1 files changed, 63 insertions, 0 deletions
diff --git a/third_party/rust/clap_builder/src/parser/error.rs b/third_party/rust/clap_builder/src/parser/error.rs
new file mode 100644
index 0000000000..77b0bb0fc0
--- /dev/null
+++ b/third_party/rust/clap_builder/src/parser/error.rs
@@ -0,0 +1,63 @@
+use crate::util::AnyValueId;
+
+/// Violation of [`ArgMatches`][crate::ArgMatches] assumptions
+#[derive(Clone, Debug)]
+#[allow(missing_copy_implementations)] // We might add non-Copy types in the future
+#[non_exhaustive]
+pub enum MatchesError {
+    /// Failed to downcast `AnyValue` to the specified type
+    #[non_exhaustive]
+    Downcast {
+        /// Type for value stored in [`ArgMatches`][crate::ArgMatches]
+        actual: AnyValueId,
+        /// The target type to downcast to
+        expected: AnyValueId,
+    },
+    /// Argument not defined in [`Command`][crate::Command]
+    #[non_exhaustive]
+    UnknownArgument {
+        // Missing `id` but blocked on a public id type which will hopefully come with `unstable-v4`
+    },
+}
+
+impl MatchesError {
+    #[cfg_attr(debug_assertions, track_caller)]
+    pub(crate) fn unwrap<T>(id: &str, r: Result<T, MatchesError>) -> T {
+        let err = match r {
+            Ok(t) => {
+                return t;
+            }
+            Err(err) => err,
+        };
+        panic!("Mismatch between definition and access of `{id}`. {err}",)
+    }
+}
+
+impl std::error::Error for MatchesError {}
+
+impl std::fmt::Display for MatchesError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            Self::Downcast { actual, expected } => {
+                writeln!(
+                    f,
+                    "Could not downcast to {expected:?}, need to downcast to {actual:?}"
+                )
+            }
+            Self::UnknownArgument {} => {
+                writeln!(f, "Unknown argument or group id.  Make sure you are using the argument id and not the short or long flags")
+            }
+        }
+    }
+}
+
+#[test]
+fn check_auto_traits() {
+    static_assertions::assert_impl_all!(
+        MatchesError: Send,
+        Sync,
+        std::panic::RefUnwindSafe,
+        std::panic::UnwindSafe,
+        Unpin
+    );
+}