lib.meta: Avoid attrset allocation in platformMatch

Benchmarks (`nix-instantiate ./. -A python3`)

- Before
``` json
{
  "cpuTime": 0.30625399947166443,
  "envs": {
    "bytes": 4484216,
    "elements": 221443,
    "number": 169542
  },
  "gc": {
    "heapSize": 402915328,
    "totalBytes": 53091024
  },
  "list": {
    "bytes": 749424,
    "concats": 4242,
    "elements": 93678
  },
  "nrAvoided": 253991,
  "nrFunctionCalls": 149848,
  "nrLookups": 49614,
  "nrOpUpdateValuesCopied": 1588326,
  "nrOpUpdates": 10106,
  "nrPrimOpCalls": 130356,
  "nrThunks": 359013,
  "sets": {
    "bytes": 30432320,
    "elements": 1860540,
    "number": 41480
  },
  "sizes": {
    "Attr": 16,
    "Bindings": 16,
    "Env": 16,
    "Value": 24
  },
  "symbols": {
    "bytes": 236218,
    "number": 24459
  },
  "values": {
    "bytes": 10504632,
    "number": 437693
  }
}
```

- After
```
{
  "cpuTime": 0.29695799946784973,
  "envs": {
    "bytes": 3296712,
    "elements": 169055,
    "number": 121517
  },
  "gc": {
    "heapSize": 402915328,
    "totalBytes": 49044992
  },
  "list": {
    "bytes": 504928,
    "concats": 4242,
    "elements": 63116
  },
  "nrAvoided": 175403,
  "nrFunctionCalls": 110554,
  "nrLookups": 44907,
  "nrOpUpdateValuesCopied": 1588326,
  "nrOpUpdates": 10106,
  "nrPrimOpCalls": 82330,
  "nrThunks": 306625,
  "sets": {
    "bytes": 29943328,
    "elements": 1843076,
    "number": 28382
  },
  "sizes": {
    "Attr": 16,
    "Bindings": 16,
    "Env": 16,
    "Value": 24
  },
  "symbols": {
    "bytes": 236218,
    "number": 24459
  },
  "values": {
    "bytes": 9037752,
    "number": 376573
  }
}
```

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index 06cb5e763e2c2..8f4a37149d92c 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -1948,4 +1948,24 @@ runTests {
   testGetExe'FailureSecondArg = testingThrow (
     getExe' { type = "derivation"; } "dir/executable"
   );
+
+  testPlatformMatch = {
+    expr = meta.platformMatch { system = "x86_64-linux"; } "x86_64-linux";
+    expected = true;
+  };
+
+  testPlatformMatchAttrs = {
+    expr = meta.platformMatch (systems.elaborate "x86_64-linux") (systems.elaborate "x86_64-linux").parsed;
+    expected = true;
+  };
+
+  testPlatformMatchNoMatch = {
+    expr = meta.platformMatch { system = "x86_64-darwin"; } "x86_64-linux";
+    expected = false;
+  };
+
+  testPlatformMatchMissingSystem = {
+    expr = meta.platformMatch { } "x86_64-linux";
+    expected = false;
+  };
 }