From d6482510f4ac179506370f421b2c3a3802f574f9 Mon Sep 17 00:00:00 2001
From: Andrew Cann <shum@canndrew.org>
Date: Wed, 14 Sep 2016 23:58:58 +0800
Subject: [PATCH] Expand `is_uninhabited` to recurse into datatypes

---
 src/librustc/ty/mod.rs | 45 +++++++++++++++++++++++++++++++++++++-----
 src/librustc/ty/sty.rs | 15 ++++++++++++--
 2 files changed, 53 insertions(+), 7 deletions(-)

diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index e94e93158c4..5737f776422 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -16,6 +16,7 @@ pub use self::IntVarValue::*;
 pub use self::LvaluePreference::*;
 pub use self::fold::TypeFoldable;
 
+use std::collections::{hash_map, HashMap};
 use dep_graph::{self, DepNode};
 use hir::map as ast_map;
 use middle;
@@ -1389,6 +1390,20 @@ impl<'tcx> serialize::UseSpecializedEncodable for AdtDef<'tcx> {
 
 impl<'tcx> serialize::UseSpecializedDecodable for AdtDef<'tcx> {}
 
+impl<'a, 'gcx, 'tcx> AdtDefData<'tcx, 'static> {
+    #[inline]
+    pub fn is_uninhabited_recurse(&'tcx self,
+                                  visited: &mut HashMap<(DefId, &'tcx Substs<'tcx>), ()>, 
+                                  cx: TyCtxt<'a, 'gcx, 'tcx>,
+                                  substs: &'tcx Substs<'tcx>) -> bool {
+        match visited.entry((self.did, substs)) {
+            hash_map::Entry::Occupied(_) => return true,
+            hash_map::Entry::Vacant(ve) => ve.insert(()),
+        };
+        self.variants.iter().all(|v| v.is_uninhabited_recurse(visited, cx, substs, self.is_union()))
+    }
+}
+
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub enum AdtKind { Struct, Union, Enum }
 
@@ -1531,11 +1546,6 @@ impl<'a, 'gcx, 'tcx, 'container> AdtDefData<'gcx, 'container> {
         self.variants.iter().flat_map(VariantDefData::fields_iter)
     }
 
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.variants.is_empty()
-    }
-
     #[inline]
     pub fn is_univariant(&self) -> bool {
         self.variants.len() == 1
@@ -1795,6 +1805,21 @@ impl<'tcx, 'container> VariantDefData<'tcx, 'container> {
     }
 }
 
+impl<'a, 'gcx, 'tcx> VariantDefData<'tcx, 'static> {
+    #[inline]
+    pub fn is_uninhabited_recurse(&'tcx self,
+                                  visited: &mut HashMap<(DefId, &'tcx Substs<'tcx>), ()>,
+                                  cx: TyCtxt<'a, 'gcx, 'tcx>,
+                                  substs: &'tcx Substs<'tcx>,
+                                  is_union: bool) -> bool {
+        if is_union {
+            self.fields.iter().all(|f| f.is_uninhabited_recurse(visited, cx, substs))
+        } else {
+            self.fields.iter().any(|f| f.is_uninhabited_recurse(visited, cx, substs))
+        }
+    }
+}
+
 impl<'a, 'gcx, 'tcx, 'container> FieldDefData<'tcx, 'container> {
     pub fn new(did: DefId,
                name: Name,
@@ -1820,6 +1845,16 @@ impl<'a, 'gcx, 'tcx, 'container> FieldDefData<'tcx, 'container> {
     }
 }
 
+impl<'a, 'gcx, 'tcx> FieldDefData<'tcx, 'static> {
+    #[inline]
+    pub fn is_uninhabited_recurse(&'tcx self,
+                                  visited: &mut HashMap<(DefId, &'tcx Substs<'tcx>), ()>,
+                                  tcx: TyCtxt<'a, 'gcx, 'tcx>,
+                                  substs: &'tcx Substs<'tcx>) -> bool {
+        self.ty(tcx, substs).is_uninhabited_recurse(visited, tcx)
+    }
+}
+
 /// Records the substitutions used to translate the polytype for an
 /// item into the monotype of an item reference.
 #[derive(Clone, RustcEncodable, RustcDecodable)]
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index af45a323eee..53230eaf59d 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -21,6 +21,7 @@ use util::common::ErrorReported;
 use collections::enum_set::{self, EnumSet, CLike};
 use std::fmt;
 use std::ops;
+use std::collections::HashMap;
 use syntax::abi;
 use syntax::ast::{self, Name};
 use syntax::symbol::{keywords, InternedString};
@@ -929,14 +930,24 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
-    pub fn is_uninhabited(&self, cx: TyCtxt) -> bool {
+    pub fn is_uninhabited(&self, cx: TyCtxt<'a, 'gcx, 'tcx>) -> bool {
+        let mut visited = HashMap::new();
+        self.is_uninhabited_recurse(&mut visited, cx)
+    }
+
+    pub fn is_uninhabited_recurse(&self,
+                                  visited: &mut HashMap<(DefId, &'tcx Substs<'tcx>), ()>,
+                                  cx: TyCtxt<'a, 'gcx, 'tcx>) -> bool {
         // FIXME(#24885): be smarter here, the AdtDefData::is_empty method could easily be made
         // more complete.
         match self.sty {
-            TyAdt(def, _) => def.is_empty(),
+            TyAdt(def, substs) => {
+                def.is_uninhabited_recurse(visited, cx, substs)
+            },
 
             TyNever => true,
             TyTuple(ref tys) => tys.iter().any(|ty| ty.is_uninhabited(cx)),
+            TyArray(ty, len) => len > 0 && ty.is_uninhabited(cx),
 
             // FIXME(canndrew): this line breaks core::fmt
             //TyRef(_, ref tm) => tm.ty.is_uninhabited(cx),