Rollup merge of rust-lang#67224 - nikomatsakis:revert-stabilization-of-never-type, r=centril

Revert stabilization of never type

Fixes rust-lang#66757

I decided to keep the separate `never-type-fallback` feature gate, but tried to otherwise revert rust-lang#65355. Seemed pretty clean.

( cc @Centril, author of rust-lang#65355, you may want to check this over briefly )

Author: Centril

src/libcore/clone.rs

@@ -195,7 +195,7 @@ mod impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Clone for ! {
         #[inline]
         fn clone(&self) -> Self {

src/libcore/cmp.rs

@@ -1141,24 +1141,24 @@ mod impls {
 
     ord_impl! { char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialEq for ! {
         fn eq(&self, _: &!) -> bool {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Eq for ! {}
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialOrd for ! {
         fn partial_cmp(&self, _: &!) -> Option<Ordering> {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Ord for ! {
         fn cmp(&self, _: &!) -> Ordering {
             *self

src/libcore/convert/mod.rs

@@ -40,6 +40,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::fmt;
+
 mod num;
 
 #[unstable(feature = "convert_float_to_int", issue = "67057")]
@@ -429,7 +431,9 @@ pub trait TryInto<T>: Sized {
 /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
 /// is implemented and cannot fail -- the associated `Error` type for
-/// calling `T::try_from()` on a value of type `T` is [`!`].
+/// calling `T::try_from()` on a value of type `T` is [`Infallible`].
+/// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
+/// equivalent.
 ///
 /// `TryFrom<T>` can be implemented as follows:
 ///
@@ -478,6 +482,7 @@ pub trait TryInto<T>: Sized {
 /// [`TryInto`]: trait.TryInto.html
 /// [`i32::MAX`]: ../../std/i32/constant.MAX.html
 /// [`!`]: ../../std/primitive.never.html
+/// [`Infallible`]: enum.Infallible.html
 #[stable(feature = "try_from", since = "1.34.0")]
 pub trait TryFrom<T>: Sized {
     /// The type returned in the event of a conversion error.
@@ -633,9 +638,9 @@ impl AsRef<str> for str {
 // THE NO-ERROR ERROR TYPE
 ////////////////////////////////////////////////////////////////////////////////
 
-/// A type alias for [the `!` “never” type][never].
+/// The error type for errors that can never happen.
 ///
-/// `Infallible` represents types of errors that can never happen since `!` has no valid values.
+/// 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`].
 ///
@@ -652,15 +657,91 @@ impl AsRef<str> for str {
 /// }
 /// ```
 ///
-/// # Eventual deprecation
+/// # 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 {}
+/// ```
 ///
-/// Previously, `Infallible` was defined as `enum Infallible {}`.
-/// Now that it is merely a type alias to `!`, we will eventually deprecate `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")]
-pub type Infallible = !;
+#[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
+    }
+}

src/libcore/fmt/mod.rs

@@ -1940,14 +1940,14 @@ macro_rules! fmt_refs {
 
 fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
 
-#[stable(feature = "never_type", since = "1.41.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Debug for ! {
     fn fmt(&self, _: &mut Formatter<'_>) -> Result {
         *self
     }
 }
 
-#[stable(feature = "never_type", since = "1.41.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Display for ! {
     fn fmt(&self, _: &mut Formatter<'_>) -> Result {
         *self

src/libcore/lib.rs

@@ -87,7 +87,7 @@
 #![feature(iter_once_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(nll)]
 #![feature(exhaustive_patterns)]
 #![feature(no_core)]

src/libcore/marker.rs

@@ -776,7 +776,7 @@ mod copy_impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.41.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Copy for ! {}
 
     #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/num/mod.rs

@@ -4,6 +4,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::convert::Infallible;
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem;
@@ -4724,8 +4725,18 @@ impl fmt::Display for TryFromIntError {
 }
 
 #[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 {
+        // 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 {}
     }
 }

src/librustc/lib.rs

@@ -37,7 +37,7 @@
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![cfg_attr(windows, feature(libc))]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(overlapping_marker_traits)]
 #![feature(extern_types)]

src/librustc_codegen_utils/lib.rs

@@ -8,7 +8,7 @@
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(core_intrinsics)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(nll)]
 #![feature(in_band_lifetimes)]
 

src/librustc_error_codes/error_codes/E0725.md

@@ -4,8 +4,8 @@ command line flags.
 Erroneous code example:
 
 ```ignore (can't specify compiler flags from doctests)
-#![feature(specialization)] // error: the feature `specialization` is not in
-                            // the list of allowed features
+#![feature(never_type)] // error: the feature `never_type` is not in
+                        // the list of allowed features
 ```
 
 Delete the offending feature attribute, or add it to the list of allowed

src/librustc_feature/accepted.rs

@@ -253,8 +253,6 @@ declare_features! (
     (accepted, const_constructor, "1.40.0", Some(61456), None),
     /// Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests.
     (accepted, cfg_doctest, "1.40.0", Some(62210), None),
-    /// Allows the `!` type. Does not imply 'exhaustive_patterns' any more.
-    (accepted, never_type, "1.41.0", Some(35121), None),
     /// Allows relaxing the coherence rules such that
     /// `impl<T> ForeignTrait<LocalType> for ForeignType<T>` is permitted.
     (accepted, re_rebalance_coherence, "1.41.0", Some(55437), None),

src/librustc_feature/active.rs

@@ -307,6 +307,9 @@ declare_features! (
     /// Allows `X..Y` patterns.
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
 
+    /// Allows the `!` type. Does not imply 'exhaustive_patterns' (below) any more.
+    (active, never_type, "1.13.0", Some(35121), None),
+
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (active, exhaustive_patterns, "1.13.0", Some(51085), None),
 

src/librustc_mir/lib.rs

@@ -18,7 +18,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(drain_filter)]
 #![feature(exhaustive_patterns)]
 #![feature(iter_order_by)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(specialization)]
 #![feature(try_trait)]
 #![feature(unicode_internals)]

src/librustc_typeck/lib.rs

@@ -67,7 +67,7 @@ This API is completely unstable and subject to change.
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
 #![feature(slice_patterns)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 
 #![recursion_limit="256"]
 

src/librustdoc/lib.rs

@@ -14,7 +14,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(const_fn)]
 #![feature(drain_filter)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(unicode_internals)]
 
 #![recursion_limit="256"]

src/libserialize/lib.rs

@@ -11,7 +11,7 @@ Core encoding and decoding interfaces.
 #![feature(box_syntax)]
 #![feature(core_intrinsics)]
 #![feature(specialization)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(nll)]
 #![feature(associated_type_bounds)]
 #![cfg_attr(test, feature(test))]

src/libstd/error.rs

@@ -465,7 +465,7 @@ impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
     }
 }
 
-#[stable(feature = "never_type", since = "1.41.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {
     fn description(&self) -> &str { *self }
 }
@@ -551,6 +551,13 @@ impl Error for string::FromUtf16Error {
     }
 }
 
+#[stable(feature = "str_parse_error2", since = "1.8.0")]
+impl Error for string::ParseError {
+    fn description(&self) -> &str {
+        match *self {}
+    }
+}
+
 #[stable(feature = "decode_utf16", since = "1.9.0")]
 impl Error for char::DecodeUtf16Error {
     fn description(&self) -> &str {

src/libstd/lib.rs

@@ -281,7 +281,7 @@
 #![feature(maybe_uninit_ref)]
 #![feature(maybe_uninit_slice)]
 #![feature(needs_panic_runtime)]
-#![cfg_attr(bootstrap, feature(never_type))]
+#![feature(never_type)]
 #![feature(nll)]
 #![cfg_attr(bootstrap, feature(on_unimplemented))]
 #![feature(optin_builtin_traits)]

src/libstd/primitive_docs.rs

@@ -71,6 +71,7 @@ mod prim_bool { }
 /// write:
 ///
 /// ```
+/// #![feature(never_type)]
 /// # fn foo() -> u32 {
 /// let x: ! = {
 ///     return 123
@@ -200,6 +201,7 @@ mod prim_bool { }
 /// for example:
 ///
 /// ```
+/// #![feature(never_type)]
 /// # use std::fmt;
 /// # trait Debug {
 /// # fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
@@ -237,7 +239,7 @@ mod prim_bool { }
 /// [`Default`]: default/trait.Default.html
 /// [`default()`]: default/trait.Default.html#tymethod.default
 ///
-#[stable(feature = "never_type", since = "1.41.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 mod prim_never { }
 
 #[doc(primitive = "char")]