backward compatibility with feature gate

Cargo.toml

@@ -18,7 +18,11 @@ features = ["std"]
 
 [dependencies]
 
+[build-dependencies]
+rustc_version = "0.2"
+
 [features]
 default = ["std"]
 std = []
 i128 = []
+int_to_from_bytes = []

build.rs

@@ -1,3 +1,5 @@
+extern crate rustc_version;
+
 use std::env;
 use std::io::Write;
 use std::process::{Command, Stdio};
@@ -8,6 +10,13 @@ fn main() {
     } else if env::var_os("CARGO_FEATURE_I128").is_some() {
         panic!("i128 support was not detected!");
     }
+
+    match rustc_version::version() {
+        Ok(ref version) if version.major >= 1 && version.minor >= 32 => {
+            println!("cargo:rustc-cfg=int_to_from_bytes");
+        }
+        _ => {}
+    }
 }
 
 /// Test if a code snippet can be compiled

src/int.rs

@@ -1,4 +1,4 @@
-use core::mem::size_of;
+use core::mem::transmute;
 use core::ops::{BitAnd, BitOr, BitXor, Not, Shl, Shr};
 
 use bounds::Bounded;
@@ -380,7 +380,7 @@ pub trait PrimInt:
 }
 
 macro_rules! prim_int_impl {
-    ($T:ty, $S:ty, $U:ty) => {
+    ($T:ty, $S:ty, $U:ty, $L:expr) => {
         impl PrimInt for $T {
             #[inline]
             fn count_ones(self) -> u32 {
@@ -462,55 +462,97 @@ macro_rules! prim_int_impl {
                 <$T>::pow(self, exp)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn to_be_bytes(self) -> Self::Bytes {
                 <$T>::to_be_bytes(self)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn to_le_bytes(self) -> Self::Bytes {
                 <$T>::to_le_bytes(self)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn to_ne_bytes(self) -> Self::Bytes {
                 <$T>::to_ne_bytes(self)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn from_be_bytes(bytes: Self::Bytes) -> Self {
                 <$T>::from_be_bytes(bytes)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn from_le_bytes(bytes: Self::Bytes) -> Self {
                 <$T>::from_le_bytes(bytes)
             }
 
+            #[cfg(feature = "int_to_from_bytes")]
             #[inline]
             fn from_ne_bytes(bytes: Self::Bytes) -> Self {
                 <$T>::from_ne_bytes(bytes)
             }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn to_be_bytes(self) -> Self::Bytes {
+                <$T>::to_ne_bytes(<$T>::to_be(self))
+            }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn to_le_bytes(self) -> Self::Bytes {
+                <$T>::to_ne_bytes(<$T>::to_le(self))
+            }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn to_ne_bytes(self) -> Self::Bytes {
+                unsafe { transmute(self) }
+            }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn from_be_bytes(bytes: Self::Bytes) -> Self {
+                Self::from_be(Self::from_ne_bytes(bytes))
+            }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn from_le_bytes(bytes: Self::Bytes) -> Self {
+                Self::from_le(Self::from_ne_bytes(bytes))
+            }
+
+            #[cfg(not(feature = "int_to_from_bytes"))]
+            #[inline]
+            fn from_ne_bytes(bytes: Self::Bytes) -> Self {
+                unsafe { transmute(bytes) }
+            }
         }
 
         impl Layout for $T {
-            type Bytes = [u8; size_of::<$T>()];
+            type Bytes = [u8; $L];
         }
     };
 }
 
 // prim_int_impl!(type, signed, unsigned);
-prim_int_impl!(u8, i8, u8);
-prim_int_impl!(u16, i16, u16);
-prim_int_impl!(u32, i32, u32);
-prim_int_impl!(u64, i64, u64);
+prim_int_impl!(u8, i8, u8, 1);
+prim_int_impl!(u16, i16, u16, 2);
+prim_int_impl!(u32, i32, u32, 4);
+prim_int_impl!(u64, i64, u64, 8);
 #[cfg(has_i128)]
-prim_int_impl!(u128, i128, u128);
-prim_int_impl!(usize, isize, usize);
-prim_int_impl!(i8, i8, u8);
-prim_int_impl!(i16, i16, u16);
-prim_int_impl!(i32, i32, u32);
-prim_int_impl!(i64, i64, u64);
+prim_int_impl!(u128, i128, u128, 16);
+prim_int_impl!(usize, isize, usize, 8);
+prim_int_impl!(i8, i8, u8, 1);
+prim_int_impl!(i16, i16, u16, 2);
+prim_int_impl!(i32, i32, u32, 4);
+prim_int_impl!(i64, i64, u64, 8);
 #[cfg(has_i128)]
-prim_int_impl!(i128, i128, u128);
-prim_int_impl!(isize, isize, usize);
+prim_int_impl!(i128, i128, u128, 16);
+prim_int_impl!(isize, isize, usize, 8);