Prefer candidates with no external constrains in new solver

Author: compiler-errors

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -1,5 +1,7 @@
 //! Code shared by trait and projection goals for candidate assembly.
 
+use crate::solve::CanonicalResponseExt;
+
 #[cfg(doc)]
 use super::trait_goals::structural_traits::*;
 use super::{EvalCtxt, SolverMode};
@@ -18,7 +20,7 @@ use std::fmt::Debug;
 ///
 /// It consists of both the `source`, which describes how that goal would be proven,
 /// and the `result` when using the given `source`.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Copy)]
 pub(super) struct Candidate<'tcx> {
     pub(super) source: CandidateSource,
     pub(super) result: CanonicalResponse<'tcx>,
@@ -510,12 +512,19 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         &mut self,
         mut candidates: Vec<Candidate<'tcx>>,
     ) -> QueryResult<'tcx> {
-        match candidates.len() {
-            0 => return Err(NoSolution),
-            1 => return Ok(candidates.pop().unwrap().result),
+        match &candidates[..] {
+            [] => return Err(NoSolution),
+            [candidate] => return Ok(candidate.result),
             _ => {}
         }
 
+        if let Some(candidate) = candidates.iter().find(|candidate| {
+            candidate.result.value.certainty == Certainty::Yes
+                && candidate.result.has_no_inference_or_external_constraints()
+        }) {
+            return Ok(candidate.result);
+        }
+
         if candidates.len() > 1 {
             let mut i = 0;
             'outer: while i < candidates.len() {

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -11,6 +11,7 @@
 
 // FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
 
+use itertools::Itertools;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::canonical::{Canonical, CanonicalVarValues};
 use rustc_infer::traits::query::NoSolution;
@@ -300,9 +301,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         // FIXME(-Ztrait-solver=next): We should instead try to find a `Certainty::Yes` response with
         // a subset of the constraints that all the other responses have.
-        let one = candidates[0];
-        if candidates[1..].iter().all(|resp| resp == &one) {
-            return Ok(one);
+        if candidates.iter().all_equal() {
+            return Ok(candidates[0]);
         }
 
         if let Some(response) = candidates.iter().find(|response| {

tests/ui/traits/new-solver/prefer-candidate-no-constraints.rs

@@ -0,0 +1,22 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+trait Foo {}
+
+impl<T> Foo for T {}
+
+trait Bar {}
+
+struct Wrapper<'a, T>(&'a T);
+
+impl<'a, T> Bar for Wrapper<'a, T> where &'a T: Foo {}
+// We need to satisfy `&'a T: Foo` when checking that this impl is WF
+// that can either be satisfied via the param-env, or via an impl.
+//
+// When satisfied via the param-env, since each lifetime is canonicalized
+// separately, we end up getting extra region constraints.
+//
+// However, when satisfied via the impl, there are no region constraints,
+// and we can short-circuit a response with no external constraints.
+
+fn main() {}