvalidSize,
 
+    #[error("frame too large")]
+    FrameTooLarge,
+
     #[error("io error: {0}")]
     Io(Arc<std::io::Error>),
 }
@@ -162,7 +166,7 @@ impl Settings {
 
             let size = size.into_inner();
             if size > MAX_FRAME_SIZE {
-                return Err(std::io::Error::other("frame too large"))?;
+                return Err(SettingsError::FrameTooLarge);
             }
 
             let mut payload = stream.take(size);
@@ -377,8 +381,8 @@ mod tests {
         let mut cursor = Cursor::new(wire);
         let err = Settings::read(&mut cursor).await.unwrap_err();
         assert!(
-            matches!(err, SettingsError::Io(_)),
-            "expected Io, got {err:?}"
+            matches!(err, SettingsError::FrameTooLarge),
+            "expected FrameTooLarge, got {err:?}"
         );
     }