Auto merge of #56219 - arielb1:never-coerce-box, r=nikomatsakis

trigger unsized coercions keyed on Sized bounds

This PR causes unsized coercions to not be disabled by `$0: Unsize<dyn
Object>` coercion obligations when we have an `$0: Sized` obligation somewhere.

This should be mostly backwards-compatible, because in these cases not doing the unsize coercion should have caused the `$0: Sized` obligation to fail.

Note that `X: Unsize<dyn Object>` obligations can't fail *as obligations* if `X: Sized` holds, so this still maintains some version of monotonicity (I think that an unsized coercion can't be converted to no coercion by unifying type variables).

Fixes #49593 (unblocking never_type).

r? @eddyb
cc @nikomatsakis

src/librustc/infer/mod.rs

@@ -1275,6 +1275,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         self.inlined_shallow_resolve(typ)
     }
 
+    pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
+        self.type_variables.borrow_mut().root_var(var)
+    }
+
     pub fn resolve_type_vars_if_possible<T>(&self, value: &T) -> T
     where
         T: TypeFoldable<'tcx>,

src/librustc_typeck/check/closure.rs

@@ -20,7 +20,7 @@ use rustc::infer::LateBoundRegionConversionTime;
 use rustc::infer::type_variable::TypeVariableOrigin;
 use rustc::traits::Obligation;
 use rustc::traits::error_reporting::ArgKind;
-use rustc::ty::{self, ToPolyTraitRef, Ty, GenericParamDefKind};
+use rustc::ty::{self, Ty, GenericParamDefKind};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::subst::Substs;
 use std::cmp;
@@ -219,13 +219,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         &self,
         expected_vid: ty::TyVid,
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
-        let fulfillment_cx = self.fulfillment_cx.borrow();
-        // Here `expected_ty` is known to be a type inference variable.
-
-        let expected_sig = fulfillment_cx
-            .pending_obligations()
-            .iter()
-            .filter_map(|obligation| {
+        let expected_sig = self.obligations_for_self_ty(expected_vid)
+            .find_map(|(_, obligation)| {
                 debug!(
                     "deduce_expectations_from_obligations: obligation.predicate={:?}",
                     obligation.predicate
@@ -234,52 +229,21 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 if let ty::Predicate::Projection(ref proj_predicate) = obligation.predicate {
                     // Given a Projection predicate, we can potentially infer
                     // the complete signature.
-                    let trait_ref = proj_predicate.to_poly_trait_ref(self.tcx);
-                    self.self_type_matches_expected_vid(trait_ref, expected_vid)
-                        .and_then(|_| {
-                            self.deduce_sig_from_projection(
-                                Some(obligation.cause.span),
-                                proj_predicate
-                            )
-                        })
+                    self.deduce_sig_from_projection(
+                        Some(obligation.cause.span),
+                        proj_predicate
+                    )
                 } else {
                     None
                 }
-            })
-            .next();
+            });
 
         // Even if we can't infer the full signature, we may be able to
         // infer the kind. This can occur if there is a trait-reference
         // like `F : Fn<A>`. Note that due to subtyping we could encounter
         // many viable options, so pick the most restrictive.
-        let expected_kind = fulfillment_cx
-            .pending_obligations()
-            .iter()
-            .filter_map(|obligation| {
-                let opt_trait_ref = match obligation.predicate {
-                    ty::Predicate::Projection(ref data) => Some(data.to_poly_trait_ref(self.tcx)),
-                    ty::Predicate::Trait(ref data) => Some(data.to_poly_trait_ref()),
-                    ty::Predicate::Subtype(..) => None,
-                    ty::Predicate::RegionOutlives(..) => None,
-                    ty::Predicate::TypeOutlives(..) => None,
-                    ty::Predicate::WellFormed(..) => None,
-                    ty::Predicate::ObjectSafe(..) => None,
-                    ty::Predicate::ConstEvaluatable(..) => None,
-
-                    // N.B., this predicate is created by breaking down a
-                    // `ClosureType: FnFoo()` predicate, where
-                    // `ClosureType` represents some `Closure`. It can't
-                    // possibly be referring to the current closure,
-                    // because we haven't produced the `Closure` for
-                    // this closure yet; this is exactly why the other
-                    // code is looking for a self type of a unresolved
-                    // inference variable.
-                    ty::Predicate::ClosureKind(..) => None,
-                };
-                opt_trait_ref
-                    .and_then(|tr| self.self_type_matches_expected_vid(tr, expected_vid))
-                    .and_then(|tr| self.tcx.lang_items().fn_trait_kind(tr.def_id()))
-            })
+        let expected_kind = self.obligations_for_self_ty(expected_vid)
+            .filter_map(|(tr, _)| self.tcx.lang_items().fn_trait_kind(tr.def_id()))
             .fold(None, |best, cur| {
                 Some(best.map_or(cur, |best| cmp::min(best, cur)))
             });
@@ -339,22 +303,6 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         Some(ExpectedSig { cause_span, sig })
     }
 
-    fn self_type_matches_expected_vid(
-        &self,
-        trait_ref: ty::PolyTraitRef<'tcx>,
-        expected_vid: ty::TyVid,
-    ) -> Option<ty::PolyTraitRef<'tcx>> {
-        let self_ty = self.shallow_resolve(trait_ref.self_ty());
-        debug!(
-            "self_type_matches_expected_vid(trait_ref={:?}, self_ty={:?})",
-            trait_ref, self_ty
-        );
-        match self_ty.sty {
-            ty::Infer(ty::TyVar(v)) if expected_vid == v => Some(trait_ref),
-            _ => None,
-        }
-    }
-
     fn sig_of_closure(
         &self,
         expr_def_id: DefId,

src/librustc_typeck/check/coercion.rs

@@ -579,7 +579,33 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
             };
             match selcx.select(&obligation.with(trait_ref)) {
                 // Uncertain or unimplemented.
-                Ok(None) |
+                Ok(None) => {
+                    if trait_ref.def_id() == unsize_did {
+                        let trait_ref = self.resolve_type_vars_if_possible(&trait_ref);
+                        let self_ty = trait_ref.skip_binder().self_ty();
+                        let unsize_ty = trait_ref.skip_binder().input_types().nth(1).unwrap();
+                        debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_ref);
+                        match (&self_ty.sty, &unsize_ty.sty) {
+                            (ty::Infer(ty::TyVar(v)),
+                             ty::Dynamic(..)) if self.type_var_is_sized(*v) => {
+                                debug!("coerce_unsized: have sized infer {:?}", v);
+                                coercion.obligations.push(obligation);
+                                // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
+                                // for unsizing.
+                            }
+                            _ => {
+                                // Some other case for `$0: Unsize<Something>`. Note that we
+                                // hit this case even if `Something` is a sized type, so just
+                                // don't do the coercion.
+                                debug!("coerce_unsized: ambiguous unsize");
+                                return Err(TypeError::Mismatch);
+                            }
+                        }
+                    } else {
+                        debug!("coerce_unsized: early return - ambiguous");
+                        return Err(TypeError::Mismatch);
+                    }
+                }
                 Err(traits::Unimplemented) => {
                     debug!("coerce_unsized: early return - can't prove obligation");
                     return Err(TypeError::Mismatch);

src/librustc_typeck/check/mod.rs

@@ -114,8 +114,8 @@ use rustc::mir::interpret::{ConstValue, GlobalId};
 use rustc::ty::subst::{CanonicalUserSubsts, UnpackedKind, Subst, Substs,
                        UserSelfTy, UserSubsts};
 use rustc::traits::{self, ObligationCause, ObligationCauseCode, TraitEngine};
-use rustc::ty::{self, AdtKind, Ty, TyCtxt, GenericParamDefKind, Visibility, ToPredicate,
-                RegionKind};
+use rustc::ty::{self, AdtKind, Ty, TyCtxt, GenericParamDefKind, RegionKind, Visibility,
+                ToPolyTraitRef, ToPredicate};
 use rustc::ty::adjustment::{Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::query::Providers;
@@ -143,6 +143,7 @@ use require_c_abi_if_variadic;
 use session::{CompileIncomplete, config, Session};
 use TypeAndSubsts;
 use lint;
+use util::captures::Captures;
 use util::common::{ErrorReported, indenter};
 use util::nodemap::{DefIdMap, DefIdSet, FxHashMap, FxHashSet, NodeMap};
 
@@ -2732,6 +2733,72 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         method.sig.output()
     }
 
+    fn self_type_matches_expected_vid(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        expected_vid: ty::TyVid,
+    ) -> bool {
+        let self_ty = self.shallow_resolve(trait_ref.self_ty());
+        debug!(
+            "self_type_matches_expected_vid(trait_ref={:?}, self_ty={:?}, expected_vid={:?})",
+            trait_ref, self_ty, expected_vid
+        );
+        match self_ty.sty {
+            ty::Infer(ty::TyVar(found_vid)) => {
+                // FIXME: consider using `sub_root_var` here so we
+                // can see through subtyping.
+                let found_vid = self.root_var(found_vid);
+                debug!("self_type_matches_expected_vid - found_vid={:?}", found_vid);
+                expected_vid == found_vid
+            }
+            _ => false
+        }
+    }
+
+    fn obligations_for_self_ty<'b>(&'b self, self_ty: ty::TyVid)
+        -> impl Iterator<Item=(ty::PolyTraitRef<'tcx>, traits::PredicateObligation<'tcx>)>
+           + Captures<'gcx> + 'b
+    {
+        // FIXME: consider using `sub_root_var` here so we
+        // can see through subtyping.
+        let ty_var_root = self.root_var(self_ty);
+        debug!("obligations_for_self_ty: self_ty={:?} ty_var_root={:?} pending_obligations={:?}",
+               self_ty, ty_var_root,
+               self.fulfillment_cx.borrow().pending_obligations());
+
+        self.fulfillment_cx
+            .borrow()
+            .pending_obligations()
+            .into_iter()
+            .filter_map(move |obligation| match obligation.predicate {
+                ty::Predicate::Projection(ref data) =>
+                    Some((data.to_poly_trait_ref(self.tcx), obligation)),
+                ty::Predicate::Trait(ref data) =>
+                    Some((data.to_poly_trait_ref(), obligation)),
+                ty::Predicate::Subtype(..) => None,
+                ty::Predicate::RegionOutlives(..) => None,
+                ty::Predicate::TypeOutlives(..) => None,
+                ty::Predicate::WellFormed(..) => None,
+                ty::Predicate::ObjectSafe(..) => None,
+                ty::Predicate::ConstEvaluatable(..) => None,
+                // N.B., this predicate is created by breaking down a
+                // `ClosureType: FnFoo()` predicate, where
+                // `ClosureType` represents some `Closure`. It can't
+                // possibly be referring to the current closure,
+                // because we haven't produced the `Closure` for
+                // this closure yet; this is exactly why the other
+                // code is looking for a self type of a unresolved
+                // inference variable.
+                ty::Predicate::ClosureKind(..) => None,
+            }).filter(move |(tr, _)| self.self_type_matches_expected_vid(*tr, ty_var_root))
+    }
+
+    fn type_var_is_sized(&self, self_ty: ty::TyVid) -> bool {
+        self.obligations_for_self_ty(self_ty).any(|(tr, _)| {
+            Some(tr.def_id()) == self.tcx.lang_items().sized_trait()
+        })
+    }
+
     /// Generic function that factors out common logic from function calls,
     /// method calls and overloaded operators.
     fn check_argument_types(&self,

src/test/ui/coercion/coerce-issue-49593-box-never.rs

@@ -0,0 +1,62 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// 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.
+
+// compile-pass
+
+#![feature(never_type)]
+#![allow(unreachable_code)]
+
+use std::error::Error;
+use std::mem;
+
+fn raw_ptr_box<T>(t: T) -> *mut T {
+    panic!()
+}
+
+fn foo(x: !) -> Box<dyn Error> {
+    /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
+}
+
+fn foo_raw_ptr(x: !) -> *mut dyn Error {
+    /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
+}
+
+fn no_coercion(d: *mut dyn Error) -> *mut dyn Error {
+    /* an unsize coercion won't compile here, and it is indeed not used
+       because there is nothing requiring the _ to be Sized */
+    d as *mut _
+}
+
+trait Xyz {}
+struct S;
+struct T;
+impl Xyz for S {}
+impl Xyz for T {}
+
+fn foo_no_never() {
+    let mut x /* : Option<S> */ = None;
+    let mut first_iter = false;
+    loop {
+        if !first_iter {
+            let y: Box<dyn Xyz>
+                = /* Box<$0> is coerced to Box<Xyz> here */ Box::new(x.unwrap());
+        }
+
+        x = Some(S);
+        first_iter = true;
+    }
+
+    let mut y : Option<S> = None;
+    // assert types are equal
+    mem::swap(&mut x, &mut y);
+}
+
+fn main() {
+}