Perform obligation deduplication to avoid buggy ExistentialMismatch

Fix #59326.

Author: estebank

src/librustc_middle/ty/relate.rs

@@ -617,12 +617,22 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
         a: &Self,
         b: &Self,
     ) -> RelateResult<'tcx, Self> {
-        if a.len() != b.len() {
+        let tcx = relation.tcx();
+
+        // FIXME: this is wasteful, but want to do a perf run to see how slow it is.
+        // We need to perform this deduplication as we sometimes generate duplicate projections
+        // in `a`.
+        let mut a_v: Vec<_> = a.into_iter().collect();
+        let mut b_v: Vec<_> = b.into_iter().collect();
+        a_v.sort_by(|a, b| a.stable_cmp(tcx, b));
+        a_v.dedup();
+        b_v.sort_by(|a, b| a.stable_cmp(tcx, b));
+        b_v.dedup();
+        if a_v.len() != b_v.len() {
             return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
         }
 
-        let tcx = relation.tcx();
-        let v = a.iter().zip(b.iter()).map(|(ep_a, ep_b)| {
+        let v = a_v.into_iter().zip(b_v.into_iter()).map(|(ep_a, ep_b)| {
             use crate::ty::ExistentialPredicate::*;
             match (ep_a, ep_b) {
                 (Trait(ref a), Trait(ref b)) => Ok(Trait(relation.relate(a, b)?)),

src/test/ui/issues/issue-59326.rs

@@ -0,0 +1,26 @@
+// check-pass
+trait Service {
+    type S;
+}
+
+trait Framing {
+    type F;
+}
+
+impl Framing for () {
+    type F = ();
+}
+
+trait HttpService<F: Framing>: Service<S = F::F> {}
+
+type BoxService = Box<dyn HttpService<(), S = ()>>;
+
+fn build_server<F: FnOnce() -> BoxService>(_: F) {}
+
+fn make_server<F: Framing>() -> Box<dyn HttpService<F, S = F::F>> {
+    unimplemented!()
+}
+
+fn main() {
+    build_server(|| make_server())
+}