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.)

Author: bors

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;
 

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)