Swap the 2 variants of Option<T>

That allows us to have a better path from Result<T, E> to Option<T>
in the general case, without niche-filling optimisations.

Author: nox

src/libcore/option.rs

@@ -146,23 +146,23 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use iter::{FromIterator, FusedIterator, TrustedLen};
-use {mem, ops};
+use {cmp, intrinsics, mem, ops};
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
 // `Iterator`, which is one. The compiler assumes that the `next` method of
 // `Iterator` is an enumeration with one type parameter and two variants,
 // which basically means it must be `Option`.
 
 /// The `Option` type. See [the module level documentation](index.html) for more.
-#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)] // PartialOrd and Ord by hand below.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub enum Option<T> {
-    /// No value
-    #[stable(feature = "rust1", since = "1.0.0")]
-    None,
     /// Some value `T`
     #[stable(feature = "rust1", since = "1.0.0")]
     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
+    /// No value
+    #[stable(feature = "rust1", since = "1.0.0")]
+    None,
 }
 
 /////////////////////////////////////////////////////////////////////////////
@@ -981,6 +981,107 @@ impl<T> From<T> for Option<T> {
     }
 }
 
+// The Option<T> type used to be defined as { None, Some(T) }, but for codegen
+// reasons we reversed it in #49499 to reduce the amount of work that needs
+// to be done for Result<T, ()> <-> Option<T> conversions. Keeping the derived
+// Ord and PartialOrd implementations would make that swap a breaking change.
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Ord for Option<T>
+where
+    T: Ord,
+{
+    #[inline]
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this.cmp(other),
+                _ => cmp::Ordering::Equal,
+            }
+        } else {
+            other_discr.cmp(&self_discr)
+        }
+    }
+}
+
+// See comment on the Ord impl above.
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> PartialOrd for Option<T>
+where
+    T: PartialOrd,
+{
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this.partial_cmp(other),
+                _ => Some(cmp::Ordering::Equal),
+            }
+        } else {
+            other_discr.partial_cmp(&self_discr)
+        }
+    }
+
+    #[inline]
+    fn lt(&self, other: &Self) -> bool {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this < other,
+                _ => false,
+            }
+        } else {
+            other_discr < self_discr
+        }
+    }
+
+    #[inline]
+    fn le(&self, other: &Self) -> bool {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this <= other,
+                _ => true,
+            }
+        } else {
+            other_discr <= self_discr
+        }
+    }
+
+    #[inline]
+    fn gt(&self, other: &Self) -> bool {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this > other,
+                _ => false,
+            }
+        } else {
+            other_discr > self_discr
+        }
+    }
+
+    #[inline]
+    fn ge(&self, other: &Self) -> bool {
+        let self_discr = unsafe { intrinsics::discriminant_value(self) } as isize;
+        let other_discr = unsafe { intrinsics::discriminant_value(other) } as isize;
+        if self_discr == other_discr {
+            match (self, other) {
+                (&Some(ref this), &Some(ref other)) => this >= other,
+                _ => true,
+            }
+        } else {
+            other_discr >= self_discr
+        }
+    }
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // The Option Iterators
 /////////////////////////////////////////////////////////////////////////////

src/test/compile-fail/issue-2111.rs

@@ -10,7 +10,7 @@
 
 fn foo(a: Option<usize>, b: Option<usize>) {
   match (a,b) {
-  //~^ ERROR: non-exhaustive patterns: `(None, None)` not covered
+  //~^ ERROR: non-exhaustive patterns: `(Some(_), Some(_))` not covered
     (Some(a), Some(b)) if a == b => { }
     (Some(_), None) |
     (None, Some(_)) => { }

src/test/mir-opt/match_false_edges.rs

@@ -55,7 +55,7 @@ fn main() {
 //      _2 = std::option::Option<i32>::Some(const 42i32,);
 //      _3 = discriminant(_2);
 //      _6 = discriminant(_2);
-//      switchInt(move _6) -> [0isize: bb6, 1isize: bb4, otherwise: bb8];
+//      switchInt(move _6) -> [0isize: bb4, 1isize: bb6, otherwise: bb8];
 //  }
 //  bb1: {
 //      resume;
@@ -119,7 +119,7 @@ fn main() {
 //      _2 = std::option::Option<i32>::Some(const 42i32,);
 //      _3 = discriminant(_2);
 //      _6 = discriminant(_2);
-//      switchInt(move _6) -> [0isize: bb5, 1isize: bb4, otherwise: bb8];
+//      switchInt(move _6) -> [0isize: bb4, 1isize: bb5, otherwise: bb8];
 //  }
 //  bb1: {
 //      resume;
@@ -183,7 +183,7 @@ fn main() {
 //     _2 = std::option::Option<i32>::Some(const 1i32,);
 //     _3 = discriminant(_2);
 //     _8 = discriminant(_2);
-//     switchInt(move _8) -> [1isize: bb4, otherwise: bb5];
+//     switchInt(move _8) -> [0isize: bb4, otherwise: bb5];
 // }
 // bb1: {
 //     resume;