Enforce object safety

closes #17670

[breaking-change]

Traits must be object-safe if they are to be used in trait objects. This might require splitting a trait into object-safe and non-object-safe parts.

Some standard library traits in std::io have been split - Reader has new traits BytesReader (for the bytes method) and AsRefReader (for by_ref), Writer has new trait AsRefWriter (for by_ref). All these new traits have blanket impls, so any type which implements Reader or Writer (respectively) will have an implmentation of the new traits. To fix your code, you just need to `use` the new trait.

diff --git a/src/librustc/middle/typeck/check/method.rs b/src/librustc/middle/typeck/check/method.rs
index 7527160c825ae17cc7f7229e50ef23069ef9a59a..31364748423ca2fb797b5cd00af3b6f58ab11da2 100644 (file)
--- a/src/librustc/middle/typeck/check/method.rs
+++ b/src/librustc/middle/typeck/check/method.rs
@@ -1336,16 +1336,6 @@ fn confirm_candidate(&self, rcvr_ty: ty::t, candidate: &Candidate)
                self.ty_to_string(rcvr_ty),
                candidate.repr(self.tcx()));
 
-        let mut rcvr_substs = candidate.rcvr_substs.clone();
-
-        if !self.enforce_object_limitations(candidate) {
-            // Here we change `Self` from `Trait` to `err` in the case that
-            // this is an illegal object method. This is necessary to prevent
-            // the user from getting strange, derivative errors when the method
-            // takes an argument/return-type of type `Self` etc.
-            rcvr_substs.types.get_mut_slice(SelfSpace)[0] = ty::mk_err();
-        }
-
         self.enforce_drop_trait_limitations(candidate);
 
         // Determine the values for the generic parameters of the method.
@@ -1554,71 +1544,6 @@ fn fixup_derefs_on_method_receiver_if_necessary(
         }
     }
 
-    fn enforce_object_limitations(&self, candidate: &Candidate) -> bool {
-        /*!
-         * There are some limitations to calling functions through an
-         * object, because (a) the self type is not known
-         * (that's the whole point of a trait instance, after all, to
-         * obscure the self type) and (b) the call must go through a
-         * vtable and hence cannot be monomorphized.
-         */
-
-        match candidate.origin {
-            MethodStatic(..) |
-            MethodTypeParam(..) |
-            MethodStaticUnboxedClosure(..) => {
-                return true; // not a call to a trait instance
-            }
-            MethodTraitObject(..) => {}
-        }
-
-        match candidate.method_ty.explicit_self {
-            ty::StaticExplicitSelfCategory => { // reason (a) above
-                self.tcx().sess.span_err(
-                    self.span,
-                    "cannot call a method without a receiver \
-                     through an object");
-                return false;
-            }
-
-            ty::ByValueExplicitSelfCategory |
-            ty::ByReferenceExplicitSelfCategory(..) |
-            ty::ByBoxExplicitSelfCategory => {}
-        }
-
-        // reason (a) above
-        let check_for_self_ty = |ty| -> bool {
-            if ty::type_has_self(ty) {
-                span_err!(self.tcx().sess, self.span, E0038,
-                    "cannot call a method whose type contains a \
-                     self-type through an object");
-                false
-            } else {
-                true
-            }
-        };
-        let ref sig = candidate.method_ty.fty.sig;
-        for &input_ty in sig.inputs[1..].iter() {
-            if !check_for_self_ty(input_ty) {
-                return false;
-            }
-        }
-        if let ty::FnConverging(result_type) = sig.output {
-            if !check_for_self_ty(result_type) {
-                return false;
-            }
-        }
-
-        if candidate.method_ty.generics.has_type_params(subst::FnSpace) {
-            // reason (b) above
-            span_err!(self.tcx().sess, self.span, E0039,
-                "cannot call a generic method through an object");
-            return false;
-        }
-
-        true
-    }
-
     fn enforce_drop_trait_limitations(&self, candidate: &Candidate) {
         // No code can call the finalize method explicitly.
         let bad = match candidate.origin {

diff --git a/src/librustc/middle/typeck/check/mod.rs b/src/librustc/middle/typeck/check/mod.rs
index 8843be3cf816f387c0745d0df42d5c45da4a4343..7a5ce9a528cda1f6b5a6a0ec2e7f24b777aaafc5 100644 (file)
--- a/src/librustc/middle/typeck/check/mod.rs
+++ b/src/librustc/middle/typeck/check/mod.rs
@@ -1687,6 +1687,7 @@ fn register_unsize_obligations(&self,
                 self.register_unsize_obligations(span, &**u)
             }
             ty::UnsizeVtable(ref ty_trait, self_ty) => {
+                vtable2::check_object_safety(self.tcx(), ty_trait, span);
                 // If the type is `Foo+'a`, ensures that the type
                 // being cast to `Foo+'a` implements `Foo`:
                 vtable::register_object_cast_obligations(self,

diff --git a/src/librustc/middle/typeck/check/vtable.rs b/src/librustc/middle/typeck/check/vtable.rs
index a5624dcc2fcd3f9b503d2579d4663e60d4eed690..639ba9bdf49486d777e71621329538302a94f10b 100644 (file)
--- a/src/librustc/middle/typeck/check/vtable.rs
+++ b/src/librustc/middle/typeck/check/vtable.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use middle::subst::{SelfSpace};
+use middle::subst::{SelfSpace, FnSpace};
 use middle::traits;
 use middle::traits::{SelectionError, OutputTypeParameterMismatch, Overflow, Unimplemented};
 use middle::traits::{Obligation, obligation_for_builtin_bound};
@@ -46,6 +46,7 @@ pub fn check_object_cast(fcx: &FnCtxt,
 
             // Ensure that if ~T is cast to ~Trait, then T : Trait
             push_cast_obligation(fcx, cast_expr, object_trait, referent_ty);
+            check_object_safety(fcx.tcx(), object_trait, source_expr.span);
         }
 
         (&ty::ty_rptr(referent_region, ty::mt { ty: referent_ty,
@@ -68,6 +69,8 @@ pub fn check_object_cast(fcx: &FnCtxt,
                                infer::RelateObjectBound(source_expr.span),
                                target_region,
                                referent_region);
+
+                check_object_safety(fcx.tcx(), object_trait, source_expr.span);
             }
         }
 
@@ -128,6 +131,70 @@ fn push_cast_obligation(fcx: &FnCtxt,
     }
 }
 
+// TODO comment
+pub fn check_object_safety(tcx: &ty::ctxt, object_trait: &ty::TyTrait, span: Span) {
+    let trait_items = ty::trait_items(tcx, object_trait.def_id);
+    for item in trait_items.iter() {
+        match *item {
+            ty::MethodTraitItem(ref m) => check_object_safety_of_method(tcx, &**m, span),
+            ty::TypeTraitItem(_) => {}
+        }
+    }
+
+    // TODO error messages
+    fn check_object_safety_of_method(tcx: &ty::ctxt, method: &ty::Method, span: Span) {
+        /*!
+         * There are some limitations to calling functions through an
+         * object, because (a) the self type is not known
+         * (that's the whole point of a trait instance, after all, to
+         * obscure the self type) and (b) the call must go through a
+         * vtable and hence cannot be monomorphized.
+         */
+
+        match method.explicit_self {
+            ty::ByValueExplicitSelfCategory => { // reason (a) above
+                tcx.sess.span_err(
+                    span,
+                    "cannot call a method with a by-value receiver \
+                     through a trait object");
+            }
+
+            ty::StaticExplicitSelfCategory |
+            ty::ByReferenceExplicitSelfCategory(..) |
+            ty::ByBoxExplicitSelfCategory => {}
+        }
+
+        // reason (a) above
+        let check_for_self_ty = |ty| {
+            if ty::type_has_self(ty) {
+                span_err!(tcx.sess, span, E0038,
+                    "cannot call a method whose type contains a \
+                     self-type through an object: {}", ::util::ppaux::ty_to_string(tcx, ty));
+                true
+            } else {
+                false
+            }
+        };
+        let ref sig = method.fty.sig;
+        let mut found_self_ty = false;
+        for &input_ty in sig.inputs.tail().iter() {
+            if check_for_self_ty(input_ty) {
+                found_self_ty = true;
+                break;
+            }
+        }
+        if !found_self_ty {
+            check_for_self_ty(sig.output);
+        }
+
+        if method.generics.has_type_params(FnSpace) {
+            // reason (b) above
+            span_err!(tcx.sess, span, E0039,
+                "cannot call a generic method through an object");
+        }
+    }
+}
+
 pub fn register_object_cast_obligations(fcx: &FnCtxt,
                                         span: Span,
                                         object_trait: &ty::TyTrait,