Recursive solver: Exit early on ambiguity

If we have an ambiguous solution while going through program clauses, it's not
going to become less ambiguous, so there's no use in continuing to go through
everything.

This gives a 10-20% speedup in rust-analyzer analysis.

Author: flodiebold

chalk-solve/src/recursive.rs

@@ -362,6 +362,11 @@ impl<'me, I: Interner> Solver<'me, I> {
         for program_clause in clauses {
             debug_heading!("clause={:?}", program_clause);
 
+            // If we have a completely ambiguous answer, it's not going to get better, so stop
+            if cur_solution == Some((Solution::Ambig(Guidance::Unknown), ClausePriority::High)) {
+                return (Ok(Solution::Ambig(Guidance::Unknown)), ClausePriority::High);
+            }
+
             match program_clause.data(self.program.interner()) {
                 ProgramClauseData::Implies(implication) => {
                     let res = self.solve_via_implication(
@@ -515,6 +520,12 @@ fn combine_with_priorities<I: Interner>(
     match (prio_a, prio_b, a, b) {
         (ClausePriority::High, ClausePriority::Low, higher, lower)
         | (ClausePriority::Low, ClausePriority::High, lower, higher) => {
+            // if we have a high-priority solution and a low-priority solution,
+            // the high-priority solution overrides *if* they are both for the
+            // same inputs -- we don't want a more specific high-priority
+            // solution overriding a general low-priority one. Currently inputs
+            // only matter for projections; in a goal like `AliasEq(<?0 as
+            // Trait>::Type = ?1)`, ?0 is the input.
             let inputs_higher = calculate_inputs(interner, domain_goal, &higher);
             let inputs_lower = calculate_inputs(interner, domain_goal, &lower);
             if inputs_higher == inputs_lower {