correctly dedup ExistentialPredicates

Author: lcnr

compiler/rustc_middle/src/ty/context.rs

@@ -2419,7 +2419,8 @@ impl<'tcx> TyCtxt<'tcx> {
         eps: &[ExistentialPredicate<'tcx>],
     ) -> &'tcx List<ExistentialPredicate<'tcx>> {
         assert!(!eps.is_empty());
-        assert!(eps.windows(2).all(|w| w[0].stable_cmp(self, &w[1]) != Ordering::Greater));
+        // Do not allow duplicate existential predicates.
+        assert!(eps.windows(2).all(|w| w[0].stable_cmp(self, &w[1]) == Ordering::Less));
         self._intern_existential_predicates(eps)
     }
 

compiler/rustc_middle/src/ty/relate.rs

@@ -598,20 +598,11 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::ExistentialPredicate<'tcx>> {
     ) -> RelateResult<'tcx, Self> {
         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() {
+        if a.len() != b.len() {
             return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
         }
 
-        let v = a_v.into_iter().zip(b_v.into_iter()).map(|(ep_a, ep_b)| {
+        let v = a.into_iter().zip(b.into_iter()).map(|(ep_a, ep_b)| {
             use crate::ty::ExistentialPredicate::*;
             match (ep_a, ep_b) {
                 (Trait(a), Trait(b)) => Ok(Trait(relation.relate(a, b)?)),

compiler/rustc_typeck/src/astconv/mod.rs

@@ -35,6 +35,7 @@ use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 use rustc_trait_selection::traits::wf::object_region_bounds;
 
 use smallvec::SmallVec;
+use std::cmp::Ordering;
 use std::collections::BTreeSet;
 use std::iter;
 use std::slice;
@@ -1192,7 +1193,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             )
             .collect::<SmallVec<[_; 8]>>();
         v.sort_by(|a, b| a.stable_cmp(tcx, b));
-        v.dedup();
+        v.dedup_by(|a, b| a.stable_cmp(tcx, b) == Ordering::Equal);
         let existential_predicates = ty::Binder::bind(tcx.mk_existential_predicates(v.into_iter()));
 
         // Use explicitly-specified region bound.

src/test/ui/associated-types/associated-types-overridden-binding-2.rs

@@ -1,8 +1,8 @@
+// check-pass
 #![feature(trait_alias)]
 
 trait I32Iterator = Iterator<Item = i32>;
 
 fn main() {
     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();
-    //~^ ERROR type mismatch
 }