libserialize: add `error()` to `Decoder`

A quick and dirty fix for #15036 until we get serious decoder reform.

Right now it is impossible for a Decodable to signal a decode error,
for example if it has only finitely many allowed values, is a string
which must be encoded a certain way, needs a valid checksum, etc. For
example in the libuuid implementation of Decodable an Option is
unwrapped, meaning that a decode of a malformed UUID will cause the
task to fail.

Since this adds a method to the `Decoder` trait, all users will need
to update their implementations to add it. The strategy used for the
current implementations for JSON and EBML is to add a new entry to
the error enum `ApplicationError(String)` which stores the string
provided to `.error()`.

[breaking-change]

2 files changed, 8 insertions, 0 deletions

diff --git a/src/libserialize/json.rs b/src/libserialize/json.rs
index 0752c68f4d0..58d69e38cc6 100644
--- a/src/libserialize/json.rs
+++ b/src/libserialize/json.rs
@@ -257,6 +257,7 @@ pub enum DecoderError {
     ExpectedError(String, String),
     MissingFieldError(String),
     UnknownVariantError(String),
+    ApplicationError(String)
 }
 
 /// Returns a readable error string for a given error code.
@@ -2071,6 +2072,10 @@ impl ::Decoder<DecoderError> for Decoder {
         debug!("read_map_elt_val(idx={})", idx);
         f(self)
     }
+
+    fn error(&mut self, err: &str) -> DecoderError {
+        ApplicationError(err.to_string())
+    }
 }
 
 /// A trait for converting values to JSON
diff --git a/src/libserialize/serialize.rs b/src/libserialize/serialize.rs
index 03d9445b9b9..95c677bcbd7 100644
--- a/src/libserialize/serialize.rs
+++ b/src/libserialize/serialize.rs
@@ -163,6 +163,9 @@ pub trait Decoder<E> {
     fn read_map<T>(&mut self, f: |&mut Self, uint| -> Result<T, E>) -> Result<T, E>;
     fn read_map_elt_key<T>(&mut self, idx: uint, f: |&mut Self| -> Result<T, E>) -> Result<T, E>;
     fn read_map_elt_val<T>(&mut self, idx: uint, f: |&mut Self| -> Result<T, E>) -> Result<T, E>;
+
+    // Failure
+    fn error(&mut self, err: &str) -> E;
 }
 
 pub trait Encodable<S:Encoder<E>, E> {