Speed up extracting default package pname

In order to speed up evalutation of flakes setting `packages.default`,
this trys to compute the pname in multiple ways, ordered by speed. The
pname can be set manually, which is fastest. To avoid computing a new
nixpkgs and flakelight overlays, the second option just uses callPackage
from stock nixpkgs. If that fails, the prior, most accurate, method is
used which creates a new nixpkgs with all the overlays available.

API_GUIDE.md

@@ -288,6 +288,12 @@ and have build checks in `checks.${system}`.
 
 `packages` can be set to attrs of package definitions.
 
+By default, the `packages.default` package's name (its attribute name in
+the package set and overlay) is automatically determined from the derivation's
+`pname`. In order to use a different attribute name from the package pname,
+to set it in cases where it cannot be automatically determined, or to speed up
+uncached evaluation, the flakelight `pname` option can be set.
+
 To set the default package, you can set the options as follows:
 
 ```nix
@@ -347,6 +353,25 @@ The above will export `packages.${system}.default`, `packages.${system}.pkg2`,
 `packages.${system}.pkg3` attributes, add `pkg1`, `pkg2`, and `pkg3` to
 `overlays.default`, and export corresponding build checks.
 
+To use the first example, but manually specify the package name:
+
+```nix
+{
+  inputs.flakelight.url = "github:nix-community/flakelight";
+  outputs = { flakelight, ... }:
+    flakelight ./. {
+      pname = "pkgs-attribute-name";
+      package = { stdenv }:
+        stdenv.mkDerivation {
+          pname = "package-name";
+          version = "0.0.1";
+          src = ./.;
+          installPhase = "make DESTDIR=$out install";
+        };
+    };
+}
+```
+
 ### devShell
 
 The devshell options allow you to configure `devShells.${system}.default`. It is

builtinModules/packages.nix

@@ -4,10 +4,10 @@
 
 { config, lib, inputs, flakelight, genSystems, ... }:
 let
-  inherit (builtins) parseDrvName;
+  inherit (builtins) parseDrvName tryEval;
-  inherit (lib) filterAttrs mapAttrs mapAttrs' mkIf mkMerge mkOption
+  inherit (lib) filterAttrs findFirst mapAttrs mapAttrs' mkIf mkMerge
-    nameValuePair optionalAttrs;
+    mkOption nameValuePair optionalAttrs;
-  inherit (lib.types) lazyAttrsOf nullOr uniq;
+  inherit (lib.types) lazyAttrsOf nullOr str uniq;
   inherit (flakelight) supportedSystem;
   inherit (flakelight.types) overlay packageDef;
 
@@ -26,6 +26,11 @@ in
       default = { };
     };
 
+    pname = mkOption {
+      type = nullOr str;
+      default = null;
+    };
+
     packageOverlay = mkOption {
       internal = true;
       type = uniq overlay;
@@ -42,11 +47,21 @@ in
       packageOverlay = final: prev:
         let
           getName = pkg: pkg.pname or (parseDrvName pkg.name).name;
-          defaultPkgName = getName (import inputs.nixpkgs {
+          inherit (prev.stdenv.hostPlatform) system;
-            inherit (prev.stdenv.hostPlatform) system;
+          baseNixpkgs = inputs.nixpkgs.legacyPackages.${system};
-            inherit (config.nixpkgs) config;
+
-            overlays = config.withOverlays ++ [ (final: _: genPkgs final) ];
+          defaultPkgName = findFirst (x: (tryEval x).success)
-          }).default;
+            (throw ("Could not determine the name of the default package; " +
+              "please set the `pname` flakelight option to the intended name."))
+            [
+              (assert config.pname != null; config.pname)
+              (getName (baseNixpkgs.callPackage config.packages.default { }))
+              (getName (import inputs.nixpkgs {
+                inherit (prev.stdenv.hostPlatform) system;
+                inherit (config.nixpkgs) config;
+                overlays = config.withOverlays ++ [ (final: _: genPkgs final) ];
+              }).default)
+            ];
         in
         (optionalAttrs (config.packages ? default) {
           ${defaultPkgName} = genPkg final config.packages.default;