Rollup merge of rust-lang#39009 - canndrew:default-unit-warnings, r=nikomatsakis

Add warning for () to ! switch

With feature(never_type) enabled diverging type variables will default to `!` instead of `()`. This can cause breakages where a trait is resolved on such a type.

This PR emits a future-compatibility warning when it sees this happen.

Author: frewsxcv

src/librustc/lint/builtin.rs

@@ -192,6 +192,13 @@ declare_lint! {
     "lifetimes or labels named `'_` were erroneously allowed"
 }
 
+declare_lint! {
+    pub RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
+    Warn,
+    "attempt to resolve a trait on an expression whose type cannot be inferred but which \
+     currently defaults to ()"
+}
+
 declare_lint! {
     pub SAFE_EXTERN_STATICS,
     Warn,
@@ -272,6 +279,7 @@ impl LintPass for HardwiredLints {
             SUPER_OR_SELF_IN_GLOBAL_PATH,
             HR_LIFETIME_IN_ASSOC_TYPE,
             LIFETIME_UNDERSCORE,
+            RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
             SAFE_EXTERN_STATICS,
             PATTERNS_IN_FNS_WITHOUT_BODY,
             EXTRA_REQUIREMENT_IN_IMPL,

src/librustc/middle/mem_categorization.rs

@@ -1199,7 +1199,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
           PatKind::Tuple(ref subpats, ddpos) => {
             // (p1, ..., pN)
             let expected_len = match self.pat_ty(&pat)?.sty {
-                ty::TyTuple(ref tys) => tys.len(),
+                ty::TyTuple(ref tys, _) => tys.len(),
                 ref ty => span_bug!(pat.span, "tuple pattern unexpected type {:?}", ty),
             };
             for (i, subpat) in subpats.iter().enumerate_and_adjust(expected_len, ddpos) {

src/librustc/mir/tcx.rs

@@ -163,7 +163,7 @@ impl<'tcx> Rvalue<'tcx> {
                 let lhs_ty = lhs.ty(mir, tcx);
                 let rhs_ty = rhs.ty(mir, tcx);
                 let ty = op.ty(tcx, lhs_ty, rhs_ty);
-                let ty = tcx.intern_tup(&[ty, tcx.types.bool]);
+                let ty = tcx.intern_tup(&[ty, tcx.types.bool], false);
                 Some(ty)
             }
             &Rvalue::UnaryOp(_, ref operand) => {
@@ -184,7 +184,8 @@ impl<'tcx> Rvalue<'tcx> {
                     }
                     AggregateKind::Tuple => {
                         Some(tcx.mk_tup(
-                            ops.iter().map(|op| op.ty(mir, tcx))
+                            ops.iter().map(|op| op.ty(mir, tcx)),
+                            false
                         ))
                     }
                     AggregateKind::Adt(def, _, substs, _) => {

src/librustc/traits/select.rs

@@ -52,6 +52,7 @@ use std::mem;
 use std::rc::Rc;
 use syntax::abi::Abi;
 use hir;
+use lint;
 use util::nodemap::FxHashMap;
 
 struct InferredObligationsSnapshotVecDelegate<'tcx> {
@@ -407,19 +408,62 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         debug!("select({:?})", obligation);
         assert!(!obligation.predicate.has_escaping_regions());
 
+        let tcx = self.tcx();
         let dep_node = obligation.predicate.dep_node();
-        let _task = self.tcx().dep_graph.in_task(dep_node);
+        let _task = tcx.dep_graph.in_task(dep_node);
 
         let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
-        match self.candidate_from_obligation(&stack)? {
-            None => Ok(None),
+        let ret = match self.candidate_from_obligation(&stack)? {
+            None => None,
             Some(candidate) => {
                 let mut candidate = self.confirm_candidate(obligation, candidate)?;
                 let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
                 candidate.nested_obligations_mut().extend(inferred_obligations);
-                Ok(Some(candidate))
+                Some(candidate)
             },
+        };
+
+        // Test whether this is a `()` which was produced by defaulting a
+        // diverging type variable with `!` disabled. If so, we may need
+        // to raise a warning.
+        if obligation.predicate.skip_binder().self_ty().is_defaulted_unit() {
+            let mut raise_warning = true;
+            // Don't raise a warning if the trait is implemented for ! and only
+            // permits a trivial implementation for !. This stops us warning
+            // about (for example) `(): Clone` becoming `!: Clone` because such
+            // a switch can't cause code to stop compiling or execute
+            // differently.
+            let mut never_obligation = obligation.clone();
+            let def_id = never_obligation.predicate.skip_binder().trait_ref.def_id;
+            never_obligation.predicate = never_obligation.predicate.map_bound(|mut trait_pred| {
+                // Swap out () with ! so we can check if the trait is impld for !
+                {
+                    let mut trait_ref = &mut trait_pred.trait_ref;
+                    let unit_substs = trait_ref.substs;
+                    let mut never_substs = Vec::with_capacity(unit_substs.len());
+                    never_substs.push(From::from(tcx.types.never));
+                    never_substs.extend(&unit_substs[1..]);
+                    trait_ref.substs = tcx.intern_substs(&never_substs);
+                }
+                trait_pred
+            });
+            if let Ok(Some(..)) = self.select(&never_obligation) {
+                if !tcx.trait_relevant_for_never(def_id) {
+                    // The trait is also implemented for ! and the resulting
+                    // implementation cannot actually be invoked in any way.
+                    raise_warning = false;
+                }
+            }
+
+            if raise_warning {
+                tcx.sess.add_lint(lint::builtin::RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
+                                  obligation.cause.body_id,
+                                  obligation.cause.span,
+                                  format!("code relies on type inference rules which are likely \
+                                           to change"));
+            }
         }
+        Ok(ret)
     }
 
     ///////////////////////////////////////////////////////////////////////////
@@ -1744,15 +1788,15 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
 
             ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) => Never,
 
-            ty::TyTuple(tys) => {
+            ty::TyTuple(tys, _) => {
                 Where(ty::Binder(tys.last().into_iter().cloned().collect()))
             }
 
             ty::TyAdt(def, substs) => {
                 let sized_crit = def.sized_constraint(self.tcx());
                 // (*) binder moved here
                 Where(ty::Binder(match sized_crit.sty {
-                    ty::TyTuple(tys) => tys.to_vec().subst(self.tcx(), substs),
+                    ty::TyTuple(tys, _) => tys.to_vec().subst(self.tcx(), substs),
                     ty::TyBool => vec![],
                     _ => vec![sized_crit.subst(self.tcx(), substs)]
                 }))
@@ -1799,7 +1843,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 Where(ty::Binder(vec![element_ty]))
             }
 
-            ty::TyTuple(tys) => {
+            ty::TyTuple(tys, _) => {
                 // (*) binder moved here
                 Where(ty::Binder(tys.to_vec()))
             }
@@ -1874,7 +1918,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 vec![element_ty]
             }
 
-            ty::TyTuple(ref tys) => {
+            ty::TyTuple(ref tys, _) => {
                 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
                 tys.to_vec()
             }

src/librustc/traits/util.rs

@@ -489,7 +489,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         let arguments_tuple = match tuple_arguments {
             TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
             TupleArgumentsFlag::Yes =>
-                self.intern_tup(sig.skip_binder().inputs()),
+                self.intern_tup(sig.skip_binder().inputs(), false),
         };
         let trait_ref = ty::TraitRef {
             def_id: fn_trait_def_id,

src/librustc/ty/contents.rs

@@ -201,7 +201,7 @@ impl<'a, 'tcx> ty::TyS<'tcx> {
                         |ty| tc_ty(tcx, &ty, cache))
                 }
 
-                ty::TyTuple(ref tys) => {
+                ty::TyTuple(ref tys, _) => {
                     TypeContents::union(&tys[..],
                                         |ty| tc_ty(tcx, *ty, cache))
                 }

src/librustc/ty/context.rs

@@ -1384,23 +1384,24 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.mk_ty(TySlice(ty))
     }
 
-    pub fn intern_tup(self, ts: &[Ty<'tcx>]) -> Ty<'tcx> {
-        self.mk_ty(TyTuple(self.intern_type_list(ts)))
+    pub fn intern_tup(self, ts: &[Ty<'tcx>], defaulted: bool) -> Ty<'tcx> {
+        self.mk_ty(TyTuple(self.intern_type_list(ts), defaulted))
     }
 
-    pub fn mk_tup<I: InternAs<[Ty<'tcx>], Ty<'tcx>>>(self, iter: I) -> I::Output {
-        iter.intern_with(|ts| self.mk_ty(TyTuple(self.intern_type_list(ts))))
+    pub fn mk_tup<I: InternAs<[Ty<'tcx>], Ty<'tcx>>>(self, iter: I,
+                                                     defaulted: bool) -> I::Output {
+        iter.intern_with(|ts| self.mk_ty(TyTuple(self.intern_type_list(ts), defaulted)))
     }
 
     pub fn mk_nil(self) -> Ty<'tcx> {
-        self.intern_tup(&[])
+        self.intern_tup(&[], false)
     }
 
     pub fn mk_diverging_default(self) -> Ty<'tcx> {
         if self.sess.features.borrow().never_type {
             self.types.never
         } else {
-            self.mk_nil()
+            self.intern_tup(&[], true)
         }
     }
 

src/librustc/ty/error.rs

@@ -178,7 +178,7 @@ impl<'a, 'gcx, 'lcx, 'tcx> ty::TyS<'tcx> {
         match self.sty {
             ty::TyBool | ty::TyChar | ty::TyInt(_) |
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyStr | ty::TyNever => self.to_string(),
-            ty::TyTuple(ref tys) if tys.is_empty() => self.to_string(),
+            ty::TyTuple(ref tys, _) if tys.is_empty() => self.to_string(),
 
             ty::TyAdt(def, _) => format!("{} `{}`", def.descr(), tcx.item_path_str(def.did)),
             ty::TyArray(_, n) => format!("array of {} elements", n),
@@ -209,7 +209,7 @@ impl<'a, 'gcx, 'lcx, 'tcx> ty::TyS<'tcx> {
                     |p| format!("trait {}", tcx.item_path_str(p.def_id())))
             }
             ty::TyClosure(..) => "closure".to_string(),
-            ty::TyTuple(_) => "tuple".to_string(),
+            ty::TyTuple(..) => "tuple".to_string(),
             ty::TyInfer(ty::TyVar(_)) => "inferred type".to_string(),
             ty::TyInfer(ty::IntVar(_)) => "integral variable".to_string(),
             ty::TyInfer(ty::FloatVar(_)) => "floating-point variable".to_string(),

src/librustc/ty/fast_reject.rs

@@ -72,7 +72,7 @@ pub fn simplify_type<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
             Some(ClosureSimplifiedType(def_id))
         }
         ty::TyNever => Some(NeverSimplifiedType),
-        ty::TyTuple(ref tys) => {
+        ty::TyTuple(ref tys, _) => {
             Some(TupleSimplifiedType(tys.len()))
         }
         ty::TyFnDef(.., ref f) | ty::TyFnPtr(ref f) => {

src/librustc/ty/flags.rs

@@ -151,7 +151,7 @@ impl FlagComputation {
                 self.add_ty(m.ty);
             }
 
-            &ty::TyTuple(ref ts) => {
+            &ty::TyTuple(ref ts, _) => {
                 self.add_tys(&ts[..]);
             }
 

src/librustc/ty/inhabitedness/mod.rs

@@ -178,7 +178,7 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
             },
 
             TyNever => DefIdForest::full(tcx),
-            TyTuple(ref tys) => {
+            TyTuple(ref tys, _) => {
                 DefIdForest::union(tcx, tys.iter().map(|ty| {
                     ty.uninhabited_from(visited, tcx)
                 }))

src/librustc/ty/item_path.rs

@@ -319,9 +319,9 @@ pub fn characteristic_def_id_of_type(ty: Ty) -> Option<DefId> {
         ty::TyRawPtr(mt) |
         ty::TyRef(_, mt) => characteristic_def_id_of_type(mt.ty),
 
-        ty::TyTuple(ref tys) => tys.iter()
-                                   .filter_map(|ty| characteristic_def_id_of_type(ty))
-                                   .next(),
+        ty::TyTuple(ref tys, _) => tys.iter()
+                                      .filter_map(|ty| characteristic_def_id_of_type(ty))
+                                      .next(),
 
         ty::TyFnDef(def_id, ..) |
         ty::TyClosure(def_id, _) => Some(def_id),

src/librustc/ty/layout.rs

@@ -791,7 +791,7 @@ impl<'a, 'gcx, 'tcx> Struct {
                     Some(&variant.memory_index[..]))
             }
             // Can we use one of the fields in this tuple?
-            (&Univariant { ref variant, .. }, &ty::TyTuple(tys)) => {
+            (&Univariant { ref variant, .. }, &ty::TyTuple(tys, _)) => {
                 Struct::non_zero_field_paths(infcx, tys.iter().cloned(),
                     Some(&variant.memory_index[..]))
             }
@@ -1157,7 +1157,7 @@ impl<'a, 'gcx, 'tcx> Layout {
                 Univariant { variant: st, non_zero: false }
             }
 
-            ty::TyTuple(tys) => {
+            ty::TyTuple(tys, _) => {
                 // FIXME(camlorn): if we ever allow unsized tuples, this needs to be checked.
                 // See the univariant case below to learn how.
                 let st = Struct::new(dl,

src/librustc/ty/mod.rs

@@ -197,6 +197,17 @@ impl AssociatedItem {
             AssociatedKind::Type => Def::AssociatedTy(self.def_id),
         }
     }
+
+    /// Tests whether the associated item admits a non-trivial implementation
+    /// for !
+    pub fn relevant_for_never<'tcx>(&self) -> bool {
+        match self.kind {
+            AssociatedKind::Const => true,
+            AssociatedKind::Type => true,
+            // FIXME(canndrew): Be more thorough here, check if any argument is uninhabited.
+            AssociatedKind::Method => !self.method_has_self_argument,
+        }
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Copy, RustcEncodable, RustcDecodable)]
@@ -1603,7 +1614,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
             _ if tys.references_error() => tcx.types.err,
             0 => tcx.types.bool,
             1 => tys[0],
-            _ => tcx.intern_tup(&tys[..])
+            _ => tcx.intern_tup(&tys[..], false)
         };
 
         let old = tcx.adt_sized_constraint.borrow().get(&self.did).cloned();
@@ -1638,7 +1649,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
                 vec![ty]
             }
 
-            TyTuple(ref tys) => {
+            TyTuple(ref tys, _) => {
                 match tys.last() {
                     None => vec![],
                     Some(ty) => self.sized_constraint_for_ty(tcx, stack, ty)
@@ -1652,7 +1663,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
                        .subst(tcx, substs);
                 debug!("sized_constraint_for_ty({:?}) intermediate = {:?}",
                        ty, adt_ty);
-                if let ty::TyTuple(ref tys) = adt_ty.sty {
+                if let ty::TyTuple(ref tys, _) = adt_ty.sty {
                     tys.iter().flat_map(|ty| {
                         self.sized_constraint_for_ty(tcx, stack, ty)
                     }).collect()
@@ -2010,6 +2021,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    pub fn trait_relevant_for_never(self, did: DefId) -> bool {
+        self.associated_items(did).any(|item| {
+            item.relevant_for_never()
+        })
+    }
+
     pub fn custom_coerce_unsized_kind(self, did: DefId) -> adjustment::CustomCoerceUnsized {
         self.custom_coerce_unsized_kinds.memoize(did, || {
             let (kind, src) = if did.krate != LOCAL_CRATE {

src/librustc/ty/relate.rs

@@ -447,10 +447,11 @@ pub fn super_relate_tys<'a, 'gcx, 'tcx, R>(relation: &mut R,
             Ok(tcx.mk_slice(t))
         }
 
-        (&ty::TyTuple(as_), &ty::TyTuple(bs)) =>
+        (&ty::TyTuple(as_, a_defaulted), &ty::TyTuple(bs, b_defaulted)) =>
         {
             if as_.len() == bs.len() {
-                Ok(tcx.mk_tup(as_.iter().zip(bs).map(|(a, b)| relation.relate(a, b)))?)
+                let defaulted = a_defaulted || b_defaulted;
+                Ok(tcx.mk_tup(as_.iter().zip(bs).map(|(a, b)| relation.relate(a, b)), defaulted)?)
             } else if !(as_.is_empty() || bs.is_empty()) {
                 Err(TypeError::TupleSize(
                     expected_found(relation, &as_.len(), &bs.len())))

src/librustc/ty/structural_impls.rs

@@ -474,7 +474,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyAdt(tid, substs) => ty::TyAdt(tid, substs.fold_with(folder)),
             ty::TyDynamic(ref trait_ty, ref region) =>
                 ty::TyDynamic(trait_ty.fold_with(folder), region.fold_with(folder)),
-            ty::TyTuple(ts) => ty::TyTuple(ts.fold_with(folder)),
+            ty::TyTuple(ts, defaulted) => ty::TyTuple(ts.fold_with(folder), defaulted),
             ty::TyFnDef(def_id, substs, f) => {
                 ty::TyFnDef(def_id,
                             substs.fold_with(folder),
@@ -511,7 +511,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyAdt(_, substs) => substs.visit_with(visitor),
             ty::TyDynamic(ref trait_ty, ref reg) =>
                 trait_ty.visit_with(visitor) || reg.visit_with(visitor),
-            ty::TyTuple(ts) => ts.visit_with(visitor),
+            ty::TyTuple(ts, _) => ts.visit_with(visitor),
             ty::TyFnDef(_, substs, ref f) => {
                 substs.visit_with(visitor) || f.visit_with(visitor)
             }