First pass at making "alloc" its own feature

Part of this means that we don't actually need to allocate to write to Serde
any more, which is a win in its own right.

Cargo.toml

@@ -27,6 +27,8 @@ name = "chrono"
 default = ["clock", "std"]
 alloc = []
 std = []
+serde_alloc = ["serde/alloc"]
+serde_std = ["serde/std"]
 clock = ["time", "std"]
 wasmbind = ["wasm-bindgen", "js-sys"]
 
@@ -35,7 +37,7 @@ time = { version = "0.1.39", optional = true }
 num-integer = { version = "0.1.36", default-features = false }
 num-traits = { version = "0.2", default-features = false }
 rustc-serialize = { version = "0.3.20", optional = true }
-serde = { version = "1.0.99", default-features = false, features = ["alloc"], optional = true }
+serde = { version = "1.0.99", default-features = false, optional = true }
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))'.dependencies]
 wasm-bindgen = { version = "0.2", optional = true }

src/datetime.rs

@@ -10,8 +10,10 @@ use core::ops::{Add, Sub};
 use std::time::{SystemTime, UNIX_EPOCH};
 use oldtime::Duration as OldDuration;
 
-#[cfg(any(feature = "alloc", feature = "std", test))]
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
 use alloc::string::{String, ToString};
+#[cfg(feature = "std")]
+use std::string::ToString;
 
 use {Weekday, Timelike, Datelike};
 #[cfg(feature="clock")]
@@ -921,26 +923,50 @@ pub mod rustc_serialize {
 #[cfg(feature = "serde")]
 pub mod serde {
     use core::fmt;
-    #[cfg(not(any(feature = "std", test)))]
-    use alloc::format;
+    // #[cfg(any(test, feature = "alloc"))]
+    // use alloc::format;
     use super::DateTime;
     #[cfg(feature="clock")]
     use offset::Local;
     use offset::{LocalResult, TimeZone, Utc, FixedOffset};
     use serdelib::{ser, de};
 
+    enum SerdeError<V: fmt::Display, D: fmt::Display> {
+        NonExistent { timestamp: V },
+        Ambiguous { timestamp: V, min: D, max: D }
+    }
+
+    impl<V: fmt::Display, D: fmt::Display> fmt::Debug for SerdeError<V, D> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            write!(f, "ChronoSerdeError({})", self)
+        }
+    }
+
+    // impl<V: fmt::Display, D: fmt::Debug> core::error::Error for SerdeError<V, D> {}
+    impl<V: fmt::Display, D: fmt::Display> fmt::Display for SerdeError<V, D> {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            match self {
+                &SerdeError::NonExistent { ref timestamp } => write!(
+                    f, "value is not a legal timestamp: {}", timestamp),
+                &SerdeError::Ambiguous { ref timestamp, ref min, ref max } => write!(
+                        f, "value is an ambiguous timestamp: {}, could be either of {}, {}",
+                        timestamp, min, max),
+            }
+        }
+    }
+
     // try!-like function to convert a LocalResult into a serde-ish Result
     fn serde_from<T, E, V>(me: LocalResult<T>, ts: &V) -> Result<T, E>
-        where E: de::Error,
-                V: fmt::Display,
-                T: fmt::Display,
+    where
+        E: de::Error,
+        V: fmt::Display,
+        T: fmt::Display,
     {
         match me {
             LocalResult::None => Err(E::custom(
-                format!("value is not a legal timestamp: {}", ts))),
+                SerdeError::NonExistent::<_, u8> { timestamp: ts })),
             LocalResult::Ambiguous(min, max) => Err(E::custom(
-                format!("value is an ambiguous timestamp: {}, could be either of {}, {}",
-                        ts, min, max))),
+                SerdeError::Ambiguous { timestamp: ts, min: min, max: max })),
             LocalResult::Single(val) => Ok(val)
         }
     }
@@ -1418,7 +1444,7 @@ pub mod serde {
         fn visit_str<E>(self, value: &str) -> Result<DateTime<FixedOffset>, E>
             where E: de::Error
         {
-            value.parse().map_err(|err| E::custom(format!("{}", err)))
+            value.parse().map_err(|err: ::format::ParseError| E::custom(err))
         }
     }
 

src/format/mod.rs

@@ -21,9 +21,9 @@ use core::fmt;
 use core::str::FromStr;
 #[cfg(any(feature = "std", test))]
 use std::error::Error;
-#[cfg(any(feature = "alloc", feature = "std", test))]
+#[cfg(feature = "alloc")]
 use alloc::boxed::Box;
-#[cfg(any(feature = "alloc", feature = "std", test))]
+#[cfg(feature = "alloc")]
 use alloc::string::{String, ToString};
 
 #[cfg(not(any(feature = "alloc", feature = "std", test)))]

src/lib.rs

@@ -405,11 +405,11 @@
     trivially_copy_pass_by_ref,
 ))]
 
-#[cfg(not(any(feature = "std", test)))]
+#[cfg(feature = "alloc")]
 extern crate alloc;
 #[cfg(any(feature = "std", test))]
 extern crate std as core;
-#[cfg(any(feature = "std", test))]
+#[cfg(all(feature = "std", not(feature="alloc")))]
 extern crate std as alloc;
 
 #[cfg(feature="clock")]
@@ -654,6 +654,20 @@ impl Weekday {
     }
 }
 
+impl fmt::Display for Weekday {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(match *self {
+            Weekday::Mon => "Mon",
+            Weekday::Tue => "Tue",
+            Weekday::Wed => "Wed",
+            Weekday::Thu => "Thu",
+            Weekday::Fri => "Fri",
+            Weekday::Sat => "Sat",
+            Weekday::Sun => "Sun",
+        })
+    }
+}
+
 /// Any weekday can be represented as an integer from 0 to 6, which equals to
 /// [`Weekday::num_days_from_monday`](#method.num_days_from_monday) in this implementation.
 /// Do not heavily depend on this though; use explicit methods whenever possible.
@@ -707,15 +721,13 @@ impl fmt::Debug for ParseWeekdayError {
 mod weekday_serde {
     use super::Weekday;
     use core::fmt;
-    #[cfg(not(any(feature = "std", test)))]
-    use alloc::format;
     use serdelib::{ser, de};
 
     impl ser::Serialize for Weekday {
         fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
             where S: ser::Serializer
         {
-            serializer.serialize_str(&format!("{:?}", self))
+            serializer.collect_str(&self)
         }
     }
 

src/naive/date.rs

@@ -1605,8 +1605,6 @@ mod rustc_serialize {
 #[cfg(feature = "serde")]
 mod serde {
     use core::fmt;
-    #[cfg(not(any(feature = "std", test)))]
-    use alloc::format;
     use super::NaiveDate;
     use serdelib::{ser, de};
 
@@ -1640,11 +1638,19 @@ mod serde {
             write!(formatter, "a formatted date string")
         }
 
+        #[cfg(any(feature = "std", test))]
         fn visit_str<E>(self, value: &str) -> Result<NaiveDate, E>
             where E: de::Error
         {
             value.parse().map_err(|err| E::custom(format!("{}", err)))
         }
+
+        #[cfg(not(any(feature = "std", test)))]
+        fn visit_str<E>(self, value: &str) -> Result<NaiveDate, E>
+            where E: de::Error
+        {
+            value.parse().map_err(|err| E::custom(err))
+        }
     }
 
     impl<'de> de::Deserialize<'de> for NaiveDate {

src/naive/datetime.rs

@@ -1668,7 +1668,7 @@ pub mod rustc_serialize {
 #[cfg(feature = "serde")]
 pub mod serde {
     use core::fmt;
-    #[cfg(not(any(feature = "std", test)))]
+    #[cfg(feature = "alloc")]
     use alloc::format;
     use super::{NaiveDateTime};
     use serdelib::{ser, de};

src/naive/time.rs

@@ -1416,7 +1416,7 @@ mod rustc_serialize {
 #[cfg(feature = "serde")]
 mod serde {
     use core::fmt;
-    #[cfg(not(any(feature = "std", test)))]
+    #[cfg(feature = "alloc")]
     use alloc::format;
     use super::NaiveTime;
     use serdelib::{ser, de};