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)
         }
     }
 

src/librustc/infer/freshen.rs

@@ -168,6 +168,7 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
             ty::TyTrait(..) |
             ty::TyStruct(..) |
             ty::TyClosure(..) |
+            ty::TyNever |
             ty::TyTuple(..) |
             ty::TyProjection(..) |
             ty::TyParam(..) |

src/librustc/infer/mod.rs

@@ -607,7 +607,6 @@ impl_trans_normalize!('gcx,
     Ty<'gcx>,
     &'gcx Substs<'gcx>,
     ty::FnSig<'gcx>,
-    ty::FnOutput<'gcx>,
     &'gcx ty::BareFnTy<'gcx>,
     ty::ClosureSubsts<'gcx>,
     ty::PolyTraitRef<'gcx>

src/librustc/middle/expr_use_visitor.rs

@@ -717,6 +717,7 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
         let adj = infcx.adjustments().get(&expr.id).map(|x| x.clone());
         if let Some(adjustment) = adj {
             match adjustment {
+                adjustment::AdjustNeverToAny(..) |
                 adjustment::AdjustReifyFnPointer |
                 adjustment::AdjustUnsafeFnPointer |
                 adjustment::AdjustMutToConstPointer => {

src/librustc/middle/intrinsicck.rs

@@ -161,10 +161,9 @@ impl<'a, 'gcx, 'tcx, 'v> Visitor<'v> for ExprVisitor<'a, 'gcx, 'tcx> {
                     let typ = self.infcx.tcx.node_id_to_type(expr.id);
                     match typ.sty {
                         ty::TyFnDef(_, _, ref bare_fn_ty) if bare_fn_ty.abi == RustIntrinsic => {
-                            if let ty::FnConverging(to) = bare_fn_ty.sig.0.output {
-                                let from = bare_fn_ty.sig.0.inputs[0];
-                                self.check_transmute(expr.span, from, to, expr.id);
-                            }
+                            let from = bare_fn_ty.sig.0.inputs[0];
+                            let to = bare_fn_ty.sig.0.output;
+                            self.check_transmute(expr.span, from, to, expr.id);
                         }
                         _ => {
                             span_bug!(expr.span, "transmute wasn't a bare fn?!");

src/librustc/middle/liveness.rs

@@ -112,7 +112,7 @@ use self::VarKind::*;
 use dep_graph::DepNode;
 use hir::def::*;
 use hir::pat_util;
-use ty::{self, TyCtxt, ParameterEnvironment};
+use ty::{self, Ty, TyCtxt, ParameterEnvironment};
 use traits::{self, Reveal};
 use ty::subst::Subst;
 use lint;
@@ -1111,8 +1111,9 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
           }
 
           hir::ExprCall(ref f, ref args) => {
+            // FIXME(canndrew): This is_never should really be an is_uninhabited
             let diverges = !self.ir.tcx.is_method_call(expr.id) &&
-                self.ir.tcx.expr_ty_adjusted(&f).fn_ret().diverges();
+                self.ir.tcx.expr_ty_adjusted(&f).fn_ret().0.is_never();
             let succ = if diverges {
                 self.s.exit_ln
             } else {
@@ -1125,7 +1126,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
           hir::ExprMethodCall(_, _, ref args) => {
             let method_call = ty::MethodCall::expr(expr.id);
             let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty;
-            let succ = if method_ty.fn_ret().diverges() {
+            // FIXME(canndrew): This is_never should really be an is_uninhabited
+            let succ = if method_ty.fn_ret().0.is_never() {
                 self.s.exit_ln
             } else {
                 succ
@@ -1454,7 +1456,7 @@ fn check_fn(_v: &Liveness,
 }
 
 impl<'a, 'tcx> Liveness<'a, 'tcx> {
-    fn fn_ret(&self, id: NodeId) -> ty::PolyFnOutput<'tcx> {
+    fn fn_ret(&self, id: NodeId) -> ty::Binder<Ty<'tcx>> {
         let fn_ty = self.ir.tcx.node_id_to_type(id);
         match fn_ty.sty {
             ty::TyClosure(closure_def_id, substs) =>
@@ -1477,55 +1479,44 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 self.ir.tcx.region_maps.call_site_extent(id, body.id),
                 &self.fn_ret(id));
 
-        match fn_ret {
-            ty::FnConverging(t_ret)
-                    if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
-
-                let param_env = ParameterEnvironment::for_item(self.ir.tcx, id);
-                let t_ret_subst = t_ret.subst(self.ir.tcx, &param_env.free_substs);
-                let is_nil = self.ir.tcx.infer_ctxt(None, Some(param_env),
-                                                    Reveal::All).enter(|infcx| {
-                    let cause = traits::ObligationCause::dummy();
-                    traits::fully_normalize(&infcx, cause, &t_ret_subst).unwrap().is_nil()
-                });
-
-                // for nil return types, it is ok to not return a value expl.
-                if !is_nil {
-                    let ends_with_stmt = match body.expr {
-                        None if !body.stmts.is_empty() =>
-                            match body.stmts.last().unwrap().node {
-                                hir::StmtSemi(ref e, _) => {
-                                    self.ir.tcx.expr_ty(&e) == t_ret
-                                },
-                                _ => false
+        if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
+            let param_env = ParameterEnvironment::for_item(self.ir.tcx, id);
+            let t_ret_subst = fn_ret.subst(self.ir.tcx, &param_env.free_substs);
+            let is_nil = self.ir.tcx.infer_ctxt(None, Some(param_env),
+                                                Reveal::All).enter(|infcx| {
+                let cause = traits::ObligationCause::dummy();
+                traits::fully_normalize(&infcx, cause, &t_ret_subst).unwrap().is_nil()
+            });
+
+            // for nil return types, it is ok to not return a value expl.
+            if !is_nil {
+                let ends_with_stmt = match body.expr {
+                    None if !body.stmts.is_empty() =>
+                        match body.stmts.last().unwrap().node {
+                            hir::StmtSemi(ref e, _) => {
+                                self.ir.tcx.expr_ty(&e) == fn_ret
                             },
-                        _ => false
+                            _ => false
+                        },
+                    _ => false
+                };
+                let mut err = struct_span_err!(self.ir.tcx.sess,
+                                               sp,
+                                               E0269,
+                                               "not all control paths return a value");
+                if ends_with_stmt {
+                    let last_stmt = body.stmts.last().unwrap();
+                    let original_span = original_sp(self.ir.tcx.sess.codemap(),
+                                                    last_stmt.span, sp);
+                    let span_semicolon = Span {
+                        lo: original_span.hi - BytePos(1),
+                        hi: original_span.hi,
+                        expn_id: original_span.expn_id
                     };
-                    let mut err = struct_span_err!(self.ir.tcx.sess,
-                                                   sp,
-                                                   E0269,
-                                                   "not all control paths return a value");
-                    if ends_with_stmt {
-                        let last_stmt = body.stmts.last().unwrap();
-                        let original_span = original_sp(self.ir.tcx.sess.codemap(),
-                                                        last_stmt.span, sp);
-                        let span_semicolon = Span {
-                            lo: original_span.hi - BytePos(1),
-                            hi: original_span.hi,
-                            expn_id: original_span.expn_id
-                        };
-                        err.span_help(span_semicolon, "consider removing this semicolon:");
-                    }
-                    err.emit();
+                    err.span_help(span_semicolon, "consider removing this semicolon:");
                 }
+                err.emit();
             }
-            ty::FnDiverging
-                if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => {
-                    span_err!(self.ir.tcx.sess, sp, E0270,
-                        "computation may converge in a function marked as diverging");
-                }
-
-            _ => {}
         }
     }
 

src/librustc/middle/mem_categorization.rs

@@ -451,6 +451,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
                         self.cat_expr_autoderefd(expr, autoderefs)
                     }
 
+                    adjustment::AdjustNeverToAny(..) |
                     adjustment::AdjustReifyFnPointer |
                     adjustment::AdjustUnsafeFnPointer |
                     adjustment::AdjustMutToConstPointer |
@@ -922,7 +923,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         let base_cmt = match method_ty {
             Some(method_ty) => {
                 let ref_ty =
-                    self.tcx().no_late_bound_regions(&method_ty.fn_ret()).unwrap().unwrap();
+                    self.tcx().no_late_bound_regions(&method_ty.fn_ret()).unwrap();
                 self.cat_rvalue_node(node.id(), node.span(), ref_ty)
             }
             None => base_cmt
@@ -1244,7 +1245,6 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         // to skip past the binder.
         self.tcx().no_late_bound_regions(&method_ty.fn_ret())
            .unwrap()
-           .unwrap() // overloaded ops do not diverge, either
     }
 }