Auto merge of #50121 - pnkfelix:revert-stabilization-of-never-type-et-al, r=alexcrichton

Revert stabilization of never_type (!) et al

Fix #49691

I *think* this correctly adopts @nikomatsakis 's desired fix of:
 * reverting stabilization of `!` and `TryFrom`, and
 * returning to the previous fallback semantics (i.e. it is once again dependent on whether the crate has opted into `#[feature(never_type)]`,
 * **without** attempting to put back in the previous future-proofing warnings regarding the change in fallback semantics.

(I'll be away from computers for a week starting now, so any updates to this PR should be either pushed into it, or someone else should adopt the task of polishing this fix and put up their own PR.)

11 files changed, 27 insertions, 29 deletions

diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 87144c27c9e..3d24f8902bd 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -59,7 +59,7 @@ unsafe impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
 }
 
 /// The error type returned when a conversion from a slice to an array fails.
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 #[derive(Debug, Copy, Clone)]
 pub struct TryFromSliceError(());
 
@@ -148,7 +148,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[stable(feature = "try_from", since = "1.26.0")]
+            #[unstable(feature = "try_from", issue = "33417")]
             impl<'a, T> TryFrom<&'a [T]> for &'a [T; $N] {
                 type Error = TryFromSliceError;
 
@@ -162,7 +162,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[stable(feature = "try_from", since = "1.26.0")]
+            #[unstable(feature = "try_from", issue = "33417")]
             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 150562a4a9b..803a924eb3a 100644
--- a/src/libcore/char/convert.rs
+++ b/src/libcore/char/convert.rs
@@ -204,7 +204,7 @@ impl FromStr for char {
 }
 
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl TryFrom<u32> for char {
     type Error = CharTryFromError;
 
@@ -219,11 +219,11 @@ impl TryFrom<u32> for char {
 }
 
 /// The error type returned when a conversion from u32 to char fails.
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub struct CharTryFromError(());
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 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 9edc0c88756..210eceebc51 100644
--- a/src/libcore/char/mod.rs
+++ b/src/libcore/char/mod.rs
@@ -40,7 +40,7 @@ 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;
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 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/clone.rs b/src/libcore/clone.rs
index 58a8439162c..f79f7351698 100644
--- a/src/libcore/clone.rs
+++ b/src/libcore/clone.rs
@@ -179,7 +179,7 @@ mod impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Clone for ! {
         #[inline]
         fn clone(&self) -> Self {
diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index 3ae9b05b865..c91aa06609d 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -881,24 +881,24 @@ mod impls {
 
     ord_impl! { char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialEq for ! {
         fn eq(&self, _: &!) -> bool {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Eq for ! {}
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialOrd for ! {
         fn partial_cmp(&self, _: &!) -> Option<Ordering> {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Ord for ! {
         fn cmp(&self, _: &!) -> Ordering {
             *self
diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index 63721395784..7324df95bc5 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -322,26 +322,22 @@ pub trait From<T>: Sized {
 ///
 /// [`TryFrom`]: trait.TryFrom.html
 /// [`Into`]: trait.Into.html
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 pub trait TryInto<T>: Sized {
     /// The type returned in the event of a conversion error.
-    #[stable(feature = "try_from", since = "1.26.0")]
     type Error;
 
     /// Performs the conversion.
-    #[stable(feature = "try_from", since = "1.26.0")]
     fn try_into(self) -> Result<T, Self::Error>;
 }
 
 /// Attempt to construct `Self` via a conversion.
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 pub trait TryFrom<T>: Sized {
     /// The type returned in the event of a conversion error.
-    #[stable(feature = "try_from", since = "1.26.0")]
     type Error;
 
     /// Performs the conversion.
-    #[stable(feature = "try_from", since = "1.26.0")]
     fn try_from(value: T) -> Result<Self, Self::Error>;
 }
 
@@ -409,7 +405,7 @@ impl<T> From<T> for T {
 
 
 // TryFrom implies TryInto
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl<T, U> TryInto<U> for T where U: TryFrom<T>
 {
     type Error = U::Error;
@@ -421,7 +417,7 @@ impl<T, U> TryInto<U> for T where U: TryFrom<T>
 
 // Infallible conversions are semantically equivalent to fallible conversions
 // with an uninhabited error type.
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl<T, U> TryFrom<U> for T where T: From<U> {
     type Error = !;
 
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 277bef2bf66..a8430f14410 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1780,14 +1780,14 @@ macro_rules! fmt_refs {
 
 fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Debug for ! {
     fn fmt(&self, _: &mut Formatter) -> Result {
         *self
     }
 }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Display for ! {
     fn fmt(&self, _: &mut Formatter) -> Result {
         *self
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index ea7a46f44ae..ac1a8091eb3 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -82,6 +82,7 @@
 #![feature(iterator_repeat_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(macro_at_most_once_rep)]
 #![feature(no_core)]
diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs
index 885aabe0806..feb689dbc1f 100644
--- a/src/libcore/marker.rs
+++ b/src/libcore/marker.rs
@@ -630,7 +630,7 @@ mod copy_impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Copy for ! {}
 
     #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index f2e8caaad14..801e2328a4b 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -4192,7 +4192,7 @@ 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.
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 #[derive(Debug, Copy, Clone)]
 pub struct TryFromIntError(());
 
@@ -4207,14 +4207,14 @@ impl TryFromIntError {
     }
 }
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl fmt::Display for TryFromIntError {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         self.__description().fmt(fmt)
     }
 }
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl From<!> for TryFromIntError {
     fn from(never: !) -> TryFromIntError {
         never
@@ -4224,7 +4224,7 @@ impl From<!> for TryFromIntError {
 // only negative bounds
 macro_rules! try_from_lower_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[stable(feature = "try_from", since = "1.26.0")]
+        #[unstable(feature = "try_from", issue = "33417")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
@@ -4243,7 +4243,7 @@ macro_rules! try_from_lower_bounded {
 // unsigned to signed (only positive bound)
 macro_rules! try_from_upper_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[stable(feature = "try_from", since = "1.26.0")]
+        #[unstable(feature = "try_from", issue = "33417")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
@@ -4262,7 +4262,7 @@ macro_rules! try_from_upper_bounded {
 // all other cases
 macro_rules! try_from_both_bounded {
     ($source:ty, $($target:ty),*) => {$(
-        #[stable(feature = "try_from", since = "1.26.0")]
+        #[unstable(feature = "try_from", issue = "33417")]
         impl TryFrom<$source> for $target {
             type Error = TryFromIntError;
 
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index bb875c7219a..3b080689cb3 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -36,6 +36,7 @@
 #![feature(str_internals)]
 #![feature(test)]
 #![feature(trusted_len)]
+#![feature(try_from)]
 #![feature(try_trait)]
 #![feature(exact_chunks)]
 #![cfg_attr(stage0, feature(atomic_nand))]