Auto merge of #28921 - petrochenkov:intconv, r=alexcrichton

Part of https://github.com/rust-lang/rfcs/pull/1218#issuecomment-146615171

r? @aturon

src/libcore/num/mod.rs

@@ -17,6 +17,7 @@ use self::wrapping::OverflowingOps;
 
 use char::CharExt;
 use cmp::{Eq, PartialOrd};
+use convert::From;
 use fmt;
 use intrinsics;
 use marker::{Copy, Sized};
@@ -1471,3 +1472,45 @@ impl fmt::Display for ParseIntError {
 }
 
 pub use num::dec2flt::ParseFloatError;
+
+// Conversion traits for primitive integer types
+// Conversions T -> T are covered by a blanket impl and therefore excluded
+// Some conversions from and to usize/isize are not implemented due to portability concerns
+macro_rules! impl_from {
+    ($Small: ty, $Large: ty) => {
+        #[stable(feature = "lossless_int_conv", since = "1.5.0")]
+        impl From<$Small> for $Large {
+            #[stable(feature = "lossless_int_conv", since = "1.5.0")]
+            #[inline]
+            fn from(small: $Small) -> $Large {
+                small as $Large
+            }
+        }
+    }
+}
+
+// Unsigned -> Unsigned
+impl_from! { u8, u16 }
+impl_from! { u8, u32 }
+impl_from! { u8, u64 }
+impl_from! { u8, usize }
+impl_from! { u16, u32 }
+impl_from! { u16, u64 }
+impl_from! { u32, u64 }
+
+// Signed -> Signed
+impl_from! { i8, i16 }
+impl_from! { i8, i32 }
+impl_from! { i8, i64 }
+impl_from! { i8, isize }
+impl_from! { i16, i32 }
+impl_from! { i16, i64 }
+impl_from! { i32, i64 }
+
+// Unsigned -> Signed
+impl_from! { u8, i16 }
+impl_from! { u8, i32 }
+impl_from! { u8, i64 }
+impl_from! { u16, i32 }
+impl_from! { u16, i64 }
+impl_from! { u32, i64 }

src/libcoretest/num/mod.rs

@@ -137,4 +137,44 @@ mod tests {
         assert_eq!("+".parse::<i8>().ok(), None);
         assert_eq!("".parse::<u8>().ok(), None);
     }
+
+    macro_rules! test_impl_from {
+        ($fn_name: ident, $Small: ty, $Large: ty) => {
+            #[test]
+            fn $fn_name() {
+                let small_max = <$Small>::max_value();
+                let small_min = <$Small>::min_value();
+                let large_max: $Large = small_max.into();
+                let large_min: $Large = small_min.into();
+                assert_eq!(large_max as $Small, small_max);
+                assert_eq!(large_min as $Small, small_min);
+            }
+        }
+    }
+
+    // Unsigned -> Unsigned
+    test_impl_from! { test_u8u16, u8, u16 }
+    test_impl_from! { test_u8u32, u8, u32 }
+    test_impl_from! { test_u8u64, u8, u64 }
+    test_impl_from! { test_u8usize, u8, usize }
+    test_impl_from! { test_u16u32, u16, u32 }
+    test_impl_from! { test_u16u64, u16, u64 }
+    test_impl_from! { test_u32u64, u32, u64 }
+
+    // Signed -> Signed
+    test_impl_from! { test_i8i16, i8, i16 }
+    test_impl_from! { test_i8i32, i8, i32 }
+    test_impl_from! { test_i8i64, i8, i64 }
+    test_impl_from! { test_i8isize, i8, isize }
+    test_impl_from! { test_i16i32, i16, i32 }
+    test_impl_from! { test_i16i64, i16, i64 }
+    test_impl_from! { test_i32i64, i32, i64 }
+
+    // Unsigned -> Signed
+    test_impl_from! { test_u8i16, u8, i16 }
+    test_impl_from! { test_u8i32, u8, i32 }
+    test_impl_from! { test_u8i64, u8, i64 }
+    test_impl_from! { test_u16i32, u16, i32 }
+    test_impl_from! { test_u16i64, u16, i64 }
+    test_impl_from! { test_u32i64, u32, i64 }
 }