Don't constrain projection predicates with inference vars in GAT substs

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2471,7 +2471,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     // `T`
                     substs: self.tcx.mk_substs_trait(
                         trait_pred.self_ty().skip_binder(),
-                        self.fresh_substs_for_item(span, item_def_id),
+                        &self.fresh_substs_for_item(span, item_def_id)[1..],
                     ),
                     // `Future::Output`
                     item_def_id,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1073,6 +1073,16 @@ fn project<'cx, 'tcx>(
         return Ok(Projected::Progress(Progress::error(selcx.tcx())));
     }
 
+    // If the obligation contains any inference types or consts in associated
+    // type substs, then we don't assemble any candidates.
+    // This isn't really correct, but otherwise we can end up in a case where
+    // we constrain inference variables by selecting a single predicate, when
+    // we need to stay general. See issue #91762.
+    let (_, predicate_own_substs) = obligation.predicate.trait_ref_and_own_substs(selcx.tcx());
+    if predicate_own_substs.iter().any(|g| g.has_infer_types_or_consts()) {
+        return Err(ProjectionError::TooManyCandidates);
+    }
+
     let mut candidates = ProjectionCandidateSet::None;
 
     // Make sure that the following procedures are kept in order. ParamEnv

src/test/ui/generic-associated-types/issue-74824.rs

@@ -17,6 +17,7 @@ impl<T> UnsafeCopy for T {}
 fn main() {
     let b = Box::new(42usize);
     let copy = <()>::copy(&b);
+    //~^ type annotations needed
 
     let raw_b = Box::deref(&b) as *const _;
     let raw_copy = Box::deref(&copy) as *const _;

src/test/ui/generic-associated-types/issue-74824.stderr

@@ -27,6 +27,13 @@ help: consider restricting type parameter `T`
 LL |     type Copy<T: std::clone::Clone>: Copy = Box<T>;
    |                +++++++++++++++++++
 
-error: aborting due to 2 previous errors
+error[E0282]: type annotations needed
+  --> $DIR/issue-74824.rs:19:16
+   |
+LL |     let copy = <()>::copy(&b);
+   |                ^^^^^^^^^^ cannot infer type for type parameter `T` declared on the associated function `copy`
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0282.
+For more information about an error, try `rustc --explain E0277`.

src/test/ui/generic-associated-types/issue-91762.rs

@@ -0,0 +1,30 @@
+// check-fail
+
+// FIXME(generic_associated_types): We almost certaintly want this to pass, but
+// it's particularly difficult currently, because we need a way of specifying
+// that `<Self::Base as Functor>::With<T> = Self` without using that when we have
+// a `U`. See `https://github.com/rust-lang/rust/pull/92728` for a (hacky)
+// solution. This might be better to just wait for Chalk.
+
+#![feature(generic_associated_types)]
+
+pub trait Functor {
+    type With<T>;
+
+    fn fmap<T, U>(this: Self::With<T>) -> Self::With<U>;
+}
+
+pub trait FunctorExt<T>: Sized {
+    type Base: Functor<With<T> = Self>;
+
+    fn fmap<U>(self) {
+        let arg: <Self::Base as Functor>::With<T>;
+        let ret: <Self::Base as Functor>::With<U>;
+
+        arg = self;
+        ret = <Self::Base as Functor>::fmap(arg);
+        //~^ type annotations needed
+    }
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-91762.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-91762.rs:25:15
+   |
+LL |         ret = <Self::Base as Functor>::fmap(arg);
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the associated function `fmap`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.