trait_sel: don't require predicates of aliases hold

No longer require that we prove that the predicates of aliases hold when
checking the well-formedness of the alias. This permits more uses of GATs
and changes the output of yet more tests.

Author: davidtwco

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -144,8 +144,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
         idx: usize,
     ) -> Result<PredicateObligations<'tcx>, SelectionError<'tcx>> {
-        let tcx = self.tcx();
-
         let placeholder_trait_predicate =
             self.infcx.enter_forall_and_leak_universe(obligation.predicate).trait_ref;
         let placeholder_self_ty = self.infcx.shallow_resolve(placeholder_trait_predicate.self_ty());
@@ -194,28 +192,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .map_err(|_| SelectionError::Unimplemented)?,
         );
 
-        // FIXME(compiler-errors): I don't think this is needed.
-        if let ty::Alias(ty::Projection, alias_ty) = placeholder_self_ty.kind() {
-            let predicates = tcx.predicates_of(alias_ty.def_id).instantiate_own(tcx, alias_ty.args);
-            for (predicate, _) in predicates {
-                let normalized = normalize_with_depth_to(
-                    self,
-                    obligation.param_env,
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    predicate,
-                    &mut obligations,
-                );
-                obligations.push(Obligation::with_depth(
-                    self.tcx(),
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    obligation.param_env,
-                    normalized,
-                ));
-            }
-        }
-
         Ok(obligations)
     }
 

tests/ui/dyn-compatibility/assoc_type_bounds_sized_used.rs

@@ -9,7 +9,8 @@ trait Bop {
 
 fn bop<T: Bop + ?Sized>() {
     let _ = <T as Bop>::Bar::default();
-    //~^ ERROR: trait bounds were not satisfied
+    //~^ ERROR: the size for values of type `T` cannot be known at compilation time
+    //~| ERROR: the size for values of type `T` cannot be known at compilation time
     //~| ERROR: the size for values of type `T` cannot be known at compilation time
 }
 

tests/ui/dyn-compatibility/assoc_type_bounds_sized_used.stderr

@@ -24,21 +24,58 @@ help: consider relaxing the implicit `Sized` restriction
 LL |     type Bar: Default + ?Sized
    |                       ++++++++
 
-error[E0599]: the function or associated item `default` exists for associated type `<T as Bop>::Bar`, but its trait bounds were not satisfied
-  --> $DIR/assoc_type_bounds_sized_used.rs:11:30
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/assoc_type_bounds_sized_used.rs:11:13
    |
+LL | fn bop<T: Bop + ?Sized>() {
+   |        - this type parameter needs to be `Sized`
 LL |     let _ = <T as Bop>::Bar::default();
-   |                              ^^^^^^^ function or associated item cannot be called on `<T as Bop>::Bar` due to unsatisfied trait bounds
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+note: required by a bound in `Bop::Bar`
+  --> $DIR/assoc_type_bounds_sized_used.rs:7:15
+   |
+LL |     type Bar: Default
+   |          --- required by a bound in this associated type
+LL |     where
+LL |         Self: Sized;
+   |               ^^^^^ required by this bound in `Bop::Bar`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn bop<T: Bop + ?Sized>() {
+LL + fn bop<T: Bop>() {
    |
-   = note: the following trait bounds were not satisfied:
-           `T: Sized`
-           which is required by `<T as Bop>::Bar: Default`
-help: consider restricting the type parameter to satisfy the trait bound
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Bar: Default + ?Sized
+   |                       ++++++++
+
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/assoc_type_bounds_sized_used.rs:11:13
    |
-LL | fn bop<T: Bop + ?Sized>() where T: Sized {
-   |                           ++++++++++++++
+LL | fn bop<T: Bop + ?Sized>() {
+   |        - this type parameter needs to be `Sized`
+LL |     let _ = <T as Bop>::Bar::default();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+note: required by a bound in `Bop::Bar`
+  --> $DIR/assoc_type_bounds_sized_used.rs:7:15
+   |
+LL |     type Bar: Default
+   |          --- required by a bound in this associated type
+LL |     where
+LL |         Self: Sized;
+   |               ^^^^^ required by this bound in `Bop::Bar`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn bop<T: Bop + ?Sized>() {
+LL + fn bop<T: Bop>() {
+   |
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     type Bar: Default + ?Sized
+   |                       ++++++++
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0277, E0599.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/generic-associated-types/bugs/issue-100013.stderr

@@ -1,25 +1,3 @@
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:15:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'_, '_>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:22:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
 error: lifetime may not live long enough
   --> $DIR/issue-100013.rs:23:17
    |
@@ -33,16 +11,5 @@ LL |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
    |
    = help: consider adding the following bound: `'a: 'b`
 
-error: lifetime bound not satisfied
-  --> $DIR/issue-100013.rs:29:5
-   |
-LL | /     async { // a coroutine checked for autotrait impl `Send`
-LL | |         let x = None::<I::Future<'a, 'b>>; // a type referencing GAT
-LL | |         async {}.await; // a yield point
-LL | |     }
-   | |_____^
-   |
-   = note: this is a known limitation that will be removed in the future (see issue #100013 <https://github.com/rust-lang/rust/issues/100013> for more information)
-
-error: aborting due to 4 previous errors
+error: aborting due to 1 previous error
 

tests/ui/generic-associated-types/issue-92096.rs

@@ -1,3 +1,4 @@
+//@ check-pass
 //@ edition:2018
 
 use std::future::Future;
@@ -15,14 +16,6 @@ where
     C: Client + Send + Sync,
 {
     async move { c.connect().await }
-    //~^ ERROR `C` does not live long enough
-    //
-    // FIXME(#71723). This is because we infer at some point a value of
-    //
-    // impl Future<Output = <C as Client>::Connection<'_>>
-    //
-    // and then we somehow fail the WF check because `where C: 'a` is not known,
-    // but I'm not entirely sure how that comes about.
 }
 
 fn main() {}

tests/ui/generic-associated-types/issue-92096.stderr

@@ -1,4 +1,4 @@
-//@ check-fail
+//@ check-pass
 
 pub trait Trait {
     type Assoc<'a> where Self: 'a;
@@ -11,7 +11,7 @@ where
 
 pub struct Type;
 
-impl<T: Trait> Foo<T> for Type //~ ERROR: the parameter type `T` may not live long enough
+impl<T: Trait> Foo<T> for Type
 where
     for<'a> T::Assoc<'a>: Clone
 {}

tests/ui/generic-associated-types/issue-93262.rs

@@ -0,0 +1,21 @@
+//@ check-pass
+//@ compile-flags: --crate-type=lib
+//@ revisions: old next
+//@[next] compile-flags: -Znext-solver
+
+use std::marker::PhantomData;
+
+pub trait ZeroMapKV<'a> {
+    type Container;
+}
+
+pub trait ZeroFrom<'zf, C: ?Sized> {}
+
+pub struct ZeroMap<'a, K: ZeroMapKV<'a>>(PhantomData<&'a K>);
+
+impl<'zf, 's, K> ZeroFrom<'zf, ZeroMap<'s, K>> for ZeroMap<'zf, K>
+where
+    K: for<'b> ZeroMapKV<'b>,
+    <K as ZeroMapKV<'zf>>::Container: ZeroFrom<'zf, <K as ZeroMapKV<'s>>::Container>,
+{
+}