Don't emit alias-eq goal if projections are structurally equal after resolving vars

Author: compiler-errors

compiler/rustc_infer/src/infer/combine.rs

@@ -128,7 +128,12 @@ impl<'tcx> InferCtxt<'tcx> {
             (_, ty::Alias(AliasKind::Projection, _)) | (ty::Alias(AliasKind::Projection, _), _)
                 if self.tcx.trait_solver_next() =>
             {
-                relation.register_type_relate_obligation(a, b);
+                // The new solver can be somewhat lazy with resolving nested variables,
+                // so we want to avoid emitting a trivial alias-eq goal (which becomes
+                // non-trivial after canonicalization) if the types are identical.
+                if self.resolve_vars_if_possible(a) != self.resolve_vars_if_possible(b) {
+                    relation.register_type_relate_obligation(a, b);
+                }
                 Ok(a)
             }
 

tests/ui/traits/new-solver/iter-filter-projection.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+use std::{iter, slice};
+
+struct Attr;
+
+fn test<'a, T: Iterator<Item = &'a Attr>>() {}
+
+fn main() {
+    test::<iter::Filter<slice::Iter<'_, Attr>, fn(&&Attr) -> bool>>();
+}