Implement RFC 1542

cc #33417

src/libcore/convert.rs

@@ -20,18 +20,19 @@
 //! - Impl the `As*` traits for reference-to-reference conversions
 //! - Impl the `Into` trait when you want to consume the value in the conversion
 //! - The `From` trait is the most flexible, useful for value _and_ reference conversions
+//! - The `TryFrom` and `TryInto` traits behave like `From` and `Into`, but allow for the
+//!   conversion to fail
 //!
-//! As a library author, you should prefer implementing `From<T>` rather than
+//! As a library author, you should prefer implementing `From<T>` or `TryFrom<T>` rather than
-//! `Into<U>`, as `From` provides greater flexibility and offers an equivalent `Into`
+//! `Into<U>` or `TryInto<U>`, as `From` and `TryFrom` provide greater flexibility and offer
-//! implementation for free, thanks to a blanket implementation in the standard library.
+//! equivalent `Into` or `TryInto` implementations for free, thanks to a blanket implementation
-//!
+//! in the standard library.
-//! **Note: these traits must not fail**. If the conversion can fail, you must use a dedicated
-//! method which returns an `Option<T>` or a `Result<T, E>`.
 //!
 //! # Generic impl
 //!
 //! - `AsRef` and `AsMut` auto-dereference if the inner type is a reference
 //! - `From<U> for T` implies `Into<T> for U`
+//! - `TryFrom<U> for T` implies `TryInto<T> for U`
 //! - `From` and `Into` are reflexive, which means that all types can `into()`
 //!   themselves and `from()` themselves
 //!
@@ -40,6 +41,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use marker::Sized;
+use result::Result;
 
 /// A cheap, reference-to-reference conversion.
 ///
@@ -98,8 +100,8 @@ pub trait AsMut<T: ?Sized> {
 
 /// A conversion that consumes `self`, which may or may not be expensive.
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
+/// **Note: this trait must not fail**. If the conversion can fail, use `TryInto` or a dedicated
-/// returns an `Option<T>` or a `Result<T, E>`.
+/// method which returns an `Option<T>` or a `Result<T, E>`.
 ///
 /// Library authors should not directly implement this trait, but should prefer implementing
 /// the `From` trait, which offers greater flexibility and provides an equivalent `Into`
@@ -133,8 +135,8 @@ pub trait Into<T>: Sized {
 
 /// Construct `Self` via a conversion.
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
+/// **Note: this trait must not fail**. If the conversion can fail, use `TryFrom` or a dedicated
-/// returns an `Option<T>` or a `Result<T, E>`.
+/// method which returns an `Option<T>` or a `Result<T, E>`.
 ///
 /// # Examples
 ///
@@ -158,6 +160,30 @@ pub trait From<T>: Sized {
     fn from(T) -> Self;
 }
 
+/// An attempted conversion that consumes `self`, which may or may not be expensive.
+///
+/// Library authors should not directly implement this trait, but should prefer implementing
+/// the `TryFrom` trait, which offers greater flexibility and provides an equivalent `TryInto`
+/// implementation for free, thanks to a blanket implementation in the standard library.
+#[unstable(feature = "try_from", issue = "33417")]
+pub trait TryInto<T>: Sized {
+    /// The type returned in the event of a conversion error.
+    type Err;
+
+    /// Performs the conversion.
+    fn try_into(self) -> Result<T, Self::Err>;
+}
+
+/// Attempt to construct `Self` via a conversion.
+#[unstable(feature = "try_from", issue = "33417")]
+pub trait TryFrom<T>: Sized {
+    /// The type returned in the event of a conversion error.
+    type Err;
+
+    /// Performs the conversion.
+    fn try_from(T) -> Result<Self, Self::Err>;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // GENERIC IMPLS
 ////////////////////////////////////////////////////////////////////////////////
@@ -216,6 +242,17 @@ impl<T> From<T> for T {
     fn from(t: T) -> T { t }
 }
 
+
+// TryFrom implies TryInto
+#[unstable(feature = "try_from", issue = "33417")]
+impl<T, U> TryInto<U> for T where U: TryFrom<T> {
+    type Err = U::Err;
+
+    fn try_into(self) -> Result<U, U::Err> {
+        U::try_from(self)
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // CONCRETE IMPLS
 ////////////////////////////////////////////////////////////////////////////////

src/libcore/num/mod.rs

@@ -15,7 +15,7 @@
 
 use char::CharExt;
 use cmp::PartialOrd;
-use convert::From;
+use convert::{From, TryFrom};
 use fmt;
 use intrinsics;
 use marker::{Copy, Sized};
@@ -2341,9 +2341,101 @@ macro_rules! from_str_radix_int_impl {
 }
 from_str_radix_int_impl! { isize i8 i16 i32 i64 usize u8 u16 u32 u64 }
 
+/// The error type returned when a checked integral type conversion fails.
+#[unstable(feature = "try_from", issue = "33417")]
+#[derive(Debug, Copy, Clone)]
+pub struct TryFromIntError(());
+
+impl TryFromIntError {
+    #[unstable(feature = "int_error_internals",
+               reason = "available through Error trait and this method should \
+                         not be exposed publicly",
+               issue = "0")]
+    #[doc(hidden)]
+    pub fn __description(&self) -> &str {
+        "out of range integral type conversion attempted"
+    }
+}
+
+#[unstable(feature = "try_from", issue = "33417")]
+impl fmt::Display for TryFromIntError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        self.__description().fmt(fmt)
+    }
+}
+
+macro_rules! same_sign_from_int_impl {
+    ($storage:ty, $target:ty, $($source:ty),*) => {$(
+        #[stable(feature = "rust1", since = "1.0.0")]
+        impl TryFrom<$source> for $target {
+            type Err = TryFromIntError;
+
+            fn try_from(u: $source) -> Result<$target, TryFromIntError> {
+                let min = <$target as FromStrRadixHelper>::min_value() as $storage;
+                let max = <$target as FromStrRadixHelper>::max_value() as $storage;
+                if u as $storage < min || u as $storage > max {
+                    Err(TryFromIntError(()))
+                } else {
+                    Ok(u as $target)
+                }
+            }
+        }
+    )*}
+}
+
+same_sign_from_int_impl!(u64, u8, u8, u16, u32, u64, usize);
+same_sign_from_int_impl!(i64, i8, i8, i16, i32, i64, isize);
+same_sign_from_int_impl!(u64, u16, u8, u16, u32, u64, usize);
+same_sign_from_int_impl!(i64, i16, i8, i16, i32, i64, isize);
+same_sign_from_int_impl!(u64, u32, u8, u16, u32, u64, usize);
+same_sign_from_int_impl!(i64, i32, i8, i16, i32, i64, isize);
+same_sign_from_int_impl!(u64, u64, u8, u16, u32, u64, usize);
+same_sign_from_int_impl!(i64, i64, i8, i16, i32, i64, isize);
+same_sign_from_int_impl!(u64, usize, u8, u16, u32, u64, usize);
+same_sign_from_int_impl!(i64, isize, i8, i16, i32, i64, isize);
+
+macro_rules! cross_sign_from_int_impl {
+    ($unsigned:ty, $($signed:ty),*) => {$(
+        #[stable(feature = "rust1", since = "1.0.0")]
+        impl TryFrom<$unsigned> for $signed {
+            type Err = TryFromIntError;
+
+            fn try_from(u: $unsigned) -> Result<$signed, TryFromIntError> {
+                let max = <$signed as FromStrRadixHelper>::max_value() as u64;
+                if u as u64 > max {
+                    Err(TryFromIntError(()))
+                } else {
+                    Ok(u as $signed)
+                }
+            }
+        }
+
+        #[stable(feature = "rust1", since = "1.0.0")]
+        impl TryFrom<$signed> for $unsigned {
+            type Err = TryFromIntError;
+
+            fn try_from(u: $signed) -> Result<$unsigned, TryFromIntError> {
+                let max = <$unsigned as FromStrRadixHelper>::max_value() as u64;
+                if u < 0 || u as u64 > max {
+                    Err(TryFromIntError(()))
+                } else {
+                    Ok(u as $unsigned)
+                }
+            }
+        }
+    )*}
+}
+
+cross_sign_from_int_impl!(u8, i8, i16, i32, i64, isize);
+cross_sign_from_int_impl!(u16, i8, i16, i32, i64, isize);
+cross_sign_from_int_impl!(u32, i8, i16, i32, i64, isize);
+cross_sign_from_int_impl!(u64, i8, i16, i32, i64, isize);
+cross_sign_from_int_impl!(usize, i8, i16, i32, i64, isize);
+
 #[doc(hidden)]
 trait FromStrRadixHelper: PartialOrd + Copy {
     fn min_value() -> Self;
+    fn max_value() -> Self;
     fn from_u32(u: u32) -> Self;
     fn checked_mul(&self, other: u32) -> Option<Self>;
     fn checked_sub(&self, other: u32) -> Option<Self>;
@@ -2353,6 +2445,7 @@ trait FromStrRadixHelper: PartialOrd + Copy {
 macro_rules! doit {
     ($($t:ty)*) => ($(impl FromStrRadixHelper for $t {
         fn min_value() -> Self { Self::min_value() }
+        fn max_value() -> Self { Self::max_value() }
         fn from_u32(u: u32) -> Self { u as Self }
         fn checked_mul(&self, other: u32) -> Option<Self> {
             Self::checked_mul(*self, other as Self)

src/libcoretest/lib.rs

@@ -34,6 +34,7 @@
 #![feature(unboxed_closures)]
 #![feature(unicode)]
 #![feature(unique)]
+#![feature(try_from)]
 
 extern crate core;
 extern crate test;

src/libcoretest/num/mod.rs

@@ -8,10 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::convert::TryFrom;
 use core::cmp::PartialEq;
 use core::fmt::Debug;
-use core::ops::{Add, Sub, Mul, Div, Rem};
 use core::marker::Copy;
+use core::num::Float;
+use core::ops::{Add, Sub, Mul, Div, Rem};
+use core::option::Option;
+use core::option::Option::{Some, None};
 
 #[macro_use]
 mod int_macros;
@@ -48,169 +52,318 @@ pub fn test_num<T>(ten: T, two: T) where
     assert_eq!(ten.rem(two),  ten % two);
 }
 
-#[cfg(test)]
+#[test]
-mod tests {
+fn from_str_issue7588() {
-    use core::option::Option;
+    let u : Option<u8> = u8::from_str_radix("1000", 10).ok();
-    use core::option::Option::{Some, None};
+    assert_eq!(u, None);
-    use core::num::Float;
+    let s : Option<i16> = i16::from_str_radix("80000", 10).ok();
+    assert_eq!(s, None);
+}
 
-    #[test]
+#[test]
-    fn from_str_issue7588() {
+fn test_int_from_str_overflow() {
-        let u : Option<u8> = u8::from_str_radix("1000", 10).ok();
+    let mut i8_val: i8 = 127;
-        assert_eq!(u, None);
+    assert_eq!("127".parse::<i8>().ok(), Some(i8_val));
-        let s : Option<i16> = i16::from_str_radix("80000", 10).ok();
+    assert_eq!("128".parse::<i8>().ok(), None);
-        assert_eq!(s, None);
-    }
 
-    #[test]
+    i8_val = i8_val.wrapping_add(1);
-    fn test_int_from_str_overflow() {
+    assert_eq!("-128".parse::<i8>().ok(), Some(i8_val));
-        let mut i8_val: i8 = 127;
+    assert_eq!("-129".parse::<i8>().ok(), None);
-        assert_eq!("127".parse::<i8>().ok(), Some(i8_val));
-        assert_eq!("128".parse::<i8>().ok(), None);
 
-        i8_val = i8_val.wrapping_add(1);
+    let mut i16_val: i16 = 32_767;
-        assert_eq!("-128".parse::<i8>().ok(), Some(i8_val));
+    assert_eq!("32767".parse::<i16>().ok(), Some(i16_val));
-        assert_eq!("-129".parse::<i8>().ok(), None);
+    assert_eq!("32768".parse::<i16>().ok(), None);
 
-        let mut i16_val: i16 = 32_767;
+    i16_val = i16_val.wrapping_add(1);
-        assert_eq!("32767".parse::<i16>().ok(), Some(i16_val));
+    assert_eq!("-32768".parse::<i16>().ok(), Some(i16_val));
-        assert_eq!("32768".parse::<i16>().ok(), None);
+    assert_eq!("-32769".parse::<i16>().ok(), None);
 
-        i16_val = i16_val.wrapping_add(1);
+    let mut i32_val: i32 = 2_147_483_647;
-        assert_eq!("-32768".parse::<i16>().ok(), Some(i16_val));
+    assert_eq!("2147483647".parse::<i32>().ok(), Some(i32_val));
-        assert_eq!("-32769".parse::<i16>().ok(), None);
+    assert_eq!("2147483648".parse::<i32>().ok(), None);
 
-        let mut i32_val: i32 = 2_147_483_647;
+    i32_val = i32_val.wrapping_add(1);
-        assert_eq!("2147483647".parse::<i32>().ok(), Some(i32_val));
+    assert_eq!("-2147483648".parse::<i32>().ok(), Some(i32_val));
-        assert_eq!("2147483648".parse::<i32>().ok(), None);
+    assert_eq!("-2147483649".parse::<i32>().ok(), None);
 
-        i32_val = i32_val.wrapping_add(1);
+    let mut i64_val: i64 = 9_223_372_036_854_775_807;
-        assert_eq!("-2147483648".parse::<i32>().ok(), Some(i32_val));
+    assert_eq!("9223372036854775807".parse::<i64>().ok(), Some(i64_val));
-        assert_eq!("-2147483649".parse::<i32>().ok(), None);
+    assert_eq!("9223372036854775808".parse::<i64>().ok(), None);
 
-        let mut i64_val: i64 = 9_223_372_036_854_775_807;
+    i64_val = i64_val.wrapping_add(1);
-        assert_eq!("9223372036854775807".parse::<i64>().ok(), Some(i64_val));
+    assert_eq!("-9223372036854775808".parse::<i64>().ok(), Some(i64_val));
-        assert_eq!("9223372036854775808".parse::<i64>().ok(), None);
+    assert_eq!("-9223372036854775809".parse::<i64>().ok(), None);
+}
 
-        i64_val = i64_val.wrapping_add(1);
+#[test]
-        assert_eq!("-9223372036854775808".parse::<i64>().ok(), Some(i64_val));
+fn test_leading_plus() {
-        assert_eq!("-9223372036854775809".parse::<i64>().ok(), None);
+    assert_eq!("+127".parse::<u8>().ok(), Some(127));
-    }
+    assert_eq!("+9223372036854775807".parse::<i64>().ok(), Some(9223372036854775807));
+}
 
-    #[test]
+#[test]
-    fn test_leading_plus() {
+fn test_invalid() {
-        assert_eq!("+127".parse::<u8>().ok(), Some(127));
+    assert_eq!("--129".parse::<i8>().ok(), None);
-        assert_eq!("+9223372036854775807".parse::<i64>().ok(), Some(9223372036854775807));
+    assert_eq!("++129".parse::<i8>().ok(), None);
-    }
+    assert_eq!("Съешь".parse::<u8>().ok(), None);
+}
 
-    #[test]
+#[test]
-    fn test_invalid() {
+fn test_empty() {
-        assert_eq!("--129".parse::<i8>().ok(), None);
+    assert_eq!("-".parse::<i8>().ok(), None);
-        assert_eq!("++129".parse::<i8>().ok(), None);
+    assert_eq!("+".parse::<i8>().ok(), None);
-        assert_eq!("Съешь".parse::<u8>().ok(), None);
+    assert_eq!("".parse::<u8>().ok(), None);
-    }
+}
 
-    #[test]
+macro_rules! test_impl_from {
-    fn test_empty() {
+    ($fn_name: ident, $Small: ty, $Large: ty) => {
-        assert_eq!("-".parse::<i8>().ok(), None);
+        #[test]
-        assert_eq!("+".parse::<i8>().ok(), None);
+        fn $fn_name() {
-        assert_eq!("".parse::<u8>().ok(), None);
+            let small_max = <$Small>::max_value();
-    }
+            let small_min = <$Small>::min_value();
-
+            let large_max: $Large = small_max.into();
-    macro_rules! test_impl_from {
+            let large_min: $Large = small_min.into();
-        ($fn_name: ident, $Small: ty, $Large: ty) => {
+            assert_eq!(large_max as $Small, small_max);
-            #[test]
+            assert_eq!(large_min as $Small, small_min);
-            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
+// Unsigned -> Unsigned
-    test_impl_from! { test_u8u16, u8, u16 }
+test_impl_from! { test_u8u16, u8, u16 }
-    test_impl_from! { test_u8u32, u8, u32 }
+test_impl_from! { test_u8u32, u8, u32 }
-    test_impl_from! { test_u8u64, u8, u64 }
+test_impl_from! { test_u8u64, u8, u64 }
-    test_impl_from! { test_u8usize, u8, usize }
+test_impl_from! { test_u8usize, u8, usize }
-    test_impl_from! { test_u16u32, u16, u32 }
+test_impl_from! { test_u16u32, u16, u32 }
-    test_impl_from! { test_u16u64, u16, u64 }
+test_impl_from! { test_u16u64, u16, u64 }
-    test_impl_from! { test_u32u64, u32, u64 }
+test_impl_from! { test_u32u64, u32, u64 }
 
-    // Signed -> Signed
+// Signed -> Signed
-    test_impl_from! { test_i8i16, i8, i16 }
+test_impl_from! { test_i8i16, i8, i16 }
-    test_impl_from! { test_i8i32, i8, i32 }
+test_impl_from! { test_i8i32, i8, i32 }
-    test_impl_from! { test_i8i64, i8, i64 }
+test_impl_from! { test_i8i64, i8, i64 }
-    test_impl_from! { test_i8isize, i8, isize }
+test_impl_from! { test_i8isize, i8, isize }
-    test_impl_from! { test_i16i32, i16, i32 }
+test_impl_from! { test_i16i32, i16, i32 }
-    test_impl_from! { test_i16i64, i16, i64 }
+test_impl_from! { test_i16i64, i16, i64 }
-    test_impl_from! { test_i32i64, i32, i64 }
+test_impl_from! { test_i32i64, i32, i64 }
 
-    // Unsigned -> Signed
+// Unsigned -> Signed
-    test_impl_from! { test_u8i16, u8, i16 }
+test_impl_from! { test_u8i16, u8, i16 }
-    test_impl_from! { test_u8i32, u8, i32 }
+test_impl_from! { test_u8i32, u8, i32 }
-    test_impl_from! { test_u8i64, u8, i64 }
+test_impl_from! { test_u8i64, u8, i64 }
-    test_impl_from! { test_u16i32, u16, i32 }
+test_impl_from! { test_u16i32, u16, i32 }
-    test_impl_from! { test_u16i64, u16, i64 }
+test_impl_from! { test_u16i64, u16, i64 }
-    test_impl_from! { test_u32i64, u32, i64 }
+test_impl_from! { test_u32i64, u32, i64 }
 
-    // Signed -> Float
+// Signed -> Float
-    test_impl_from! { test_i8f32, i8, f32 }
+test_impl_from! { test_i8f32, i8, f32 }
-    test_impl_from! { test_i8f64, i8, f64 }
+test_impl_from! { test_i8f64, i8, f64 }
-    test_impl_from! { test_i16f32, i16, f32 }
+test_impl_from! { test_i16f32, i16, f32 }
-    test_impl_from! { test_i16f64, i16, f64 }
+test_impl_from! { test_i16f64, i16, f64 }
-    test_impl_from! { test_i32f64, i32, f64 }
+test_impl_from! { test_i32f64, i32, f64 }
 
-    // Unsigned -> Float
+// Unsigned -> Float
-    test_impl_from! { test_u8f32, u8, f32 }
+test_impl_from! { test_u8f32, u8, f32 }
-    test_impl_from! { test_u8f64, u8, f64 }
+test_impl_from! { test_u8f64, u8, f64 }
-    test_impl_from! { test_u16f32, u16, f32 }
+test_impl_from! { test_u16f32, u16, f32 }
-    test_impl_from! { test_u16f64, u16, f64 }
+test_impl_from! { test_u16f64, u16, f64 }
-    test_impl_from! { test_u32f64, u32, f64 }
+test_impl_from! { test_u32f64, u32, f64 }
 
-    // Float -> Float
+// Float -> Float
-    #[test]
+#[test]
-    fn test_f32f64() {
+fn test_f32f64() {
-        use core::f32;
+    use core::f32;
 
-        let max: f64 = f32::MAX.into();
+    let max: f64 = f32::MAX.into();
-        assert_eq!(max as f32, f32::MAX);
+    assert_eq!(max as f32, f32::MAX);
-        assert!(max.is_normal());
+    assert!(max.is_normal());
 
-        let min: f64 = f32::MIN.into();
+    let min: f64 = f32::MIN.into();
-        assert_eq!(min as f32, f32::MIN);
+    assert_eq!(min as f32, f32::MIN);
-        assert!(min.is_normal());
+    assert!(min.is_normal());
 
-        let min_positive: f64 = f32::MIN_POSITIVE.into();
+    let min_positive: f64 = f32::MIN_POSITIVE.into();
-        assert_eq!(min_positive as f32, f32::MIN_POSITIVE);
+    assert_eq!(min_positive as f32, f32::MIN_POSITIVE);
-        assert!(min_positive.is_normal());
+    assert!(min_positive.is_normal());
 
-        let epsilon: f64 = f32::EPSILON.into();
+    let epsilon: f64 = f32::EPSILON.into();
-        assert_eq!(epsilon as f32, f32::EPSILON);
+    assert_eq!(epsilon as f32, f32::EPSILON);
-        assert!(epsilon.is_normal());
+    assert!(epsilon.is_normal());
 
-        let zero: f64 = (0.0f32).into();
+    let zero: f64 = (0.0f32).into();
-        assert_eq!(zero as f32, 0.0f32);
+    assert_eq!(zero as f32, 0.0f32);
-        assert!(zero.is_sign_positive());
+    assert!(zero.is_sign_positive());
 
-        let neg_zero: f64 = (-0.0f32).into();
+    let neg_zero: f64 = (-0.0f32).into();
-        assert_eq!(neg_zero as f32, -0.0f32);
+    assert_eq!(neg_zero as f32, -0.0f32);
-        assert!(neg_zero.is_sign_negative());
+    assert!(neg_zero.is_sign_negative());
 
-        let infinity: f64 = f32::INFINITY.into();
+    let infinity: f64 = f32::INFINITY.into();
-        assert_eq!(infinity as f32, f32::INFINITY);
+    assert_eq!(infinity as f32, f32::INFINITY);
-        assert!(infinity.is_infinite());
+    assert!(infinity.is_infinite());
-        assert!(infinity.is_sign_positive());
+    assert!(infinity.is_sign_positive());
 
-        let neg_infinity: f64 = f32::NEG_INFINITY.into();
+    let neg_infinity: f64 = f32::NEG_INFINITY.into();
-        assert_eq!(neg_infinity as f32, f32::NEG_INFINITY);
+    assert_eq!(neg_infinity as f32, f32::NEG_INFINITY);
-        assert!(neg_infinity.is_infinite());
+    assert!(neg_infinity.is_infinite());
-        assert!(neg_infinity.is_sign_negative());
+    assert!(neg_infinity.is_sign_negative());
 
-        let nan: f64 = f32::NAN.into();
+    let nan: f64 = f32::NAN.into();
-        assert!(nan.is_nan());
+    assert!(nan.is_nan());
+}
+
+macro_rules! test_impl_try_from_always_ok {
+    ($fn_name:ident, $source:ty, $target: ty) => {
+        #[test]
+        fn $fn_name() {
+            let max = <$source>::max_value();
+            let min = <$source>::min_value();
+            let zero: $source = 0;
+            assert_eq!(<$target as TryFrom<$source>>::try_from(max).unwrap(),
+                       max as $target);
+            assert_eq!(<$target as TryFrom<$source>>::try_from(min).unwrap(),
+                       min as $target);
+            assert_eq!(<$target as TryFrom<$source>>::try_from(zero).unwrap(),
+                       zero as $target);
+        }
     }
 }
+
+test_impl_try_from_always_ok! { test_try_u8u8, u8, u8 }
+test_impl_try_from_always_ok! { test_try_u8u16, u8, u16 }
+test_impl_try_from_always_ok! { test_try_u8u32, u8, u32 }
+test_impl_try_from_always_ok! { test_try_u8u64, u8, u64 }
+test_impl_try_from_always_ok! { test_try_u8i16, u8, i16 }
+test_impl_try_from_always_ok! { test_try_u8i32, u8, i32 }
+test_impl_try_from_always_ok! { test_try_u8i64, u8, i64 }
+
+test_impl_try_from_always_ok! { test_try_u16u16, u16, u16 }
+test_impl_try_from_always_ok! { test_try_u16u32, u16, u32 }
+test_impl_try_from_always_ok! { test_try_u16u64, u16, u64 }
+test_impl_try_from_always_ok! { test_try_u16i32, u16, i32 }
+test_impl_try_from_always_ok! { test_try_u16i64, u16, i64 }
+
+test_impl_try_from_always_ok! { test_try_u32u32, u32, u32 }
+test_impl_try_from_always_ok! { test_try_u32u64, u32, u64 }
+test_impl_try_from_always_ok! { test_try_u32i64, u32, i64 }
+
+test_impl_try_from_always_ok! { test_try_u64u64, u64, u64 }
+
+test_impl_try_from_always_ok! { test_try_i8i8, i8, i8 }
+test_impl_try_from_always_ok! { test_try_i8i16, i8, i16 }
+test_impl_try_from_always_ok! { test_try_i8i32, i8, i32 }
+test_impl_try_from_always_ok! { test_try_i8i64, i8, i64 }
+
+test_impl_try_from_always_ok! { test_try_i16i16, i16, i16 }
+test_impl_try_from_always_ok! { test_try_i16i32, i16, i32 }
+test_impl_try_from_always_ok! { test_try_i16i64, i16, i64 }
+
+test_impl_try_from_always_ok! { test_try_i32i32, i32, i32 }
+test_impl_try_from_always_ok! { test_try_i32i64, i32, i64 }
+
+test_impl_try_from_always_ok! { test_try_i64i64, i64, i64 }
+
+macro_rules! test_impl_try_from_signed_to_unsigned_upper_ok {
+    ($fn_name:ident, $source:ty, $target:ty) => {
+        #[test]
+        fn $fn_name() {
+            let max = <$source>::max_value();
+            let min = <$source>::min_value();
+            let zero: $source = 0;
+            let neg_one: $source = -1;
+            assert_eq!(<$target as TryFrom<$source>>::try_from(max).unwrap(),
+                       max as $target);
+            assert!(<$target as TryFrom<$source>>::try_from(min).is_err());
+            assert_eq!(<$target as TryFrom<$source>>::try_from(zero).unwrap(),
+                       zero as $target);
+            assert!(<$target as TryFrom<$source>>::try_from(neg_one).is_err());
+        }
+    }
+}
+
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i8u8, i8, u8 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i8u16, i8, u16 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i8u32, i8, u32 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i8u64, i8, u64 }
+
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i16u16, i16, u16 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i16u32, i16, u32 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i16u64, i16, u64 }
+
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i32u32, i32, u32 }
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i32u64, i32, u64 }
+
+test_impl_try_from_signed_to_unsigned_upper_ok! { test_try_i64u64, i64, u64 }
+
+macro_rules! test_impl_try_from_unsigned_to_signed_upper_err {
+    ($fn_name:ident, $source:ty, $target:ty) => {
+        #[test]
+        fn $fn_name() {
+            let max = <$source>::max_value();
+            let min = <$source>::min_value();
+            let zero: $source = 0;
+            assert!(<$target as TryFrom<$source>>::try_from(max).is_err());
+            assert_eq!(<$target as TryFrom<$source>>::try_from(min).unwrap(),
+                       min as $target);
+            assert_eq!(<$target as TryFrom<$source>>::try_from(zero).unwrap(),
+                       zero as $target);
+        }
+    }
+}
+
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u8i8, u8, i8 }
+
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u16i8, u16, i8 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u16i16, u16, i16 }
+
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u32i8, u32, i8 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u32i16, u32, i16 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u32i32, u32, i32 }
+
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u64i8, u64, i8 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u64i16, u64, i16 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u64i32, u64, i32 }
+test_impl_try_from_unsigned_to_signed_upper_err! { test_try_u64i64, u64, i64 }
+
+macro_rules! test_impl_try_from_same_sign_err {
+    ($fn_name:ident, $source:ty, $target:ty) => {
+        #[test]
+        fn $fn_name() {
+            let max = <$source>::max_value();
+            let min = <$source>::min_value();
+            let zero: $source = 0;
+            let t_max = <$target>::max_value();
+            let t_min = <$target>::min_value();
+            assert!(<$target as TryFrom<$source>>::try_from(max).is_err());
+            if min != 0 {
+                assert!(<$target as TryFrom<$source>>::try_from(min).is_err());
+            }
+            assert_eq!(<$target as TryFrom<$source>>::try_from(zero).unwrap(),
+                       zero as $target);
+            assert_eq!(<$target as TryFrom<$source>>::try_from(t_max as $source)
+                            .unwrap(),
+                       t_max as $target);
+            assert_eq!(<$target as TryFrom<$source>>::try_from(t_min as $source)
+                            .unwrap(),
+                       t_min as $target);
+        }
+    }
+}
+
+test_impl_try_from_same_sign_err! { test_try_u16u8, u16, u8 }
+
+test_impl_try_from_same_sign_err! { test_try_u32u8, u32, u8 }
+test_impl_try_from_same_sign_err! { test_try_u32u16, u32, u16 }
+
+test_impl_try_from_same_sign_err! { test_try_u64u8, u64, u8 }
+test_impl_try_from_same_sign_err! { test_try_u64u16, u64, u16 }
+test_impl_try_from_same_sign_err! { test_try_u64u32, u64, u32 }
+
+test_impl_try_from_same_sign_err! { test_try_i16i8, i16, i8 }
+
+test_impl_try_from_same_sign_err! { test_try_i32i8, i32, i8 }
+test_impl_try_from_same_sign_err! { test_try_i32i16, i32, i16 }
+
+test_impl_try_from_same_sign_err! { test_try_i64i8, i64, i8 }
+test_impl_try_from_same_sign_err! { test_try_i64i16, i64, i16 }
+test_impl_try_from_same_sign_err! { test_try_i64i32, i64, i32 }

src/libstd/error.rs

@@ -159,6 +159,13 @@ impl Error for num::ParseIntError {
     }
 }
 
+#[unstable(feature = "try_from", issue = "33417")]
+impl Error for num::TryFromIntError {
+    fn description(&self) -> &str {
+        self.__description()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseFloatError {
     fn description(&self) -> &str {

src/libstd/lib.rs

@@ -270,6 +270,7 @@
 #![feature(vec_push_all)]
 #![feature(zero_one)]
 #![feature(question_mark)]
+#![feature(try_from)]
 
 // Issue# 30592: Systematically use alloc_system during stage0 since jemalloc
 // might be unavailable or disabled

src/libstd/num/mod.rs

@@ -19,7 +19,7 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::num::{Zero, One};
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use core::num::{FpCategory, ParseIntError, ParseFloatError};
+pub use core::num::{FpCategory, ParseIntError, ParseFloatError, TryFromIntError};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::num::Wrapping;