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
  }
}
```

lib/meta.nix

@@ -3,6 +3,11 @@
 
 { lib }:
 
+let
+  inherit (lib) matchAttrs any all;
+  inherit (builtins) isString;
+
+in
 rec {
 
 
@@ -83,14 +88,21 @@ rec {
      We can inject these into a pattern for the whole of a structured platform,
      and then match that.
   */
-  platformMatch = platform: elem: let
-      pattern =
-        if builtins.isString elem
-        then { system = elem; }
-        else if elem?parsed
-        then elem
-        else { parsed = elem; };
-    in lib.matchAttrs pattern platform;
+  platformMatch = platform: elem: (
+    # Check with simple string comparison if elem was a string.
+    #
+    # The majority of comparisons done with this function will be against meta.platforms
+    # which contains a simple platform string.
+    #
+    # Avoiding an attrset allocation results in significant  performance gains (~2-30) across the board in OfBorg
+    # because this is a hot path for nixpkgs.
+    if isString elem then platform ? system && elem == platform.system
+    else matchAttrs (
+      # Normalize platform attrset.
+      if elem ? parsed then elem
+      else { parsed = elem; }
+    ) platform
+  );
 
   /* Check if a package is available on a given platform.
 
@@ -102,8 +114,8 @@ rec {
        2. None of `meta.badPlatforms` pattern matches the given platform.
   */
   availableOn = platform: pkg:
-    ((!pkg?meta.platforms) || lib.any (platformMatch platform) pkg.meta.platforms) &&
-    lib.all (elem: !platformMatch platform elem) (pkg.meta.badPlatforms or []);
+    ((!pkg?meta.platforms) || any (platformMatch platform) pkg.meta.platforms) &&
+    all (elem: !platformMatch platform elem) (pkg.meta.badPlatforms or []);
 
   /* Get the corresponding attribute in lib.licenses
      from the SPDX ID.

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;
+  };
 }