Allow all of devShells config types in devShells

`devShell` accepted either a package def, a submodule value, or a
function to a submodule value. `devShells` only accepted package defs.
This brings all the options of `devShells` to any devShell configured
with `devShells`.

API_GUIDE.md

@@ -541,12 +541,36 @@ To override the devShell, you can use a package definition as such:
 ### devShells
 
 ```
-Type: (AttrsOf PackageDef) | (ModuleArgs -> (AttrsOf PackageDef))
+Type:
+  devShells: (AttrsOf (PackageDef | Cfg | (Pkgs -> Cfg)) |
+             (ModuleArgs -> (AttrsOf (PackageDef | Cfg | (Pkgs -> Cfg))))
+  Cfg.packages: [Derivation] | (Pkgs -> [Derivation])
+  Cfg.inputsFrom: [Derivation] | (Pkgs -> [Derivation])
+  Cfg.shellHook: Str | (Pkgs -> Str)
+  Cfg.env: (AttrsOf Str) | (Pkgs -> (AttrsOf Str))
+  Cfg.stdenv: Stdenv | (Pkgs -> Stdenv)
 ```
 
-The `devShells` option allows you to set additional `devShell` outputs.
+The `devShells` option allows you to set additional `devShell` outputs. The
+values each shell can be set to are the same as described above for the
+`devShell` option.
 
-For example:
+For example, using the configuration options:
+
+```nix
+{
+  inputs.flakelight.url = "github:nix-community/flakelight";
+  outputs = { flakelight, ... }:
+    flakelight ./. {
+      devShells.testing = {
+        packages = pkgs: [ pkgs.coreutils ];
+        env.TEST_VAR = "in testing shell";
+      };
+    };
+}
+```
+
+For example, using a package definition:
 
 ```nix
 {
@@ -562,7 +586,7 @@ For example:
 }
 ```
 
-The above exports `devShells.${system}.testing` outputs.
+The above flakes export `devShells.${system}.testing` outputs.
 
 ### overlays
 

builtinModules/devShells.nix

@@ -4,13 +4,11 @@
 
 { config, lib, flakelight, genSystems, moduleArgs, ... }:
 let
-  inherit (lib) filterAttrs functionArgs mapAttrs mkDefault mkIf mkMerge
-    mkOption;
+  inherit (lib) filterAttrs functionArgs mapAttrs mkIf mkMerge mkOption;
   inherit (lib.types) attrs coercedTo functionTo lazyAttrsOf lines listOf
     package str submodule;
   inherit (flakelight) supportedSystem;
-  inherit (flakelight.types) function nullable optCallWith optFunctionTo
-    packageDef;
+  inherit (flakelight.types) function nullable optCallWith optFunctionTo;
 
   devShellModule.options = {
     inputsFrom = mkOption {
@@ -39,48 +37,51 @@ let
     };
 
     overrideShell = mkOption {
-      type = nullable packageDef;
+      type = nullable package;
       internal = true;
       default = null;
     };
   };
 
   wrapFn = fn: pkgs:
-    if (functionArgs fn == { }) || !(package.check (pkgs.callPackage fn { }))
+    let val = pkgs.callPackage fn { }; in
+    if (functionArgs fn == { }) || !(package.check val)
     then fn pkgs
-    else { overrideShell = fn; };
+    else { overrideShell = val; };
+
+  devShellType = coercedTo function wrapFn
+    (coercedTo attrs (x: _: x)
+      (functionTo (submodule devShellModule)));
+
+  genDevShell = pkgs: cfg:
+    if cfg.overrideShell != null then cfg.overrideShell
+    else
+      let cfg' = mapAttrs (_: v: v pkgs) cfg; in
+      pkgs.mkShell.override { inherit (cfg') stdenv; }
+        (cfg'.env // { inherit (cfg') inputsFrom packages shellHook; });
 in
 {
   options = {
     devShell = mkOption {
       default = null;
-      type = nullable (coercedTo function wrapFn
-        (coercedTo attrs (x: _: x)
-          (functionTo (submodule devShellModule))));
+      type = nullable devShellType;
     };
 
     devShells = mkOption {
-      type = optCallWith moduleArgs (lazyAttrsOf packageDef);
+      type = optCallWith moduleArgs (lazyAttrsOf devShellType);
       default = { };
     };
   };
 
   config = mkMerge [
     (mkIf (config.devShell != null) {
-      devShells.default = mkDefault ({ pkgs, mkShell }:
-        let cfg = mapAttrs (_: v: v pkgs) (config.devShell pkgs); in
-        mkShell.override { inherit (cfg) stdenv; }
-          (cfg.env // { inherit (cfg) inputsFrom packages shellHook; }));
-    })
-
-    (mkIf (config.devShell.overrideShell or null != null) {
-      devShells.default = config.devShell.overrideShell;
+      devShells.default = config.devShell;
     })
 
     (mkIf (config.devShells != { }) {
       outputs.devShells = genSystems (pkgs:
         filterAttrs (_: supportedSystem pkgs)
-          (mapAttrs (_: v: pkgs.callPackage v { }) config.devShells));
+          (mapAttrs (_: v: genDevShell pkgs (v pkgs)) config.devShells));
     })
   ];
 }

default.nix

@@ -94,8 +94,7 @@ let
     optFunctionTo = elemType: coercedTo elemType (x: _: x)
       (functionTo elemType);
 
-    optCallWith = args: elemType: coercedTo (functionTo elemType) (x: x args)
-      elemType;
+    optCallWith = args: elemType: coercedTo function (x: x args) elemType;
 
     nullable = elemType: mkOptionType {
       name = "nullable";

tests/default.nix

@@ -370,12 +370,16 @@ in
       devShell.inputsFrom = pkgs: [ pkgs.emacs ];
       devShells = {
         shell1 = { mkShell }: mkShell { };
-        shell2 = { mkShell }: mkShell { };
+        shell2 = { packages = pkgs: [ pkgs.emacs ]; };
+        shell3 = pkgs: { packages = [ pkgs.emacs ]; };
+        shell4 = { emacs, ... }: { packages = [ emacs ]; };
       };
     })
-    (f: (f ? devShells.x86_64-linux.default)
-      && (f ? devShells.x86_64-linux.shell1)
-      && (f ? devShells.x86_64-linux.shell2));
+    (f: (lib.isDerivation f.devShells.x86_64-linux.default)
+      && (lib.isDerivation f.devShells.x86_64-linux.shell1)
+      && (lib.isDerivation f.devShells.x86_64-linux.shell2)
+      && (lib.isDerivation f.devShells.x86_64-linux.shell3)
+      && (lib.isDerivation f.devShells.x86_64-linux.shell4));
 
   devShells-override = test
     (flakelight ./empty {