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]

src/librbml/lib.rs

@@ -105,7 +105,8 @@ pub enum EbmlEncoderTag {
 pub enum Error {
     IntTooBig(uint),
     Expected(String),
-    IoError(std::io::IoError)
+    IoError(std::io::IoError),
+    ApplicationError(String)
 }
 // --------------------------------------
 
@@ -119,11 +120,11 @@ pub mod reader {
 
     use serialize;
 
-    use super::{ EsVec, EsMap, EsEnum, EsVecLen, EsVecElt, EsMapLen, EsMapKey,
-        EsEnumVid, EsU64, EsU32, EsU16, EsU8, EsInt, EsI64, EsI32, EsI16, EsI8,
-        EsBool, EsF64, EsF32, EsChar, EsStr, EsMapVal, EsEnumBody, EsUint,
-        EsOpaque, EsLabel, EbmlEncoderTag, Doc, TaggedDoc, Error, IntTooBig,
-        Expected };
+    use super::{ ApplicationError, EsVec, EsMap, EsEnum, EsVecLen, EsVecElt,
+        EsMapLen, EsMapKey, EsEnumVid, EsU64, EsU32, EsU16, EsU8, EsInt, EsI64,
+        EsI32, EsI16, EsI8, EsBool, EsF64, EsF32, EsChar, EsStr, EsMapVal,
+        EsEnumBody, EsUint, EsOpaque, EsLabel, EbmlEncoderTag, Doc, TaggedDoc,
+        Error, IntTooBig, Expected };
 
     pub type DecodeResult<T> = Result<T, Error>;
     // rbml reading
@@ -636,6 +637,10 @@ fn read_map_elt_val<T>(&mut self, idx: uint, f: |&mut Decoder<'doc>| -> DecodeRe
             debug!("read_map_elt_val(idx={})", idx);
             self.push_doc(EsMapVal, f)
         }
+
+        fn error(&mut self, err: &str) -> Error {
+            ApplicationError(err.to_string())
+        }
     }
 }
 

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 @@ fn read_map_elt_val<T>(&mut self, idx: uint, f: |&mut Decoder| -> DecodeResult<T
         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

src/libserialize/serialize.rs

@@ -163,6 +163,9 @@ fn read_tuple_struct_arg<T>(&mut self,
     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> {