Auto merge of #35162 - canndrew:bang_type_coerced, r=nikomatsakis

Implement the `!` type

This implements the never type (`!`) and hides it behind the feature gate `#[feature(never_type)]`. With the feature gate off, things should build as normal (although some error messages may be different). With the gate on, `!` is usable as a type and diverging type variables (ie. types that are unconstrained by anything in the code) will default to `!` instead of `()`.

Author: bors

src/libcore/cmp.rs

@@ -699,6 +699,39 @@ mod impls {
 
     ord_impl! { char usize u8 u16 u32 u64 isize i8 i16 i32 i64 }
 
+    // Note: This macro is a temporary hack that can be remove once we are building with a compiler
+    // that supports `!`
+    macro_rules! not_stage0 {
+        () => {
+            #[unstable(feature = "never_type", issue = "35121")]
+            impl PartialEq for ! {
+                fn eq(&self, _: &!) -> bool {
+                    *self
+                }
+            }
+
+            #[unstable(feature = "never_type", issue = "35121")]
+            impl Eq for ! {}
+
+            #[unstable(feature = "never_type", issue = "35121")]
+            impl PartialOrd for ! {
+                fn partial_cmp(&self, _: &!) -> Option<Ordering> {
+                    *self
+                }
+            }
+
+            #[unstable(feature = "never_type", issue = "35121")]
+            impl Ord for ! {
+                fn cmp(&self, _: &!) -> Ordering {
+                    *self
+                }
+            }
+        }
+    }
+
+    #[cfg(not(stage0))]
+    not_stage0!();
+
     // & pointers
 
     #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/fmt/mod.rs

@@ -1363,6 +1363,29 @@ macro_rules! fmt_refs {
 
 fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
 
+// Note: This macro is a temporary hack that can be remove once we are building with a compiler
+// that supports `!`
+macro_rules! not_stage0 {
+    () => {
+        #[unstable(feature = "never_type", issue = "35121")]
+        impl Debug for ! {
+            fn fmt(&self, _: &mut Formatter) -> Result {
+                *self
+            }
+        }
+
+        #[unstable(feature = "never_type", issue = "35121")]
+        impl Display for ! {
+            fn fmt(&self, _: &mut Formatter) -> Result {
+                *self
+            }
+        }
+    }
+}
+
+#[cfg(not(stage0))]
+not_stage0!();
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Debug for bool {
     fn fmt(&self, f: &mut Formatter) -> Result {

src/libcore/lib.rs

@@ -88,6 +88,9 @@
 #![feature(unboxed_closures)]
 #![feature(question_mark)]
 
+// NOTE: remove the cfg_attr next snapshot
+#![cfg_attr(not(stage0), feature(never_type))]
+
 #[macro_use]
 mod macros;
 

src/librustc/cfg/construct.rs

@@ -379,7 +379,8 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
 
         let func_or_rcvr_exit = self.expr(func_or_rcvr, pred);
         let ret = self.straightline(call_expr, func_or_rcvr_exit, args);
-        if fn_ty.fn_ret().diverges() {
+        // FIXME(canndrew): This is_never should probably be an is_uninhabited.
+        if fn_ty.fn_ret().0.is_never() {
             self.add_unreachable_node()
         } else {
             ret

src/librustc/hir/fold.rs

@@ -353,6 +353,7 @@ pub fn noop_fold_ty<T: Folder>(t: P<Ty>, fld: &mut T) -> P<Ty> {
                         }
                     }))
                 }
+                TyNever => node,
                 TyTup(tys) => TyTup(tys.move_map(|ty| fld.fold_ty(ty))),
                 TyPath(qself, path) => {
                     let qself = qself.map(|QSelf { ty, position }| {
@@ -515,7 +516,6 @@ pub fn noop_fold_fn_decl<T: Folder>(decl: P<FnDecl>, fld: &mut T) -> P<FnDecl> {
             output: match output {
                 Return(ty) => Return(fld.fold_ty(ty)),
                 DefaultReturn(span) => DefaultReturn(span),
-                NoReturn(span) => NoReturn(span),
             },
             variadic: variadic,
         }

src/librustc/hir/intravisit.rs

@@ -403,6 +403,7 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
             walk_list!(visitor, visit_lifetime, opt_lifetime);
             visitor.visit_ty(&mutable_type.ty)
         }
+        TyNever => {},
         TyTup(ref tuple_element_types) => {
             walk_list!(visitor, visit_ty, tuple_element_types);
         }

src/librustc/hir/lowering.rs

@@ -270,6 +270,7 @@ impl<'a> LoweringContext<'a> {
                         decl: self.lower_fn_decl(&f.decl),
                     }))
                 }
+                Never => hir::TyNever,
                 Tup(ref tys) => hir::TyTup(tys.iter().map(|ty| self.lower_ty(ty)).collect()),
                 Paren(ref ty) => {
                     return self.lower_ty(ty);
@@ -402,7 +403,6 @@ impl<'a> LoweringContext<'a> {
             output: match decl.output {
                 FunctionRetTy::Ty(ref ty) => hir::Return(self.lower_ty(ty)),
                 FunctionRetTy::Default(span) => hir::DefaultReturn(span),
-                FunctionRetTy::None(span) => hir::NoReturn(span),
             },
             variadic: decl.variadic,
         })

src/librustc/hir/mod.rs

@@ -1112,6 +1112,7 @@ pub struct BareFnTy {
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 /// The different kinds of types recognized by the compiler
 pub enum Ty_ {
+    /// A variable length array (`[T]`)
     TyVec(P<Ty>),
     /// A fixed length array (`[T; n]`)
     TyFixedLengthVec(P<Ty>, P<Expr>),
@@ -1121,6 +1122,8 @@ pub enum Ty_ {
     TyRptr(Option<Lifetime>, MutTy),
     /// A bare function (e.g. `fn(usize) -> bool`)
     TyBareFn(P<BareFnTy>),
+    /// The never type (`!`)
+    TyNever,
     /// A tuple (`(A, B, C, D,...)`)
     TyTup(HirVec<P<Ty>>),
     /// A path (`module::module::...::Type`), optionally
@@ -1283,9 +1286,6 @@ impl fmt::Debug for ImplPolarity {
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum FunctionRetTy {
-    /// Functions with return type `!`that always
-    /// raise an error or exit (i.e. never return to the caller)
-    NoReturn(Span),
     /// Return type is not specified.
     ///
     /// Functions default to `()` and
@@ -1299,7 +1299,6 @@ pub enum FunctionRetTy {
 impl FunctionRetTy {
     pub fn span(&self) -> Span {
         match *self {
-            NoReturn(span) => span,
             DefaultReturn(span) => span,
             Return(ref ty) => ty.span,
         }

src/librustc/hir/print.rs

@@ -504,6 +504,9 @@ impl<'a> State<'a> {
                 self.print_opt_lifetime(lifetime)?;
                 self.print_mt(mt)?;
             }
+            hir::TyNever => {
+                word(&mut self.s, "!")?;
+            },
             hir::TyTup(ref elts) => {
                 self.popen()?;
                 self.commasep(Inconsistent, &elts[..], |s, ty| s.print_type(&ty))?;
@@ -1959,10 +1962,6 @@ impl<'a> State<'a> {
                 self.maybe_print_comment(ty.span.lo)
             }
             hir::DefaultReturn(..) => unreachable!(),
-            hir::NoReturn(span) => {
-                self.word_nbsp("!")?;
-                self.maybe_print_comment(span.lo)
-            }
         }
     }
 
@@ -2195,7 +2194,6 @@ impl<'a> State<'a> {
         self.ibox(indent_unit)?;
         self.word_space("->")?;
         match decl.output {
-            hir::NoReturn(_) => self.word_nbsp("!")?,
             hir::DefaultReturn(..) => unreachable!(),
             hir::Return(ref ty) => self.print_type(&ty)?,
         }

src/librustc/infer/error_reporting.rs

@@ -1326,7 +1326,6 @@ impl<'a, 'gcx, 'tcx> Rebuilder<'a, 'gcx, 'tcx> {
                 self.rebuild_arg_ty_or_output(&ret_ty, lifetime, anon_nums, region_names)
             ),
             hir::DefaultReturn(span) => hir::DefaultReturn(span),
-            hir::NoReturn(span) => hir::NoReturn(span)
         }
     }