Add a Deserialize::visit_enum that tries to deserialize into each variant

This could be more efficiently done if serde communicated the
number of arguments in each variant in this function.

src/decoder/serde.rs

@@ -89,6 +89,94 @@ impl de::Deserializer for Decoder {
             self.visit(visitor)
         }
     }
+
+    fn visit_enum<V>(&mut self,
+                     _enum: &str,
+                     variants: &[&str],
+                     mut visitor: V) -> Result<V::Value, DecodeError>
+        where V: de::EnumVisitor,
+    {
+        // When decoding enums, this crate takes the strategy of trying to
+        // decode the current TOML as all of the possible variants, returning
+        // success on the first one that succeeds.
+        //
+        // Note that fidelity of the errors returned here is a little nebulous,
+        // but we try to return the error that had the relevant field as the
+        // longest field. This way we hopefully match an error against what was
+        // most likely being written down without losing too much info.
+        let mut first_error = None::<DecodeError>;
+
+        for variant in 0 .. variants.len() {
+            let mut de = VariantVisitor {
+                de: self.sub_decoder(self.toml.clone(), ""),
+                variant: variant,
+            };
+
+            match visitor.visit(&mut de) {
+                Ok(value) => {
+                    self.toml = de.de.toml;
+                    return Ok(value);
+                }
+                Err(e) => {
+                    if let Some(ref first) = first_error {
+                        let my_len = e.field.as_ref().map(|s| s.len());
+                        let first_len = first.field.as_ref().map(|s| s.len());
+                        if my_len <= first_len {
+                            continue
+                        }
+                    }
+                    first_error = Some(e);
+                }
+            }
+        }
+
+        Err(first_error.unwrap_or_else(|| self.err(DecodeErrorKind::NoEnumVariants)))
+    }
+}
+
+struct VariantVisitor {
+    de: Decoder,
+    variant: usize,
+}
+
+impl de::VariantVisitor for VariantVisitor {
+    type Error = DecodeError;
+
+    fn visit_variant<V>(&mut self) -> Result<V, DecodeError>
+        where V: de::Deserialize
+    {
+        use serde::de::value::ValueDeserializer;
+
+        let mut de = self.variant.into_deserializer();
+
+        de::Deserialize::deserialize(&mut de).map_err(|e| se2toml(e, "variant"))
+    }
+
+    fn visit_unit(&mut self) -> Result<(), DecodeError> {
+        de::Deserialize::deserialize(&mut self.de)
+    }
+
+    fn visit_newtype<T>(&mut self) -> Result<T, DecodeError>
+        where T: de::Deserialize,
+    {
+        de::Deserialize::deserialize(&mut self.de)
+    }
+
+    fn visit_tuple<V>(&mut self,
+                      _len: usize,
+                      visitor: V) -> Result<V::Value, DecodeError>
+        where V: de::Visitor,
+    {
+        de::Deserializer::visit(&mut self.de, visitor)
+    }
+
+    fn visit_struct<V>(&mut self,
+                       _fields: &'static [&'static str],
+                       visitor: V) -> Result<V::Value, DecodeError>
+        where V: de::Visitor,
+    {
+        de::Deserializer::visit(&mut self.de, visitor)
+    }
 }
 
 struct SeqDeserializer<'a, I> {