Avoid committing to autoderef in object method probing

src/librustc_typeck/check/method/probe.rs

@@ -85,6 +85,37 @@ impl<'a, 'gcx, 'tcx> Deref for ProbeContext<'a, 'gcx, 'tcx> {
 
 #[derive(Debug)]
 struct Candidate<'tcx> {
+ // Candidates are (I'm not quite sure, but they are mostly) basically
+ // some metadata on top of a `ty::AssociatedItem` (without substs).
+ //
+ // However, method probing wants to be able to evaluate the predicates
+ // for a function with the substs applied - for example, if a function
+ // has `where Self: Sized`, we don't want to consider it unless `Self`
+ // is actually `Sized`, and similarly, return-type suggestions want
+ // to consider the "actual" return type.
+ //
+ // The way this is handled is through `xform_self_ty`. It contains
+ // the receiver type of this candidate, but `xform_self_ty`,
+ // `xform_ret_ty` and `kind` (which contains the predicates) have the
+ // generic parameters of this candidate substituted with the *same set*
+ // of inference variables, which acts as some weird sort of "query".
+ //
+ // When we check out a candidate, we require `xform_self_ty` to be
+ // a subtype of the passed-in self-type, and this equates the type
+ // variables in the rest of the fields.
+ //
+ // For example, if we have this candidate:
+ // ```
+ // trait Foo {
+ // fn foo(&self) where Self: Sized;
+ // }
+ // ```
+ //
+ // Then `xform_self_ty` will be `&'erased ?X` and `kind` will contain
+ // the predicate `?X: Sized`, so if we are evaluating `Foo` for a
+ // the receiver `&T`, we'll do the subtyping which will make `?X`
+ // get the right value, then when we evaluate the predicate we'll check
+ // if `T: Sized`.
  xform_self_ty: Ty<'tcx>,
  xform_ret_ty: Option<Ty<'tcx>>,
  item: ty::AssociatedItem,
@@ -506,13 +537,28 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
  match self_ty.value.value.sty {
  ty::Dynamic(ref data, ..) => {
  if let Some(p) = data.principal() {
- let InferOk { value: instantiated_self_ty, obligations: _ } =
- self.fcx.probe_instantiate_query_response(
- self.span, &self.orig_steps_var_values, self_ty)
- .unwrap_or_else(|_| {
- span_bug!(self.span, "{:?} was applicable but now isn't?", self_ty)
- });
- self.assemble_inherent_candidates_from_object(instantiated_self_ty);
+ // Subtle: we can't use `instantiate_query_response` here: using it will
+ // commit to all of the type equalities assumed by inference going through
+ // autoderef (see the `method-probe-no-guessing` test).
+ //
+ // However, in this code, it is OK if we end up with an object type that is
+ // "more general" than the object type that we are evaluating. For *every*
+ // object type `MY_OBJECT`, a function call that goes through a trait-ref
+ // of the form `<MY_OBJECT as SuperTraitOf(MY_OBJECT)>::func` is a valid
+ // `ObjectCandidate`, and it should be discoverable "exactly" through one
+ // of the iterations in the autoderef loop, so there is no problem with it
+ // being discoverable in another one of these iterations.
+ //
+ // Using `instantiate_canonical_with_fresh_inference_vars` on our
+ // `Canonical<QueryResponse<Ty<'tcx>>>` and then *throwing away* the
+ // `CanonicalVarValues` will exactly give us such a generalization - it
+ // will still match the original object type, but it won't pollute our
+ // type variables in any form, so just do that!
+ let (QueryResponse { value: generalized_self_ty, .. }, _ignored_var_values) =
+ self.fcx.instantiate_canonical_with_fresh_inference_vars(
+ self.span, &self_ty);
+
+ self.assemble_inherent_candidates_from_object(generalized_self_ty);
  self.assemble_inherent_impl_candidates_for_type(p.def_id());
  }
  }

src/test/run-pass/methods/method-probe-no-guessing-dyn-trait.rs

@@ -0,0 +1,59 @@
+// Check that method matching does not make "guesses" depending on
+// Deref impls that don't eventually end up being picked.
+
+use std::ops::Deref;
+
+// An impl with less derefs will get called over an impl with more derefs,
+// so `(t: Foo<_>).my_fn()` will use `<Foo<u32> as MyTrait1>::my_fn(t)`,
+// and does *not* force the `_` to equal `()`, because the Deref impl
+// was *not* used.
+
+trait MyTrait1 {
+ fn my_fn(&self) {}
+}
+
+impl MyTrait1 for Foo<u32> {}
+
+struct Foo<T>(T);
+
+impl Deref for Foo<()> {
+ type Target = dyn MyTrait1 + 'static;
+ fn deref(&self) -> &(dyn MyTrait1 + 'static) {
+ panic!()
+ }
+}
+
+// ...but if there is no impl with less derefs, the "guess" will be
+// forced, so `(t: Bar<_>).my_fn2()` is `<dyn MyTrait2 as MyTrait2>::my_fn2(*t)`,
+// and because the deref impl is used, the `_` is forced to equal `u8`.
+
+trait MyTrait2 {
+ fn my_fn2(&self) {}
+}
+
+impl MyTrait2 for u32 {}
+struct Bar<T>(T, u32);
+impl Deref for Bar<u8> {
+ type Target = dyn MyTrait2 + 'static;
+ fn deref(&self) -> &(dyn MyTrait2 + 'static) {
+ &self.1
+ }
+}
+
+// actually invoke things
+
+fn main() {
+ let mut foo: Option<Foo<_>> = None;
+ let mut bar: Option<Bar<_>> = None;
+ let mut first_iter = true;
+ loop {
+ if !first_iter {
+ foo.as_ref().unwrap().my_fn();
+ bar.as_ref().unwrap().my_fn2();
+ break;
+ }
+ foo = Some(Foo(0));
+ bar = Some(Bar(Default::default(), 0));
+ first_iter = false;
+ }
+}

src/test/ui/methods/method-deref-to-same-trait-object-with-separate-params.rs

@@ -0,0 +1,177 @@
+#![feature(arbitrary_self_types, coerce_unsized, dispatch_from_dyn, unsize, unsized_locals)]
+
+// This tests a few edge-cases around `arbitrary_self_types`. Most specifically,
+// it checks that the `ObjectCandidate` you get from method matching can't
+// match a trait with the same DefId as a supertrait but a bad type parameter.
+
+use std::marker::PhantomData;
+
+mod internal {
+ use std::ops::{CoerceUnsized, Deref, DispatchFromDyn};
+ use std::marker::{PhantomData, Unsize};
+
+ pub struct Smaht<T: ?Sized, MISC>(pub Box<T>, pub PhantomData<MISC>);
+
+ impl<T: ?Sized, MISC> Deref for Smaht<T, MISC> {
+ type Target = T;
+
+ fn deref(&self) -> &Self::Target {
+ &self.0
+ }
+ }
+ impl<T: ?Sized + Unsize<U>, U: ?Sized, MISC> CoerceUnsized<Smaht<U, MISC>>
+ for Smaht<T, MISC>
+ {}
+ impl<T: ?Sized + Unsize<U>, U: ?Sized, MISC> DispatchFromDyn<Smaht<U, MISC>>
+ for Smaht<T, MISC>
+ {}
+
+ pub trait Foo: X<u32> {}
+ pub trait X<T> {
+ fn foo(self: Smaht<Self, T>) -> T;
+ }
+
+ impl X<u32> for () {
+ fn foo(self: Smaht<Self, u32>) -> u32 {
+ 0
+ }
+ }
+
+ pub trait Marker {}
+ impl Marker for dyn Foo {}
+ impl<T: Marker + ?Sized> X<u64> for T {
+ fn foo(self: Smaht<Self, u64>) -> u64 {
+ 1
+ }
+ }
+
+ impl Deref for dyn Foo {
+ type Target = ();
+ fn deref(&self) -> &() { &() }
+ }
+
+ impl Foo for () {}
+}
+
+pub trait FinalFoo {
+ fn foo(&self) -> u8;
+}
+
+impl FinalFoo for () {
+ fn foo(&self) -> u8 { 0 }
+}
+
+mod nuisance_foo {
+ pub trait NuisanceFoo {
+ fn foo(self);
+ }
+
+ impl<T: ?Sized> NuisanceFoo for T {
+ fn foo(self) {}
+ }
+}
+
+
+fn objectcandidate_impl() {
+ let x: internal::Smaht<(), u32> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u32> = x;
+
+ // This picks `<dyn internal::Foo as X<u32>>::foo` via `ObjectCandidate`.
+ //
+ // The `TraitCandidate` is not relevant because `X` is not in scope.
+ let z = x.foo();
+
+ // Observe the type of `z` is `u32`
+ let _seetype: () = z; //~ ERROR mismatched types
+ //~| expected (), found u32
+}
+
+fn traitcandidate_impl() {
+ use internal::X;
+
+ let x: internal::Smaht<(), u64> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u64> = x;
+
+ // This picks `<dyn internal::Foo as X<u64>>::foo` via `TraitCandidate`.
+ //
+ // The `ObjectCandidate` does not apply, as it only applies to
+ // `X<u32>` (and not `X<u64>`).
+ let z = x.foo();
+
+ // Observe the type of `z` is `u64`
+ let _seetype: () = z; //~ ERROR mismatched types
+ //~| expected (), found u64
+}
+
+fn traitcandidate_impl_with_nuisance() {
+ use internal::X;
+ use nuisance_foo::NuisanceFoo;
+
+ let x: internal::Smaht<(), u64> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u64> = x;
+
+ // This picks `<dyn internal::Foo as X<u64>>::foo` via `TraitCandidate`.
+ //
+ // The `ObjectCandidate` does not apply, as it only applies to
+ // `X<u32>` (and not `X<u64>`).
+ //
+ // The NuisanceFoo impl has the same priority as the `X` impl,
+ // so we get a conflict.
+ let z = x.foo(); //~ ERROR multiple applicable items in scope
+}
+
+
+fn neither_impl() {
+ let x: internal::Smaht<(), u64> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u64> = x;
+
+ // This can't pick the `TraitCandidate` impl, because `Foo` is not
+ // imported. However, this also can't pick the `ObjectCandidate`
+ // impl, because it only applies to `X<u32>` (and not `X<u64>`).
+ //
+ // Therefore, neither of the candidates is applicable, and we pick
+ // the `FinalFoo` impl after another deref, which will return `u8`.
+ let z = x.foo();
+
+ // Observe the type of `z` is `u8`
+ let _seetype: () = z; //~ ERROR mismatched types
+ //~| expected (), found u8
+}
+
+fn both_impls() {
+ use internal::X;
+
+ let x: internal::Smaht<(), u32> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u32> = x;
+
+ // This can pick both the `TraitCandidate` and the `ObjectCandidate` impl.
+ //
+ // However, the `ObjectCandidate` is considered an "inherent candidate",
+ // and therefore has priority over both the `TraitCandidate` as well as
+ // any other "nuisance" candidate" (if present).
+ let z = x.foo();
+
+ // Observe the type of `z` is `u32`
+ let _seetype: () = z; //~ ERROR mismatched types
+ //~| expected (), found u32
+}
+
+
+fn both_impls_with_nuisance() {
+ // Similar to the `both_impls` example, except with a nuisance impl to
+ // make sure the `ObjectCandidate` indeed has a higher priority.
+
+ use internal::X;
+ use nuisance_foo::NuisanceFoo;
+
+ let x: internal::Smaht<(), u32> = internal::Smaht(Box::new(()), PhantomData);
+ let x: internal::Smaht<dyn internal::Foo, u32> = x;
+ let z = x.foo();
+
+ // Observe the type of `z` is `u32`
+ let _seetype: () = z; //~ ERROR mismatched types
+ //~| expected (), found u32
+}
+
+fn main() {
+}

src/test/ui/methods/method-deref-to-same-trait-object-with-separate-params.stderr

@@ -0,0 +1,72 @@
+error[E0308]: mismatched types
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:85:24
+ |
+LL | let _seetype: () = z; //~ ERROR mismatched types
+ | ^ expected (), found u32
+ |
+ = note: expected type `()`
+ found type `u32`
+
+error[E0308]: mismatched types
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:102:24
+ |
+LL | let _seetype: () = z; //~ ERROR mismatched types
+ | ^ expected (), found u64
+ |
+ = note: expected type `()`
+ found type `u64`
+
+error[E0034]: multiple applicable items in scope
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:120:15
+ |
+LL | let z = x.foo(); //~ ERROR multiple applicable items in scope
+ | ^^^ multiple `foo` found
+ |
+note: candidate #1 is defined in an impl of the trait `internal::X` for the type `_`
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:43:9
+ |
+LL | fn foo(self: Smaht<Self, u64>) -> u64 {
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl of the trait `nuisance_foo::NuisanceFoo` for the type `_`
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:70:9
+ |
+LL | fn foo(self) {}
+ | ^^^^^^^^^^^^
+note: candidate #3 is defined in the trait `FinalFoo`
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:57:5
+ |
+LL | fn foo(&self) -> u8;
+ | ^^^^^^^^^^^^^^^^^^^^
+ = help: to disambiguate the method call, write `FinalFoo::foo(x)` instead
+
+error[E0308]: mismatched types
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:137:24
+ |
+LL | let _seetype: () = z; //~ ERROR mismatched types
+ | ^ expected (), found u8
+ |
+ = note: expected type `()`
+ found type `u8`
+
+error[E0308]: mismatched types
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:155:24
+ |
+LL | let _seetype: () = z; //~ ERROR mismatched types
+ | ^ expected (), found u32
+ |
+ = note: expected type `()`
+ found type `u32`
+
+error[E0308]: mismatched types
+ --> $DIR/method-deref-to-same-trait-object-with-separate-params.rs:172:24
+ |
+LL | let _seetype: () = z; //~ ERROR mismatched types
+ | ^ expected (), found u32
+ |
+ = note: expected type `()`
+ found type `u32`
+
+error: aborting due to 6 previous errors
+
+Some errors occurred: E0034, E0308.
+For more information about an error, try `rustc --explain E0034`.