Merge pull request #269552 from adisbladis/lib-matchattrs-list-allocs

lib.attrsets.matchAttrs: Avoid some list allocations when walking structure

lib/attrsets.nix

@@ -897,7 +897,10 @@ rec {
     recursiveUpdateUntil (path: lhs: rhs: !(isAttrs lhs && isAttrs rhs)) lhs rhs;
 
 
-  /* Returns true if the pattern is contained in the set. False otherwise.
+  /*
+    Recurse into every attribute set of the first argument and check that:
+    - Each attribute path also exists in the second argument.
+    - If the attribute's value is not a nested attribute set, it must have the same value in the right argument.
 
      Example:
        matchAttrs { cpu = {}; } { cpu = { bits = 64; }; }
@@ -909,16 +912,24 @@ rec {
   matchAttrs =
     # Attribute set structure to match
     pattern:
-    # Attribute set to find patterns in
+    # Attribute set to check
     attrs:
     assert isAttrs pattern;
-    all id (attrValues (zipAttrsWithNames (attrNames pattern) (n: values:
+    all
-      let pat = head values; val = elemAt values 1; in
+    ( # Compare equality between `pattern` & `attrs`.
-      if length values == 1 then false
+      attr:
-      else if isAttrs pat then isAttrs val && matchAttrs pat val
+      # Missing attr, not equal.
-      else pat == val
+      attrs ? ${attr} && (
-    ) [pattern attrs]));
+        let
-
+          lhs = pattern.${attr};
+          rhs = attrs.${attr};
+        in
+        # If attrset check recursively
+        if isAttrs lhs then isAttrs rhs && matchAttrs lhs rhs
+        else lhs == rhs
+      )
+    )
+    (attrNames pattern);
 
   /* Override only the attributes that are already present in the old set
     useful for deep-overriding.

lib/tests/misc.nix

@@ -831,6 +831,26 @@ runTests {
     };
   };
 
+  testMatchAttrsMatchingExact = {
+    expr = matchAttrs { cpu = { bits = 64; }; } { cpu = { bits = 64; }; };
+    expected = true;
+  };
+
+  testMatchAttrsMismatch = {
+    expr = matchAttrs { cpu = { bits = 128; }; } { cpu = { bits = 64; }; };
+    expected = false;
+  };
+
+  testMatchAttrsMatchingImplicit = {
+    expr = matchAttrs { cpu = { }; } { cpu = { bits = 64; }; };
+    expected = true;
+  };
+
+  testMatchAttrsMissingAttrs = {
+    expr = matchAttrs { cpu = {}; } { };
+    expected = false;
+  };
+
   testOverrideExistingEmpty = {
     expr = overrideExisting {} { a = 1; };
     expected = {};