rust: Implement error types for Rust protover implementation.

This will allow us to do actual error handling intra-crate in a more
rusty manner, e.g. propogating errors in match statements, conversion
between error types, logging messages, etc.

 * FIXES part of #24031: https://bugs.torproject.org/24031.

1 files changed, 43 insertions, 0 deletions

diff --git a/src/rust/protover/errors.rs b/src/rust/protover/errors.rs
new file mode 100644
index 0000000000..56473d12e6
--- /dev/null
+++ b/src/rust/protover/errors.rs
@@ -0,0 +1,43 @@
+// Copyright (c) 2018, The Tor Project, Inc.
+// Copyright (c) 2018, isis agora lovecruft
+// See LICENSE for licensing information
+
+//! Various errors which may occur during protocol version parsing.
+
+use std::fmt;
+use std::fmt::Display;
+
+/// All errors which may occur during protover parsing routines.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
+#[allow(missing_docs)] // See Display impl for error descriptions
+pub enum ProtoverError {
+    Overlap,
+    LowGreaterThanHigh,
+    Unparseable,
+    ExceedsMax,
+    ExceedsExpansionLimit,
+    UnknownProtocol,
+    ExceedsNameLimit,
+}
+
+/// Descriptive error messages for `ProtoverError` variants.
+impl Display for ProtoverError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            ProtoverError::Overlap
+                => write!(f, "Two or more (low, high) protover ranges would overlap once expanded."),
+            ProtoverError::LowGreaterThanHigh
+                => write!(f, "The low in a (low, high) protover range was greater than high."),
+            ProtoverError::Unparseable
+                => write!(f, "The protover string was unparseable."),
+            ProtoverError::ExceedsMax
+                => write!(f, "The high in a (low, high) protover range exceeds u32::MAX."),
+            ProtoverError::ExceedsExpansionLimit
+                => write!(f, "The protover string would exceed the maximum expansion limit."),
+            ProtoverError::UnknownProtocol
+                => write!(f, "A protocol in the protover string we attempted to parse is unknown."),
+            ProtoverError::ExceedsNameLimit
+                => write!(f, "An unrecognised protocol name was too long."),
+        }
+    }
+}