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)

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};

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 {

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

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 = !;
 

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

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)]

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")]

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;
 

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))]

src/librustc/lib.rs

@@ -52,6 +52,7 @@
 #![cfg_attr(windows, feature(libc))]
 #![feature(macro_lifetime_matcher)]
 #![feature(macro_vis_matcher)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(non_exhaustive)]
 #![feature(nonzero)]

src/librustc/ty/context.rs

@@ -2374,6 +2374,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.intern_tup(&[])
     }
 
+    pub fn mk_diverging_default(self) -> Ty<'tcx> {
+        if self.features().never_type {
+            self.types.never
+        } else {
+            self.intern_tup(&[])
+        }
+    }
+
     pub fn mk_bool(self) -> Ty<'tcx> {
         self.mk_ty(TyBool)
     }

src/librustc_apfloat/lib.rs

@@ -45,6 +45,7 @@
       html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![forbid(unsafe_code)]
 
+#![feature(try_from)]
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
 #[allow(unused_extern_crates)]
 extern crate rustc_cratesio_shim;

src/librustc_mir/build/matches/simplify.rs

@@ -113,6 +113,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             PatternKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
                 let irrefutable = adt_def.variants.iter().enumerate().all(|(i, v)| {
                     i == variant_index || {
+                        self.hir.tcx().features().never_type &&
                         self.hir.tcx().features().exhaustive_patterns &&
                         self.hir.tcx().is_variant_uninhabited_from_all_modules(v, substs)
                     }

src/librustc_mir/lib.rs

@@ -33,6 +33,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(nonzero)]
 #![feature(inclusive_range_fields)]
 #![feature(crate_visibility_modifier)]
+#![feature(never_type)]
 #![cfg_attr(stage0, feature(try_trait))]
 
 extern crate arena;

src/librustc_typeck/check/mod.rs

@@ -2217,7 +2217,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     }
 
     // Tries to apply a fallback to `ty` if it is an unsolved variable.
-    // Non-numerics get replaced with !, unconstrained ints with i32,
+    // Non-numerics get replaced with ! or () (depending on whether
+    // feature(never_type) is enabled, unconstrained ints with i32,
     // unconstrained floats with f64.
     // Fallback becomes very dubious if we have encountered type-checking errors.
     // In that case, fallback to TyError.
@@ -2231,7 +2232,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             _ if self.is_tainted_by_errors() => self.tcx().types.err,
             UnconstrainedInt => self.tcx.types.i32,
             UnconstrainedFloat => self.tcx.types.f64,
-            Neither if self.type_var_diverges(ty) => self.tcx.types.never,
+            Neither if self.type_var_diverges(ty) => self.tcx.mk_diverging_default(),
             Neither => return false,
         };
         debug!("default_type_parameters: defaulting `{:?}` to `{:?}`", ty, fallback);

src/librustc_typeck/lib.rs

@@ -82,6 +82,7 @@ This API is completely unstable and subject to change.
 #![feature(slice_patterns)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(dyn_trait)]
+#![feature(never_type)]
 
 #[macro_use] extern crate log;
 #[macro_use] extern crate syntax;

src/libstd/error.rs

@@ -233,7 +233,7 @@ impl<'a> From<Cow<'a, str>> for Box<Error> {
     }
 }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {
     fn description(&self) -> &str { *self }
 }
@@ -284,14 +284,14 @@ impl Error for num::ParseIntError {
     }
 }
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl Error for num::TryFromIntError {
     fn description(&self) -> &str {
         self.__description()
     }
 }
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl Error for array::TryFromSliceError {
     fn description(&self) -> &str {
         self.__description()
@@ -365,7 +365,7 @@ impl Error for cell::BorrowMutError {
     }
 }
 
-#[stable(feature = "try_from", since = "1.26.0")]
+#[unstable(feature = "try_from", issue = "33417")]
 impl Error for char::CharTryFromError {
     fn description(&self) -> &str {
         "converted integer out of range for `char`"

src/libstd/lib.rs

@@ -275,6 +275,7 @@
 #![feature(macro_reexport)]
 #![feature(macro_vis_matcher)]
 #![feature(needs_panic_runtime)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(nonzero)]
 #![feature(num_bits_bytes)]
@@ -306,6 +307,7 @@
 #![feature(test, rustc_private)]
 #![feature(thread_local)]
 #![feature(toowned_clone_into)]
+#![feature(try_from)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(untagged_unions)]

src/libstd/primitive_docs.rs

@@ -79,6 +79,7 @@ mod prim_bool { }
 /// write:
 ///
 /// ```
+/// #![feature(never_type)]
 /// # fn foo() -> u32 {
 /// let x: ! = {
 ///     return 123
@@ -155,6 +156,7 @@ mod prim_bool { }
 /// for example:
 ///
 /// ```
+/// #![feature(never_type)]
 /// # use std::fmt;
 /// # trait Debug {
 /// # fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result;