Check for uninhabitedness instead of never

src/librustc/cfg/construct.rs

@@ -415,8 +415,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             args: I) -> CFGIndex {
         let func_or_rcvr_exit = self.expr(func_or_rcvr, pred);
         let ret = self.straightline(call_expr, func_or_rcvr_exit, args);
-        // FIXME(canndrew): This is_never should probably be an is_uninhabited.
+        if self.tables.expr_ty(call_expr).conservative_is_uninhabited() {
-        if self.tables.expr_ty(call_expr).is_never() {
             self.add_unreachable_node()
         } else {
             ret

src/librustc/middle/liveness.rs

@@ -1197,8 +1197,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::Call(ref f, ref args) => {
-                // FIXME(canndrew): This is_never should really be an is_uninhabited
+                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited() {
-                let succ = if self.tables.expr_ty(expr).is_never() {
                     self.s.exit_ln
                 } else {
                     succ
@@ -1208,8 +1207,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::MethodCall(.., ref args) => {
-                // FIXME(canndrew): This is_never should really be an is_uninhabited
+                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited() {
-                let succ = if self.tables.expr_ty(expr).is_never() {
                     self.s.exit_ln
                 } else {
                     succ

src/librustc/ty/sty.rs

@@ -1543,6 +1543,17 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
+    pub fn conservative_is_uninhabited(&self) -> bool {
+        // Checks whether a type is definitely uninhabited. This is
+        // conservative: for some types that are uninhabited we return `false`,
+        // but we only return `true` for types that are definitely uninhabited.
+        match self.sty {
+            ty::Never => true,
+            ty::Adt(def, _) => def.variants.is_empty(),
+            _ => false
+        }
+    }
+
     pub fn is_primitive(&self) -> bool {
         match self.sty {
             Bool | Char | Int(_) | Uint(_) | Float(_) => true,

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1546,8 +1546,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                 }
             }
             None => {
-                // FIXME(canndrew): This is_never should probably be an is_uninhabited
+                if !sig.output().conservative_is_uninhabited() {
-                if !sig.output().is_never() {
                     span_mirbug!(self, term, "call to converging function {:?} w/o dest", sig);
                 }
             }

src/librustc_mir/build/expr/into.rs

@@ -275,8 +275,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 exit_block.unit()
             }
             ExprKind::Call { ty, fun, args, from_hir_call } => {
-                // FIXME(canndrew): This is_never should probably be an is_uninhabited
+                let diverges = expr.ty.conservative_is_uninhabited();
-                let diverges = expr.ty.is_never();
                 let intrinsic = match ty.sty {
                     ty::FnDef(def_id, _) => {
                         let f = ty.fn_sig(this.hir.tcx());
@@ -332,7 +331,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             func: fun,
                             args,
                             cleanup: Some(cleanup),
-                            destination: if diverges {
+                            destination: if expr.ty.conservative_is_uninhabited() {
                                 None
                             } else {
                                 Some((destination.clone(), success))

src/librustc_mir/hair/pattern/check_match.rs

@@ -230,7 +230,7 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
                 } else {
-                    self.conservative_is_uninhabited(pat_ty)
+                    pat_ty.conservative_is_uninhabited()
                 };
                 if !scrutinee_is_uninhabited {
                     // We know the type is inhabited, so this must be wrong
@@ -258,15 +258,6 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
         })
     }
 
-    fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
-        // "rustc-1.0-style" uncontentious uninhabitableness check
-        match scrutinee_ty.sty {
-            ty::Never => true,
-            ty::Adt(def, _) => def.variants.is_empty(),
-            _ => false
-        }
-    }
-
     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
         let module = self.tcx.hir().get_module_parent(pat.id);
         MatchCheckCtxt::create_and_enter(self.tcx, self.param_env, module, |ref mut cx| {

src/test/debuginfo/nil-enum.rs

@@ -1,55 +1,24 @@
-// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
+// LLDB can't handle zero-sized values.
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// NOTE Instantiating an empty enum is UB. This test may break in the future.
-
-// LLDB can't handle zero-sized values
 // ignore-lldb
 
-
-// Require LLVM with DW_TAG_variant_part and a gdb that can read it.
-// gdb 8.2.0 crashes on this test case, see
-// https://sourceware.org/bugzilla/show_bug.cgi?id=23626
-// This will be fixed in the next release, which will be >= 8.2.1.
-// min-system-llvm-version: 7.0
-// min-gdb-version: 8.2.1
-
 // compile-flags:-g
 // gdb-command:run
 
-// gdb-command:print first
+// gdb-command:print *first
-// gdbr-check:$1 = nil_enum::ANilEnum {<No data fields>}
+// gdbg-check:$1 = {<No data fields>}
-
+// gdbr-check:$1 = <error reading variable>
-// gdb-command:print second
-// gdbr-check:$2 = nil_enum::AnotherNilEnum {<No data fields>}
 
 #![allow(unused_variables)]
 #![feature(omit_gdb_pretty_printer_section)]
 #![feature(maybe_uninit)]
 #![omit_gdb_pretty_printer_section]
 
-use std::mem::MaybeUninit;
+enum Void {}
 
-enum ANilEnum {}
-enum AnotherNilEnum {}
-
-// This test relies on gdbg printing the string "{<No data fields>}" for empty
-// structs (which may change some time)
-// The error from gdbr is expected since nil enums are not supposed to exist.
 fn main() {
-    unsafe {
+    let first: *const Void = 1 as *const _;
-        let first: ANilEnum = MaybeUninit::uninitialized().into_inner();
-        let second: AnotherNilEnum = MaybeUninit::uninitialized().into_inner();
 
-        zzz(); // #break
+    zzz(); // #break
-    }
 }
 
-fn zzz() {()}
+fn zzz() {}