perf: pass used_vars subset to get_loop_vars

rather than computing all loop vars and then intersecting,
only consider relevant keys when computing loop vars

reduces get_used_loop_vars from O(n_vars * n_variants) to O(n_used_vars * n_variants)

conda_build/metadata.py

@@ -2778,15 +2778,16 @@ def get_output_metadata_set(
         _check_run_constrained(output_tuples)
         return output_tuples
 
-    def get_loop_vars(self):
-        return get_vars(getattr(self.config, "input_variants", self.config.variants))
+    def get_loop_vars(self, subset=None):
+        return get_vars(
+            getattr(self.config, "input_variants", self.config.variants), subset=subset
+        )
 
     def get_used_loop_vars(self, force_top_level=False, force_global=False):
-        loop_vars = self.get_loop_vars()
         used_vars = self.get_used_vars(
             force_top_level=force_top_level, force_global=force_global
         )
-        return set(loop_vars).intersection(used_vars)
+        return self.get_loop_vars(subset=used_vars)
 
     def get_rendered_recipe_text(
         self, permit_undefined_jinja=False, extract_pattern=None

conda_build/variants.py

@@ -700,7 +700,10 @@ def get_package_variants(recipedir_or_metadata, config=None, variants=None):
     return filter_combined_spec_to_used_keys(combined_spec, specs=specs)
 
 
-def get_vars(variants: Iterable[dict[str, Any]]) -> set[str]:
+def get_vars(
+    variants: Iterable[dict[str, Any]],
+    subset: set[str] | None = None,
+) -> set[str]:
     """For purposes of naming/identifying, provide a way of identifying which variables contribute
     to the matrix dimensionality"""
     first, *others = variants
@@ -710,10 +713,12 @@ def get_vars(variants: Iterable[dict[str, Any]]) -> set[str]:
         "ignore_version",
         *ensure_list(first.get("extend_keys")),
     }
+    to_consider = set(first)
+    if subset:
+        to_consider.intersection_update(subset)
+    to_consider.difference_update(special_keys)
     return {
-        var
-        for var in set(first) - special_keys
-        if any(first[var] != other[var] for other in others)
+        var for var in to_consider if any(first[var] != other[var] for other in others)
     }