add conversion between integer types and bool through the TryFrom trait.

src/libcore/num/mod.rs

@@ -4206,6 +4206,25 @@ macro_rules! try_from_both_bounded {
     )*}
 }
 
+macro_rules! try_bool_from {
+    ($($source:ty),*) => {$(
+        #[unstable(feature = "try_from", issue = "33417")]
+        impl TryFrom<$source> for bool {
+            type Error = TryFromIntError;
+
+            #[inline]
+            fn try_from(u: $source) -> Result<bool, TryFromIntError> {
+                if u == 1 {
+                    Ok(true)
+                } else if u == 0 {
+                    Ok(false)
+                } else {
+                    Err(TryFromIntError(()))
+                }
+            }
+        }
+    )*}
+}
 macro_rules! rev {
     ($mac:ident, $source:ty, $($target:ty),*) => {$(
         $mac!($target, $source);
@@ -4223,6 +4242,9 @@ try_from_both_bounded!(i32, i16, i8);
 try_from_both_bounded!(i64, i32, i16, i8);
 try_from_both_bounded!(i128, i64, i32, i16, i8);
 
+// Integer -> Boolean
+try_bool_from! { u8, u16, u32, u64, u128, i8, i16, i32, i64, i128 }
+
 // unsigned-to-signed
 try_from_upper_bounded!(u8, i8);
 try_from_upper_bounded!(u16, i8, i16);

src/libcore/tests/num/mod.rs

@@ -527,6 +527,37 @@ assume_usize_width! {
     test_impl_try_from_signed_to_unsigned_err! { test_try_i128usize, i128, usize }
 }
 
+/// Conversions to bool is lossy.
+/// This checks that we only accept 1 for true and 0 for false.
+macro_rules! test_impl_try_bool_from_integers {
+    ($fn_name:ident, $source: ty) => {
+        #[test]
+        fn $fn_name() {
+            let max = <$source>::max_value();
+            let min = <$source>::min_value();
+            let one: $source = 1;
+            let zero: $source = 0;
+            assert!(<bool as TryFrom<$source>>::try_from(max).is_err());
+            if min != zero {
+                assert!(<bool as TryFrom<$source>>::try_from(min).is_err());
+            }
+            assert_eq!(true, <bool as TryFrom<$source>>::try_from(one).unwrap());
+            assert_eq!(false, <bool as TryFrom<$source>>::try_from(zero).unwrap());
+        }
+    }
+}
+
+test_impl_try_bool_from_integers! { test_try_bool_from_u8, u8 }
+test_impl_try_bool_from_integers! { test_try_bool_from_u16, u16 }
+test_impl_try_bool_from_integers! { test_try_bool_from_u32, u32 }
+test_impl_try_bool_from_integers! { test_try_bool_from_u64, u64 }
+test_impl_try_bool_from_integers! { test_try_bool_from_u128, u128 }
+test_impl_try_bool_from_integers! { test_try_bool_from_i8, i8 }
+test_impl_try_bool_from_integers! { test_try_bool_from_i16, i16 }
+test_impl_try_bool_from_integers! { test_try_bool_from_i32, i32 }
+test_impl_try_bool_from_integers! { test_try_bool_from_i64, i64 }
+test_impl_try_bool_from_integers! { test_try_bool_from_i128, i128 }
+
 macro_rules! test_float {
     ($modname: ident, $fty: ty, $inf: expr, $neginf: expr, $nan: expr) => { mod $modname {
         // FIXME(nagisa): these tests should test for sign of -0.0