Fix opaque types resulting from projections in function signature

src/librustc_typeck/check/mod.rs

@@ -114,7 +114,7 @@ use rustc::ty::{
 use rustc::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast
 };
-use rustc::ty::fold::TypeFoldable;
+use rustc::ty::fold::{TypeFoldable, TypeFolder};
 use rustc::ty::query::Providers;
 use rustc::ty::subst::{
     GenericArgKind, Subst, InternalSubsts, SubstsRef, UserSelfTy, UserSubsts,
@@ -872,6 +872,111 @@ fn used_trait_imports(tcx: TyCtxt<'_>, def_id: DefId) -> &DefIdSet {
     &*tcx.typeck_tables_of(def_id).used_trait_imports
 }
 
+/// Inspects the substs of opaque types, replacing any inference variables
+/// with proper generic parameter from the identity substs.
+///
+/// This is run after we normalize the function signature, to fix any inference
+/// variables introduced by the projection of associated types. This ensures that
+/// any opaque types used in the signature continue to refer to generic parameters,
+/// allowing them to be considered for defining uses in the function body
+///
+/// For example, consider this code.
+///
+/// ```rust
+/// trait MyTrait {
+///     type MyItem;
+///     fn use_it(self) -> Self::MyItem
+/// }
+/// impl<T, I> MyTrait for T where T: Iterator<Item = I> {
+///     type MyItem = impl Iterator<Item = I>;
+///     fn use_it(self) -> Self::MyItem {
+///         self
+///     }
+/// }
+/// ```
+///
+/// When we normalize the signature of `use_it` from the impl block,
+/// we will normalize `Self::MyItem` to the opaque type `impl Iterator<Item = I>`
+/// However, this projection result may contain inference variables, due
+/// to the way that projection works. We didn't have any inference variables
+/// in the signature to begin with - leaving them in will cause us to incorrectly
+/// conclude that we don't have a defining use of `MyItem`. By mapping inference
+/// variables back to the actual generic parameters, we will correctly see that
+/// we have a defining use of `MyItem`
+fn fixup_opaque_types<'tcx, T>(tcx: TyCtxt<'tcx>, val: &T) -> T where T: TypeFoldable<'tcx> {
+    struct FixupFolder<'tcx> {
+        tcx: TyCtxt<'tcx>
+    }
+
+    impl<'tcx> TypeFolder<'tcx> for FixupFolder<'tcx> {
+        fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+            self.tcx
+        }
+
+        fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+            match ty.kind {
+                ty::Opaque(def_id, substs) => {
+                    debug!("fixup_opaque_types: found type {:?}", ty);
+                    // Here, we replace any inference variables that occur within
+                    // the substs of an opaque type. By definition, any type occuring
+                    // in the substs has a corresponding generic parameter, which is what
+                    // we replace it with.
+                    // This replacement is only run on the function signature, so any
+                    // inference variables that we come across must be the rust of projection
+                    // (there's no other way for a user to get inference variables into
+                    // a function signature).
+                    if ty.needs_infer() {
+                        let new_substs = InternalSubsts::for_item(self.tcx, def_id, |param, _| {
+                            let old_param = substs[param.index as usize];
+                            match old_param.unpack() {
+                                GenericArgKind::Type(old_ty) => {
+                                    if let ty::Infer(_) = old_ty.kind {
+                                        // Replace inference type with a generic parameter
+                                        self.tcx.mk_param_from_def(param)
+                                    } else {
+                                        old_param.fold_with(self)
+                                    }
+                                },
+                                GenericArgKind::Const(old_const) => {
+                                    if let ty::ConstKind::Infer(_) = old_const.val {
+                        // This should never happen - we currently do not support
+                        // 'const projections', e.g.:
+                        // `impl<T: SomeTrait> MyTrait for T where <T as SomeTrait>::MyConst == 25`
+                        // which should be the only way for us to end up with a const inference
+                        // variable after projection. If Rust ever gains support for this kind
+                        // of projection, this should *probably* be changed to
+                        // `self.tcx.mk_param_from_def(param)`
+                                        bug!("Found infer const: `{:?}` in opaque type: {:?}",
+                                             old_const, ty);
+                                    } else {
+                                        old_param.fold_with(self)
+                                    }
+                                }
+                                GenericArgKind::Lifetime(old_region) => {
+                                    if let RegionKind::ReVar(_) = old_region {
+                                        self.tcx.mk_param_from_def(param)
+                                    } else {
+                                        old_param.fold_with(self)
+                                    }
+                                }
+                            }
+                        });
+                        let new_ty = self.tcx.mk_opaque(def_id, new_substs);
+                        debug!("fixup_opaque_types: new type: {:?}", new_ty);
+                        new_ty
+                    } else {
+                        ty
+                    }
+                },
+                _ => ty.super_fold_with(self)
+            }
+        }
+    }
+
+    debug!("fixup_opaque_types({:?})", val);
+    val.fold_with(&mut FixupFolder { tcx })
+}
+
 fn typeck_tables_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::TypeckTables<'_> {
     // Closures' tables come from their outermost function,
     // as they are part of the same "inference environment".
@@ -911,6 +1016,8 @@ fn typeck_tables_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::TypeckTables<'_> {
                                                   param_env,
                                                   &fn_sig);
 
+            let fn_sig = fixup_opaque_types(tcx, &fn_sig);
+
             let fcx = check_fn(&inh, param_env, fn_sig, decl, id, body, None).0;
             fcx
         } else {

src/librustc_typeck/collect.rs

@@ -1617,11 +1617,18 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     ty::Param(_) => true,
                     _ => false,
                 };
-                if !substs.types().all(is_param) {
-                    self.tcx.sess.span_err(
-                        span,
-                        "defining opaque type use does not fully define opaque type",
-                    );
+                let bad_substs: Vec<_> = substs.types().enumerate()
+                    .filter(|(_, ty)| !is_param(ty)).collect();
+                if !bad_substs.is_empty() {
+                    let identity_substs = InternalSubsts::identity_for_item(self.tcx, self.def_id);
+                    for (i, bad_subst) in bad_substs {
+                        self.tcx.sess.span_err(
+                            span,
+                            &format!("defining opaque type use does not fully define opaque type: \
+                            generic parameter `{}` is specified as concrete type `{}`",
+                            identity_substs.type_at(i), bad_subst)
+                        );
+                    }
                 } else if let Some((prev_span, prev_ty, ref prev_indices)) = self.found {
                     let mut ty = concrete_type.walk().fuse();
                     let mut p_ty = prev_ty.walk().fuse();
@@ -2059,6 +2066,9 @@ fn explicit_predicates_of(
                 ty::print::with_no_queries(|| {
                     let substs = InternalSubsts::identity_for_item(tcx, def_id);
                     let opaque_ty = tcx.mk_opaque(def_id, substs);
+                    debug!("explicit_predicates_of({:?}): created opaque type {:?}",
+                        def_id, opaque_ty);
+
 
                     // Collect the bounds, i.e., the `A + B + 'c` in `impl A + B + 'c`.
                     let bounds = AstConv::compute_bounds(

src/test/ui/impl-trait/type-alias-generic-param.rs

@@ -0,0 +1,22 @@
+// Regression test for issue #59342
+// Checks that we properly detect defining uses of opaque
+// types in 'item' position when generic parameters are involved
+//
+// run-pass
+#![feature(type_alias_impl_trait)]
+
+trait Meow {
+    type MeowType;
+    fn meow(self) -> Self::MeowType;
+}
+
+impl<T, I> Meow for I
+    where I: Iterator<Item = T>
+{
+    type MeowType = impl Iterator<Item = T>;
+    fn meow(self) -> Self::MeowType {
+        self
+    }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/assoc-type-const.rs

@@ -0,0 +1,36 @@
+// Tests that we properly detect defining usages when using
+// const generics in an associated opaque type
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+trait UnwrapItemsExt<const C: usize> {
+    type Iter;
+    fn unwrap_items(self) -> Self::Iter;
+}
+
+struct MyStruct<const C: usize> {}
+
+trait MyTrait<'a, const C: usize> {
+    type MyItem;
+    const MY_CONST: usize;
+}
+
+impl<'a, const C: usize> MyTrait<'a, {C}> for MyStruct<{C}> {
+    type MyItem = u8;
+    const MY_CONST: usize = C;
+}
+
+impl<'a, I, const C: usize> UnwrapItemsExt<{C}> for I
+where
+{
+    type Iter = impl MyTrait<'a, {C}>;
+
+    fn unwrap_items(self) -> Self::Iter {
+        MyStruct::<{C}> {}
+    }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/assoc-type-const.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/assoc-type-const.rs:6:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+

src/test/ui/type-alias-impl-trait/assoc-type-lifetime.rs

@@ -0,0 +1,28 @@
+// Tests that we still detect defining usages when
+// lifetimes are used in an associated opaque type
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+trait UnwrapItemsExt {
+    type Iter;
+    fn unwrap_items(self) -> Self::Iter;
+}
+
+struct MyStruct {}
+
+trait MyTrait<'a> {}
+
+impl<'a> MyTrait<'a> for MyStruct {}
+
+impl<'a, I> UnwrapItemsExt for I
+where
+{
+    type Iter = impl MyTrait<'a>;
+
+    fn unwrap_items(self) -> Self::Iter {
+        MyStruct {}
+    }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/bound_reduction2.stderr

@@ -1,4 +1,4 @@
-error: defining opaque type use does not fully define opaque type
+error: defining opaque type use does not fully define opaque type: generic parameter `V` is specified as concrete type `<T as TraitWithAssoc>::Assoc`
   --> $DIR/bound_reduction2.rs:17:1
    |
 LL | / fn foo_desugared<T: TraitWithAssoc>(_: T) -> Foo<T::Assoc> {

src/test/ui/type-alias-impl-trait/generic_nondefining_use.stderr

@@ -4,7 +4,7 @@ error: at least one trait must be specified
 LL | type Cmp<T> = impl 'static;
    |               ^^^^^^^^^^^^
 
-error: defining opaque type use does not fully define opaque type
+error: defining opaque type use does not fully define opaque type: generic parameter `T` is specified as concrete type `u32`
   --> $DIR/generic_nondefining_use.rs:11:1
    |
 LL | / fn cmp() -> Cmp<u32> {

src/test/ui/type-alias-impl-trait/issue-58887.rs

@@ -1,3 +1,5 @@
+// run-pass
+
 #![feature(type_alias_impl_trait)]
 
 trait UnwrapItemsExt {
@@ -11,11 +13,8 @@ where
     E: std::fmt::Debug,
 {
     type Iter = impl Iterator<Item = T>;
-    //~^ ERROR: could not find defining uses
 
     fn unwrap_items(self) -> Self::Iter {
-    //~^ ERROR: type parameter `T` is part of concrete type
-    //~| ERROR: type parameter `E` is part of concrete type
         self.map(|x| x.unwrap())
     }
 }

src/test/ui/type-alias-impl-trait/issue-60564.rs

@@ -18,7 +18,7 @@ where
 {
     type BitsIter = IterBitsIter<T, E, u8>;
     fn iter_bits(self, n: u8) -> Self::BitsIter {
-    //~^ ERROR type parameter `E` is part of concrete type but not used
+    //~^ ERROR defining opaque type use does not fully define opaque type
         (0u8..n)
             .rev()
             .map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())

src/test/ui/type-alias-impl-trait/issue-60564.stderr

@@ -1,8 +1,7 @@
-error: type parameter `E` is part of concrete type but not used in parameter list for the `impl Trait` type alias
-  --> $DIR/issue-60564.rs:20:49
+error: defining opaque type use does not fully define opaque type: generic parameter `I` is specified as concrete type `u8`
+  --> $DIR/issue-60564.rs:20:5
    |
-LL |       fn iter_bits(self, n: u8) -> Self::BitsIter {
-   |  _________________________________________________^
+LL | /     fn iter_bits(self, n: u8) -> Self::BitsIter {
 LL | |
 LL | |         (0u8..n)
 LL | |             .rev()

src/test/ui/type-alias-impl-trait/not_a_defining_use.stderr

@@ -1,4 +1,4 @@
-error: defining opaque type use does not fully define opaque type
+error: defining opaque type use does not fully define opaque type: generic parameter `U` is specified as concrete type `u32`
   --> $DIR/not_a_defining_use.rs:9:1
    |
 LL | / fn two<T: Debug>(t: T) -> Two<T, u32> {