Auto merge of #44174 - jimmycuadra:try-from-infallible, r=sfackler

Add blanket TryFrom impl when From is implemented.

Adds `impl<T, U> TryFrom<T> for U where U: From<T>`.

Removes `impl<'a, T> TryFrom<&'a str> for T where T: FromStr` (originally added in #40281) due to overlapping impls caused by the new blanket impl. This removal is to be discussed further on the tracking issue for TryFrom.

Refs #33417.

/cc @sfackler, @scottmcm (thank you for the help!), and @aturon

src/libcore/convert.rs

@@ -48,8 +48,25 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use str::FromStr;
+use fmt;
 
+/// A type used as the error type for implementations of fallible conversion
+/// traits in cases where conversions cannot actually fail.
+///
+/// Because `Infallible` has no variants, a value of this type can never exist.
+/// It is used only to satisfy trait signatures that expect an error type, and
+/// signals to both the compiler and the user that the error case is impossible.
+#[unstable(feature = "try_from", issue = "33417")]
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub enum Infallible {}
+
+#[unstable(feature = "try_from", issue = "33417")]
+impl fmt::Display for Infallible {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+        }
+    }
+}
 /// A cheap reference-to-reference conversion. Used to convert a value to a
 /// reference value within generic code.
 ///
@@ -417,6 +434,17 @@ impl<T, U> TryInto<U> for T where U: TryFrom<T>
     }
 }
 
+// Infallible conversions are semantically equivalent to fallible conversions
+// with an uninhabited error type.
+#[unstable(feature = "try_from", issue = "33417")]
+impl<T, U> TryFrom<U> for T where T: From<U> {
+    type Error = Infallible;
+
+    fn try_from(value: U) -> Result<Self, Self::Error> {
+        Ok(T::from(value))
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // CONCRETE IMPLS
 ////////////////////////////////////////////////////////////////////////////////
@@ -442,14 +470,3 @@ impl AsRef<str> for str {
         self
     }
 }
-
-// FromStr implies TryFrom<&str>
-#[unstable(feature = "try_from", issue = "33417")]
-impl<'a, T> TryFrom<&'a str> for T where T: FromStr
-{
-    type Error = <T as FromStr>::Err;
-
-    fn try_from(s: &'a str) -> Result<T, Self::Error> {
-        FromStr::from_str(s)
-    }
-}

src/libcore/iter/range.rs

@@ -89,6 +89,7 @@ macro_rules! step_impl_unsigned {
             }
 
             #[inline]
+            #[allow(unreachable_patterns)]
             fn add_usize(&self, n: usize) -> Option<Self> {
                 match <$t>::try_from(n) {
                     Ok(n_as_t) => self.checked_add(n_as_t),
@@ -120,6 +121,7 @@ macro_rules! step_impl_signed {
             }
 
             #[inline]
+            #[allow(unreachable_patterns)]
             fn add_usize(&self, n: usize) -> Option<Self> {
                 match <$unsigned>::try_from(n) {
                     Ok(n_as_unsigned) => {

src/libcore/num/mod.rs

@@ -12,7 +12,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use convert::TryFrom;
+use convert::{Infallible, TryFrom};
 use fmt;
 use intrinsics;
 use str::FromStr;
@@ -2507,16 +2507,24 @@ impl fmt::Display for TryFromIntError {
     }
 }
 
+#[unstable(feature = "try_from", issue = "33417")]
+impl From<Infallible> for TryFromIntError {
+    fn from(infallible: Infallible) -> TryFromIntError {
+        match infallible {
+        }
+    }
+}
+
 // no possible bounds violation
 macro_rules! try_from_unbounded {
     ($source:ty, $($target:ty),*) => {$(
         #[unstable(feature = "try_from", issue = "33417")]
         impl TryFrom<$source> for $target {
-            type Error = TryFromIntError;
+            type Error = Infallible;
 
             #[inline]
-            fn try_from(u: $source) -> Result<$target, TryFromIntError> {
-                Ok(u as $target)
+            fn try_from(value: $source) -> Result<Self, Self::Error> {
+                Ok(value as $target)
             }
         }
     )*}
@@ -2588,31 +2596,17 @@ macro_rules! rev {
 }
 
 /// intra-sign conversions
-try_from_unbounded!(u8, u8, u16, u32, u64, u128);
-try_from_unbounded!(u16, u16, u32, u64, u128);
-try_from_unbounded!(u32, u32, u64, u128);
-try_from_unbounded!(u64, u64, u128);
-try_from_unbounded!(u128, u128);
 try_from_upper_bounded!(u16, u8);
 try_from_upper_bounded!(u32, u16, u8);
 try_from_upper_bounded!(u64, u32, u16, u8);
 try_from_upper_bounded!(u128, u64, u32, u16, u8);
 
-try_from_unbounded!(i8, i8, i16, i32, i64, i128);
-try_from_unbounded!(i16, i16, i32, i64, i128);
-try_from_unbounded!(i32, i32, i64, i128);
-try_from_unbounded!(i64, i64, i128);
-try_from_unbounded!(i128, i128);
 try_from_both_bounded!(i16, i8);
 try_from_both_bounded!(i32, i16, i8);
 try_from_both_bounded!(i64, i32, i16, i8);
 try_from_both_bounded!(i128, i64, i32, i16, i8);
 
 // unsigned-to-signed
-try_from_unbounded!(u8, i16, i32, i64, i128);
-try_from_unbounded!(u16, i32, i64, i128);
-try_from_unbounded!(u32, i64, i128);
-try_from_unbounded!(u64, i128);
 try_from_upper_bounded!(u8, i8);
 try_from_upper_bounded!(u16, i8, i16);
 try_from_upper_bounded!(u32, i8, i16, i32);
@@ -2631,15 +2625,13 @@ try_from_both_bounded!(i64, u32, u16, u8);
 try_from_both_bounded!(i128, u64, u32, u16, u8);
 
 // usize/isize
-try_from_unbounded!(usize, usize);
 try_from_upper_bounded!(usize, isize);
 try_from_lower_bounded!(isize, usize);
-try_from_unbounded!(isize, isize);
 
 #[cfg(target_pointer_width = "16")]
 mod ptr_try_from_impls {
     use super::TryFromIntError;
-    use convert::TryFrom;
+    use convert::{Infallible, TryFrom};
 
     try_from_upper_bounded!(usize, u8);
     try_from_unbounded!(usize, u16, u32, u64, u128);
@@ -2651,21 +2643,21 @@ mod ptr_try_from_impls {
     try_from_both_bounded!(isize, i8);
     try_from_unbounded!(isize, i16, i32, i64, i128);
 
-    rev!(try_from_unbounded, usize, u8, u16);
+    rev!(try_from_unbounded, usize, u16);
     rev!(try_from_upper_bounded, usize, u32, u64, u128);
     rev!(try_from_lower_bounded, usize, i8, i16);
     rev!(try_from_both_bounded, usize, i32, i64, i128);
 
     rev!(try_from_unbounded, isize, u8);
     rev!(try_from_upper_bounded, isize, u16, u32, u64, u128);
-    rev!(try_from_unbounded, isize, i8, i16);
+    rev!(try_from_unbounded, isize, i16);
     rev!(try_from_both_bounded, isize, i32, i64, i128);
 }
 
 #[cfg(target_pointer_width = "32")]
 mod ptr_try_from_impls {
     use super::TryFromIntError;
-    use convert::TryFrom;
+    use convert::{Infallible, TryFrom};
 
     try_from_upper_bounded!(usize, u8, u16);
     try_from_unbounded!(usize, u32, u64, u128);
@@ -2677,21 +2669,21 @@ mod ptr_try_from_impls {
     try_from_both_bounded!(isize, i8, i16);
     try_from_unbounded!(isize, i32, i64, i128);
 
-    rev!(try_from_unbounded, usize, u8, u16, u32);
+    rev!(try_from_unbounded, usize, u16, u32);
     rev!(try_from_upper_bounded, usize, u64, u128);
     rev!(try_from_lower_bounded, usize, i8, i16, i32);
     rev!(try_from_both_bounded, usize, i64, i128);
 
     rev!(try_from_unbounded, isize, u8, u16);
     rev!(try_from_upper_bounded, isize, u32, u64, u128);
-    rev!(try_from_unbounded, isize, i8, i16, i32);
+    rev!(try_from_unbounded, isize, i16, i32);
     rev!(try_from_both_bounded, isize, i64, i128);
 }
 
 #[cfg(target_pointer_width = "64")]
 mod ptr_try_from_impls {
     use super::TryFromIntError;
-    use convert::TryFrom;
+    use convert::{Infallible, TryFrom};
 
     try_from_upper_bounded!(usize, u8, u16, u32);
     try_from_unbounded!(usize, u64, u128);
@@ -2703,14 +2695,14 @@ mod ptr_try_from_impls {
     try_from_both_bounded!(isize, i8, i16, i32);
     try_from_unbounded!(isize, i64, i128);
 
-    rev!(try_from_unbounded, usize, u8, u16, u32, u64);
+    rev!(try_from_unbounded, usize, u16, u32, u64);
     rev!(try_from_upper_bounded, usize, u128);
     rev!(try_from_lower_bounded, usize, i8, i16, i32, i64);
     rev!(try_from_both_bounded, usize, i128);
 
     rev!(try_from_unbounded, isize, u8, u16, u32);
     rev!(try_from_upper_bounded, isize, u64, u128);
-    rev!(try_from_unbounded, isize, i8, i16, i32, i64);
+    rev!(try_from_unbounded, isize, i16, i32, i64);
     rev!(try_from_both_bounded, isize, i128);
 }
 

src/libcore/str/mod.rs

@@ -18,7 +18,6 @@ use self::pattern::Pattern;
 use self::pattern::{Searcher, ReverseSearcher, DoubleEndedSearcher};
 
 use char;
-use convert::TryFrom;
 use fmt;
 use iter::{Map, Cloned, FusedIterator, TrustedLen};
 use iter_private::TrustedRandomAccess;
@@ -2198,7 +2197,7 @@ pub trait StrExt {
     #[stable(feature = "core", since = "1.6.0")]
     fn is_empty(&self) -> bool;
     #[stable(feature = "core", since = "1.6.0")]
-    fn parse<'a, T: TryFrom<&'a str>>(&'a self) -> Result<T, T::Error>;
+    fn parse<T: FromStr>(&self) -> Result<T, T::Err>;
 }
 
 // truncate `&str` to length at most equal to `max`
@@ -2518,9 +2517,7 @@ impl StrExt for str {
     fn is_empty(&self) -> bool { self.len() == 0 }
 
     #[inline]
-    fn parse<'a, T>(&'a self) -> Result<T, T::Error> where T: TryFrom<&'a str> {
-        T::try_from(self)
-    }
+    fn parse<T: FromStr>(&self) -> Result<T, T::Err> { FromStr::from_str(self) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/tests/char.rs

@@ -32,7 +32,6 @@ fn test_convert() {
 #[test]
 fn test_from_str() {
     assert_eq!(char::from_str("a").unwrap(), 'a');
-    assert_eq!(char::try_from("a").unwrap(), 'a');
     assert_eq!(char::from_str("\0").unwrap(), '\0');
     assert_eq!(char::from_str("\u{D7FF}").unwrap(), '\u{d7FF}');
     assert!(char::from_str("").is_err());

src/libcore/tests/num/mod.rs

@@ -8,10 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::convert::TryFrom;
+use core::convert::{TryFrom, TryInto};
 use core::cmp::PartialEq;
 use core::fmt::Debug;
 use core::marker::Copy;
+use core::num::TryFromIntError;
 use core::ops::{Add, Sub, Mul, Div, Rem};
 use core::option::Option;
 use core::option::Option::{Some, None};
@@ -134,6 +135,13 @@ fn test_empty() {
     assert_eq!("".parse::<u8>().ok(), None);
 }
 
+#[test]
+fn test_infallible_try_from_int_error() {
+    let func = |x: i8| -> Result<i32, TryFromIntError> { Ok(x.try_into()?) };
+
+    assert!(func(0).is_ok());
+}
+
 macro_rules! test_impl_from {
     ($fn_name: ident, $Small: ty, $Large: ty) => {
         #[test]

src/libstd/error.rs

@@ -56,6 +56,7 @@ use any::TypeId;
 use borrow::Cow;
 use cell;
 use char;
+use convert;
 use fmt::{self, Debug, Display};
 use mem::transmute;
 use num;
@@ -362,6 +363,13 @@ impl Error for char::ParseCharError {
     }
 }
 
+#[unstable(feature = "try_from", issue = "33417")]
+impl Error for convert::Infallible {
+    fn description(&self) -> &str {
+        match *self {
+        }
+    }
+}
 
 // copied from any.rs
 impl Error + 'static {