Allow Clippy lint: derive_hash_xor_eq

The implementation is identical to how #[derive] would do it, and we use the
implementation to add some documentation warning people not to use items with
nanosecond-level precision in hash maps unless they're sure that's what they
want.

src/naive/date.rs

@@ -3,7 +3,7 @@
 
 //! ISO 8601 calendar date without timezone.
 
-use std::{str, fmt, hash};
+use std::{str, fmt};
 use std::ops::{Add, Sub, AddAssign, SubAssign};
 use num::traits::ToPrimitive;
 use oldtime::Duration as OldDuration;
@@ -93,7 +93,7 @@ const MAX_BITS: usize = 44;
 /// The year number is same to that of the [calendar date](#calendar-date).
 ///
 /// This is currently the internal format of Chrono's date types.
-#[derive(PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
+#[derive(PartialEq, Eq, Hash, PartialOrd, Ord, Copy, Clone)]
 pub struct NaiveDate {
     ymdf: DateImpl, // (year << 13) | of
 }
@@ -1302,14 +1302,6 @@ impl Datelike for NaiveDate {
     }
 }
 
-/// `NaiveDate` can be used as a key to the hash maps.
-impl hash::Hash for NaiveDate {
-    fn hash<H: hash::Hasher>(&self, state: &mut H) {
-        // don't need to strip flags, as we can safely assume that it is correct
-        self.ymdf.hash(state);
-    }
-}
-
 /// An addition of `Duration` to `NaiveDate` discards the fractional days,
 /// rounding to the closest integral number of days towards `Duration::zero()`.
 ///

src/naive/datetime.rs

@@ -1103,6 +1103,7 @@ impl Timelike for NaiveDateTime {
 ///
 /// Practically this also takes account of fractional seconds, so it is not recommended.
 /// (For the obvious reason this also distinguishes leap seconds from non-leap seconds.)
+#[cfg_attr(feature = "cargo-clippy", allow(derive_hash_xor_eq))]
 impl hash::Hash for NaiveDateTime {
     fn hash<H: hash::Hasher>(&self, state: &mut H) {
         self.date.hash(state);

src/naive/time.rs

@@ -981,6 +981,7 @@ impl Timelike for NaiveTime {
 ///
 /// Practically this also takes account of fractional seconds, so it is not recommended.
 /// (For the obvious reason this also distinguishes leap seconds from non-leap seconds.)
+#[cfg_attr(feature = "cargo-clippy", allow(derive_hash_xor_eq))]
 impl hash::Hash for NaiveTime {
     fn hash<H: hash::Hasher>(&self, state: &mut H) {
         self.secs.hash(state);