Add type-checker performance tracking test for #43369

To make this test work, fix an issue in `ConstraintSystem::salvage` where a
threshold breach during solving went unnoticed due to exiting on ambiguity
before reaching the `isTooComplex` check. Address this by moving the
`isTooComplex` check to before we start processing solutions, and stick another
one in `findBestSolution` for short-circuiting while we're here.

Author: AnthonyLatsis

lib/Sema/CSRanking.cpp

@@ -1405,6 +1405,10 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
 Optional<unsigned>
 ConstraintSystem::findBestSolution(SmallVectorImpl<Solution> &viable,
                                    bool minimize) {
+  // Don't spend time filtering solutions if we already hit a threshold.
+  if (isTooComplex(viable))
+    return None;
+
   if (viable.empty())
     return None;
   if (viable.size() == 1)

lib/Sema/ConstraintSystem.cpp

@@ -3828,6 +3828,10 @@ SolutionResult ConstraintSystem::salvage() {
     // Solve the system.
     solveImpl(viable);
 
+    // If we hit a threshold, we're done.
+    if (isTooComplex(viable))
+      return SolutionResult::forTooComplex(getTooComplexRange());
+
     // Before removing any "fixed" solutions, let's check
     // if ambiguity is caused by fixes and diagnose if possible.
     if (diagnoseAmbiguityWithFixes(viable))
@@ -3873,9 +3877,6 @@ SolutionResult ConstraintSystem::salvage() {
     // Fall through to produce diagnostics.
   }
 
-  if (isTooComplex(viable))
-    return SolutionResult::forTooComplex(getTooComplexRange());
-
   // Could not produce a specific diagnostic; punt to the client.
   return SolutionResult::forUndiagnosedError();
 }

validation-test/Sema/type_checker_perf/slow/issue43369.swift

@@ -0,0 +1,23 @@
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=5
+// REQUIRES: tools-release,no_asan
+
+// Hits the default memory threshold
+// https://github.com/apple/swift/issues/43369
+
+struct A {}
+struct B {}
+
+func - (lhs: A, rhs: A) -> B {}
+func / (lhs: B, rhs: Double) -> B {}
+func - (lhs: B, rhs: B) -> B {}
+
+do {
+  let a0 = A()
+  let a1 = A()
+
+  let t0 = 0
+  let t1 = 0
+
+  // expected-error@+1 {{the compiler is unable to type-check this expression in reasonable time}}
+  let _ = (a0 - a0) / (t0 - t0) - (a1 - a1) / (t1 - t1)
+}