Add comments and tests explaining the shallow substitution rule

Author: jonas-schievink

src/librustc_typeck/check/wfcheck.rs

@@ -461,6 +461,27 @@ fn check_associated_type_defaults(
         })
         .collect::<FxHashMap<_, _>>();
 
+    /// Replaces projections of associated types with their default types.
+    ///
+    /// This does a "shallow substitution", meaning that defaults that refer to
+    /// other defaulted assoc. types will still refer to the projection
+    /// afterwards, not to the other default. For example:
+    ///
+    /// ```compile_fail
+    /// trait Tr {
+    ///     type A: Clone = Vec<Self::B>;
+    ///     type B = u8;
+    /// }
+    /// ```
+    ///
+    /// This will end up replacing the bound `Self::A: Clone` with
+    /// `Vec<Self::B>: Clone`, not with `Vec<u8>: Clone`. If we did a deep
+    /// substitution and ended up with the latter, the trait would be accepted.
+    /// If an `impl` then replaced `B` with something that isn't `Clone`,
+    /// suddenly the default for `A` is no longer valid. The shallow
+    /// substitution forces the trait to add a `B: Clone` bound to be accepted,
+    /// which means that an `impl` can replace any default without breaking
+    /// others.
     struct DefaultNormalizer<'tcx> {
         tcx: TyCtxt<'tcx>,
         map: FxHashMap<ty::ProjectionTy<'tcx>, Ty<'tcx>>,

src/test/ui/associated-types/defaults-suitability.rs

@@ -66,28 +66,43 @@ trait D where
     type Assoc = NotClone;
 }
 
-trait Foo2<T> where
-    <Self as Foo2<T>>::Bar: Clone,
+// Test behavior of the check when defaults refer to other defaults:
+
+// Shallow substitution rejects this trait since `Baz` isn't guaranteed to be
+// `Clone`.
+trait Foo2<T> {
+    type Bar: Clone = Vec<Self::Baz>;
     //~^ ERROR the trait bound `<Self as Foo2<T>>::Baz: std::clone::Clone` is not satisfied
-{
-    type Bar = Vec<Self::Baz>;
     type Baz = T;
 }
 
-trait Foo3<T: Clone> where
-    <Self as Foo3<T>>::Bar: Clone,
-    //~^ ERROR the trait bound `<Self as Foo3<T>>::Baz: std::clone::Clone` is not satisfied
+// Adding a `T: Clone` bound doesn't help since the requirement doesn't see `T`
+// because of the shallow substitution. If we did a deep substitution instead,
+// this would be accepted.
+trait Foo25<T: Clone> {
+    type Bar: Clone = Vec<Self::Baz>;
+    //~^ ERROR the trait bound `<Self as Foo25<T>>::Baz: std::clone::Clone` is not satisfied
+    type Baz = T;
+}
+
+// Adding the `Baz: Clone` bound isn't enough since the default is type
+// parameter `T`, which also might not be `Clone`.
+trait Foo3<T> where
+    Self::Bar: Clone,
+    Self::Baz: Clone,
+    //~^ ERROR the trait bound `T: std::clone::Clone` is not satisfied
 {
     type Bar = Vec<Self::Baz>;
     type Baz = T;
 }
 
+// This one finally works, with `Clone` bounds on all assoc. types and the type
+// parameter.
 trait Foo4<T> where
-    <Self as Foo4<T>>::Bar: Clone,
+    T: Clone,
 {
-    type Bar = Vec<Self::Baz>;
+    type Bar: Clone = Vec<Self::Baz>;
     type Baz: Clone = T;
-    //~^ ERROR the trait bound `T: std::clone::Clone` is not satisfied
 }
 
 fn main() {}

src/test/ui/associated-types/defaults-suitability.stderr

@@ -104,61 +104,49 @@ LL | | }
    | |_^
 
 error[E0277]: the trait bound `<Self as Foo2<T>>::Baz: std::clone::Clone` is not satisfied
-  --> $DIR/defaults-suitability.rs:70:29
+  --> $DIR/defaults-suitability.rs:74:15
    |
-LL |     <Self as Foo2<T>>::Bar: Clone,
-   |                             ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo2<T>>::Baz`
+LL |     type Bar: Clone = Vec<Self::Baz>;
+   |               ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo2<T>>::Baz`
    |
    = help: consider adding a `where <Self as Foo2<T>>::Baz: std::clone::Clone` bound
    = note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo2<T>>::Baz>`
 note: required by `Foo2`
-  --> $DIR/defaults-suitability.rs:69:1
+  --> $DIR/defaults-suitability.rs:73:1
    |
-LL | / trait Foo2<T> where
-LL | |     <Self as Foo2<T>>::Bar: Clone,
-LL | |
-LL | | {
-LL | |     type Bar = Vec<Self::Baz>;
-LL | |     type Baz = T;
-LL | | }
-   | |_^
+LL | trait Foo2<T> {
+   | ^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `<Self as Foo3<T>>::Baz: std::clone::Clone` is not satisfied
-  --> $DIR/defaults-suitability.rs:78:29
+error[E0277]: the trait bound `<Self as Foo25<T>>::Baz: std::clone::Clone` is not satisfied
+  --> $DIR/defaults-suitability.rs:83:15
    |
-LL |     <Self as Foo3<T>>::Bar: Clone,
-   |                             ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo3<T>>::Baz`
+LL |     type Bar: Clone = Vec<Self::Baz>;
+   |               ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo25<T>>::Baz`
    |
-   = help: consider adding a `where <Self as Foo3<T>>::Baz: std::clone::Clone` bound
-   = note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo3<T>>::Baz>`
-note: required by `Foo3`
-  --> $DIR/defaults-suitability.rs:77:1
+   = help: consider adding a `where <Self as Foo25<T>>::Baz: std::clone::Clone` bound
+   = note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo25<T>>::Baz>`
+note: required by `Foo25`
+  --> $DIR/defaults-suitability.rs:82:1
    |
-LL | / trait Foo3<T: Clone> where
-LL | |     <Self as Foo3<T>>::Bar: Clone,
-LL | |
-LL | | {
-LL | |     type Bar = Vec<Self::Baz>;
-LL | |     type Baz = T;
-LL | | }
-   | |_^
+LL | trait Foo25<T: Clone> {
+   | ^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `T: std::clone::Clone` is not satisfied
-  --> $DIR/defaults-suitability.rs:89:15
+  --> $DIR/defaults-suitability.rs:92:16
    |
-LL |     type Baz: Clone = T;
-   |               ^^^^^ the trait `std::clone::Clone` is not implemented for `T`
+LL |     Self::Baz: Clone,
+   |                ^^^^^ the trait `std::clone::Clone` is not implemented for `T`
    |
    = help: consider adding a `where T: std::clone::Clone` bound
-note: required by `Foo4`
-  --> $DIR/defaults-suitability.rs:85:1
-   |
-LL | / trait Foo4<T> where
-LL | |     <Self as Foo4<T>>::Bar: Clone,
-LL | | {
-LL | |     type Bar = Vec<Self::Baz>;
-LL | |     type Baz: Clone = T;
+note: required by `Foo3`
+  --> $DIR/defaults-suitability.rs:90:1
+   |
+LL | / trait Foo3<T> where
+LL | |     Self::Bar: Clone,
+LL | |     Self::Baz: Clone,
 LL | |
+...  |
+LL | |     type Baz = T;
 LL | | }
    | |_^