Resolve direct and pinned requirements first

Author: uranusjr

news/9185.feature.rst

@@ -0,0 +1,2 @@
+New resolver: Resolve direct and pinned (``==`` or ``===``) requirements first
+to improve resolver performance.

src/pip/_internal/resolution/resolvelib/provider.py

@@ -56,9 +56,33 @@ def get_preference(
         information  # type: Sequence[Tuple[Requirement, Candidate]]
     ):
         # type: (...) -> Any
+        """Produce a sort key for given requirement based on preference.
+
+        The lower the return value is, the more preferred this group of
+        arguments is.
+
+        Currently pip considers the followings in order:
+
+        * Prefer if any of the known requirements points to an explicit URL.
+        * If equal, prefer if any requirements contain `===` and `==`.
+        * If equal, prefer user-specified (non-transitive) requirements.
+        * If equal, order alphabetically for consistency (helps debuggability).
+        """
+
+        def _get_restrictive_rating(requirements):
+            # type: (Iterable[Requirement]) -> int
+            lookups = (r.get_candidate_lookup() for r in requirements)
+            cands, ireqs = zip(*lookups)
+            if any(cand is not None for cand in cands):
+                return 0
+            if any(ireq.is_pinned for ireq in ireqs if ireq is not None):
+                return 1
+            return 2
+
+        restrictive = _get_restrictive_rating(req for req, _ in information)
         transitive = all(parent is not None for _, parent in information)
         key = next(iter(candidates)).name if candidates else ""
-        return (transitive, key)
+        return (restrictive, transitive, key)
 
     def find_matches(self, requirements):
         # type: (Sequence[Requirement]) -> Iterable[Candidate]