Auto merge of #58302 - SimonSapin:tryfrom, r=alexcrichton

Stabilize TryFrom and TryInto with a convert::Infallible empty enum

This is the plan proposed in https://github.com/rust-lang/rust/issues/33417#issuecomment-423073898

6 files changed, 150 insertions, 29 deletions

diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 3a27a39af4a..9c6ecc43502 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -49,7 +49,7 @@ unsafe impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
 }
 
 /// The error type returned when a conversion from a slice to an array fails.
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 #[derive(Debug, Copy, Clone)]
 pub struct TryFromSliceError(());
 
@@ -138,7 +138,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable(feature = "try_from", issue = "33417")]
+            #[stable(feature = "try_from", since = "1.34.0")]
             impl<'a, T> TryFrom<&'a [T]> for [T; $N] where T: Copy {
                 type Error = TryFromSliceError;
 
@@ -147,7 +147,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable(feature = "try_from", issue = "33417")]
+            #[stable(feature = "try_from", since = "1.34.0")]
             impl<'a, T> TryFrom<&'a [T]> for &'a [T; $N] {
                 type Error = TryFromSliceError;
 
@@ -161,7 +161,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable(feature = "try_from", issue = "33417")]
+            #[stable(feature = "try_from", since = "1.34.0")]
             impl<'a, T> TryFrom<&'a mut [T]> for &'a mut [T; $N] {
                 type Error = TryFromSliceError;
 
diff --git a/src/libcore/char/convert.rs b/src/libcore/char/convert.rs
index 4a1a236b669..6a5abfb408f 100644
--- a/src/libcore/char/convert.rs
+++ b/src/libcore/char/convert.rs
@@ -218,7 +218,7 @@ impl FromStr for char {
 }
 
 
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 impl TryFrom<u32> for char {
     type Error = CharTryFromError;
 
@@ -233,11 +233,11 @@ impl TryFrom<u32> for char {
 }
 
 /// The error type returned when a conversion from u32 to char fails.
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub struct CharTryFromError(());
 
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 impl fmt::Display for CharTryFromError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         "converted integer out of range for `char`".fmt(f)
diff --git a/src/libcore/char/mod.rs b/src/libcore/char/mod.rs
index 15e153bdfad..f3369c4d940 100644
--- a/src/libcore/char/mod.rs
+++ b/src/libcore/char/mod.rs
@@ -30,7 +30,7 @@ pub use self::convert::{from_u32, from_digit};
 pub use self::convert::from_u32_unchecked;
 #[stable(feature = "char_from_str", since = "1.20.0")]
 pub use self::convert::ParseCharError;
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 pub use self::convert::CharTryFromError;
 #[stable(feature = "decode_utf16", since = "1.9.0")]
 pub use self::decode::{decode_utf16, DecodeUtf16, DecodeUtf16Error};
diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index b8d751cfbb6..5ecfa9cde03 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -41,6 +41,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use fmt;
+
 /// An identity function.
 ///
 /// Two things are important to note about this function:
@@ -367,22 +369,26 @@ pub trait From<T>: Sized {
 ///
 /// [`TryFrom`]: trait.TryFrom.html
 /// [`Into`]: trait.Into.html
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 pub trait TryInto<T>: Sized {
     /// The type returned in the event of a conversion error.
+    #[stable(feature = "try_from", since = "1.34.0")]
     type Error;
 
     /// Performs the conversion.
+    #[stable(feature = "try_from", since = "1.34.0")]
     fn try_into(self) -> Result<T, Self::Error>;
 }
 
 /// Attempt to construct `Self` via a conversion.
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 pub trait TryFrom<T>: Sized {
     /// The type returned in the event of a conversion error.
+    #[stable(feature = "try_from", since = "1.34.0")]
     type Error;
 
     /// Performs the conversion.
+    #[stable(feature = "try_from", since = "1.34.0")]
     fn try_from(value: T) -> Result<Self, Self::Error>;
 }
 
@@ -450,7 +456,7 @@ impl<T> From<T> for T {
 
 
 // TryFrom implies TryInto
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 impl<T, U> TryInto<U> for T where U: TryFrom<T>
 {
     type Error = U::Error;
@@ -462,9 +468,9 @@ 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")]
+#[stable(feature = "try_from", since = "1.34.0")]
 impl<T, U> TryFrom<U> for T where U: Into<T> {
-    type Error = !;
+    type Error = Infallible;
 
     fn try_from(value: U) -> Result<Self, Self::Error> {
         Ok(U::into(value))
@@ -496,3 +502,115 @@ impl AsRef<str> for str {
         self
     }
 }
+
+////////////////////////////////////////////////////////////////////////////////
+// THE NO-ERROR ERROR TYPE
+////////////////////////////////////////////////////////////////////////////////
+
+/// The error type for errors that can never happen.
+///
+/// Since this enum has no variant, a value of this type can never actually exist.
+/// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
+/// to indicate that the result is always [`Ok`].
+///
+/// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
+/// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
+///
+/// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
+/// impl<T, U> TryFrom<U> for T where U: Into<T> {
+///     type Error = Infallible;
+///
+///     fn try_from(value: U) -> Result<Self, Infallible> {
+///         Ok(U::into(value))  // Never returns `Err`
+///     }
+/// }
+/// ```
+///
+/// # Future compatibility
+///
+/// This enum has the same role as [the `!` “never” type][never],
+/// which is unstable in this version of Rust.
+/// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
+///
+/// ```ignore (illustrates future std change)
+/// pub type Infallible = !;
+/// ```
+///
+/// … and eventually deprecate `Infallible`.
+///
+///
+/// However there is one case where `!` syntax can be used
+/// before `!` is stabilized as a full-fleged type: in the position of a function’s return type.
+/// Specifically, it is possible implementations for two different function pointer types:
+///
+/// ```
+/// trait MyTrait {}
+/// impl MyTrait for fn() -> ! {}
+/// impl MyTrait for fn() -> std::convert::Infallible {}
+/// ```
+///
+/// With `Infallible` being an enum, this code is valid.
+/// However when `Infallible` becomes an alias for the never type,
+/// the two `impl`s will start to overlap
+/// and therefore will be disallowed by the language’s trait coherence rules.
+///
+/// [`Ok`]: ../result/enum.Result.html#variant.Ok
+/// [`Result`]: ../result/enum.Result.html
+/// [`TryFrom`]: trait.TryFrom.html
+/// [`Into`]: trait.Into.html
+/// [never]: ../../std/primitive.never.html
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+#[derive(Copy)]
+pub enum Infallible {}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl Clone for Infallible {
+    fn clone(&self) -> Infallible {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl fmt::Debug for Infallible {
+    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl fmt::Display for Infallible {
+    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl PartialEq for Infallible {
+    fn eq(&self, _: &Infallible) -> bool {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl Eq for Infallible {}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl PartialOrd for Infallible {
+    fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl Ord for Infallible {
+    fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
+        match *self {}
+    }
+}
+
+#[stable(feature = "convert_infallible", since = "1.34.0")]
+impl From<!> for Infallible {
+    fn from(x: !) -> Self {
+        x
+    }
+}
diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index 5b7d5f45d92..3ceba83afee 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -2,7 +2,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use convert::TryFrom;
+use convert::{TryFrom, Infallible};
 use fmt;
 use intrinsics;
 use mem;
@@ -2000,7 +2000,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -2032,7 +2031,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -2074,7 +2072,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -3767,7 +3764,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -3799,7 +3795,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -3841,7 +3836,6 @@ assert_eq!(value, ", $swap_op, ");
 When starting from a slice rather than an array, fallible conversion APIs can be used:
 
 ```
-#![feature(try_from)]
 use std::convert::TryInto;
 
 fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
@@ -4504,7 +4498,7 @@ macro_rules! from_str_radix_int_impl {
 from_str_radix_int_impl! { isize i8 i16 i32 i64 i128 usize u8 u16 u32 u64 u128 }
 
 /// The error type returned when a checked integral type conversion fails.
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct TryFromIntError(());
 
@@ -4519,24 +4513,34 @@ impl TryFromIntError {
     }
 }
 
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
 impl fmt::Display for TryFromIntError {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         self.__description().fmt(fmt)
     }
 }
 
-#[unstable(feature = "try_from", issue = "33417")]
+#[stable(feature = "try_from", since = "1.34.0")]
+impl From<Infallible> for TryFromIntError {
+    fn from(x: Infallible) -> TryFromIntError {
+        match x {}
+    }
+}
+
+#[unstable(feature = "never_type", issue = "35121")]
 impl From<!> for TryFromIntError {
     fn from(never: !) -> TryFromIntError {
-        never
+        // Match rather than coerce to make sure that code like
+        // `From<Infallible> for TryFromIntError` above will keep working
+        // when `Infallible` becomes an alias to `!`.
+        match never {}
     }
 }
 
 // no possible bounds violation
 macro_rules! try_from_unbounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[unstable(feature = "try_from", issue = "33417")]
+        #[stable(feature = "try_from", since = "1.34.0")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
@@ -4551,7 +4555,7 @@ macro_rules! try_from_unbounded {
 // only negative bounds
 macro_rules! try_from_lower_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[unstable(feature = "try_from", issue = "33417")]
+        #[stable(feature = "try_from", since = "1.34.0")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
@@ -4570,7 +4574,7 @@ macro_rules! try_from_lower_bounded {
 // unsigned to signed (only positive bound)
 macro_rules! try_from_upper_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[unstable(feature = "try_from", issue = "33417")]
+        #[stable(feature = "try_from", since = "1.34.0")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
@@ -4589,7 +4593,7 @@ macro_rules! try_from_upper_bounded {
 // all other cases
 macro_rules! try_from_both_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[unstable(feature = "try_from", issue = "33417")]
+        #[stable(feature = "try_from", since = "1.34.0")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 4cd734bad90..d0021376389 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -26,7 +26,6 @@
 #![feature(str_internals)]
 #![feature(test)]
 #![feature(trusted_len)]
-#![feature(try_from)]
 #![feature(try_trait)]
 #![feature(align_offset)]
 #![feature(reverse_bits)]