Make uninhabitedness checking more intelligent

src/librustc/cfg/construct.rs

@@ -415,7 +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);
-        if self.tables.expr_ty(call_expr).conservative_is_uninhabited() {
+        if self.tables.expr_ty(call_expr).conservative_is_uninhabited(self.tcx) {
             self.add_unreachable_node()
         } else {
             ret

src/librustc/middle/liveness.rs

@@ -1197,7 +1197,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::Call(ref f, ref args) => {
-                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited() {
+                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited(self.ir.tcx) {
                     self.s.exit_ln
                 } else {
                     succ
@@ -1207,7 +1207,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::MethodCall(.., ref args) => {
-                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited() {
+                let succ = if self.tables.expr_ty(expr).conservative_is_uninhabited(self.ir.tcx) {
                     self.s.exit_ln
                 } else {
                     succ

src/librustc/ty/sty.rs

@@ -1543,14 +1543,40 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
-    pub fn conservative_is_uninhabited(&self) -> bool {
+    pub fn conservative_is_uninhabited(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>) -> 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
+            ty::Adt(def, _) => {
+                // Any ADT is uninhabited if:
+                // (a) It has no variants (i.e. an empty `enum`);
+                // (b) Each of its variants (a single one in the case of a `struct`) has at least
+                //     one uninhabited field.
+                def.variants.iter().all(|var| {
+                    var.fields.iter().any(|field| {
+                        tcx.type_of(field.did).conservative_is_uninhabited(tcx)
+                    })
+                })
+            }
+            ty::Tuple(tys) => tys.iter().any(|ty| ty.conservative_is_uninhabited(tcx)),
+            ty::Array(ty, len) => {
+                match len.val.try_to_scalar() {
+                    // If the array is definitely non-empty, it's uninhabited if
+                    // the type of its elements is uninhabited.
+                    Some(n) if !n.is_null() => ty.conservative_is_uninhabited(tcx),
+                    _ => false
+                }
+            }
+            ty::Ref(..) => {
+                // Though references to uninhabited types are trivially uninhabited
+                // theoretically, null references are permitted in unsafe code (as
+                // long as the value is not dereferenced), so we treat all references
+                // as inhabited.
+                false
+            }
+            _ => false,
         }
     }
 

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

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

src/librustc_mir/build/expr/into.rs

@@ -331,7 +331,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             func: fun,
                             args,
                             cleanup: Some(cleanup),
-                            destination: if expr.ty.conservative_is_uninhabited() {
+                            destination: if expr.ty.conservative_is_uninhabited(this.hir.tcx()) {
                                 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 {
-                    pat_ty.conservative_is_uninhabited()
+                    pat_ty.conservative_is_uninhabited(self.tcx)
                 };
                 if !scrutinee_is_uninhabited {
                     // We know the type is inhabited, so this must be wrong