mkDerivation: add toDevShell passthru (EEP 0001)

default.nix

@@ -145,7 +145,12 @@ let
 
   config =
     lib.asserts.checkAssertWarn configEval.config.assertions configEval.config.warnings
-      configEval.config;
+      configEval.config
+    // {
+      # Injected lazily for toDevShell passthru — only forced when a user
+      # actually calls drv.toDevShell, at which point pkgs is fully resolved.
+      mkDevShell = pkgs.mkDevShell;
+    };
 
   # A few packages make a new package set to draw their dependencies from.
   # (Currently to get a cross tool chain, or forced-i686 package.) Rather than

dev-shell/default.nix

@@ -102,9 +102,16 @@ let
         name = "dev-shell";
         phases = [ "buildPhase" ];
         buildPhase = ''
-          echo "This derivation is not meant to be built, only to be used with nix-shell"
-          touch $out
+          { echo "------------------------------------------------------------";
+            echo " WARNING: the existence of this path is not guaranteed.";
+            echo " It is an internal implementation detail for mkDevShell.";
+            echo "------------------------------------------------------------";
+            echo;
+            # Record all build inputs as runtime dependencies
+            export;
+          } >> "$out"
         '';
+        preferLocalBuild = true;
         shellHook = "";
       }
       // attrs
@@ -116,13 +123,25 @@ in
   # Main function: Create a development shell with services
   mkDevShell =
     {
+      # Shell name
+      name ? "dev-shell",
+
       # Service and language configuration via modules
       modules ? [ ],
 
       # Traditional mkShell options
       packages ? [ ],
       shellHook ? "",
       buildInputs ? [ ],
+      nativeBuildInputs ? [ ],
+      propagatedBuildInputs ? [ ],
+      propagatedNativeBuildInputs ? [ ],
+
+      # Propagate all inputs from the given derivations
+      inputsFrom ? [ ],
+
+      # Environment variables to set in the shell (attrset of strings)
+      env ? { },
 
       # process-compose specific options
       processCompose ? {
@@ -189,9 +208,39 @@ in
         ) langVariables
       );
 
+      # Merge inputs from inputsFrom derivations (same logic as mkShell/to-dev-shell)
+      mergeInputs =
+        attr:
+        (args.${attr} or [ ])
+        ++ (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs attr inputsFrom)));
+
+      mergedBuildInputs = mergeInputs "buildInputs";
+      mergedNativeBuildInputs = mergeInputs "nativeBuildInputs";
+      mergedPropagatedBuildInputs = mergeInputs "propagatedBuildInputs";
+      mergedPropagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
+
+      # Merge shellHooks from inputsFrom
+      inputsFromShellHook = lib.concatStringsSep "\n" (
+        lib.catAttrs "shellHook" (lib.reverseList inputsFrom)
+      );
+
+      # Generate export statements for env variables
+      envExports = lib.concatStringsSep "\n" (
+        lib.mapAttrsToList (n: v: "export ${n}=${lib.escapeShellArg (toString v)}") env
+      );
+
       # Extract non-service options for mkShell
       shellArgs = builtins.removeAttrs args [
+        "name"
         "modules"
+        "packages"
+        "buildInputs"
+        "nativeBuildInputs"
+        "propagatedBuildInputs"
+        "propagatedNativeBuildInputs"
+        "inputsFrom"
+        "env"
+        "shellHook"
         "processCompose"
       ];
 
@@ -201,20 +250,25 @@ in
         mkdir -p ${processCompose.logDir}
         mkdir -p ${processCompose.dataDir}
 
+        ${lib.optionalString (env != { }) ''
+          # Derivation environment variables
+          ${envExports}
+        ''}
+
         ${lib.optionalString (langVariables != { }) ''
           # Language environment variables
           ${langExports}
         ''}
 
         # Display service information
         echo "================================================"
-        echo "Development Shell with Services"
+        echo "Development Shell: ${name}"
         echo "================================================"
         ${lib.optionalString (enabledServices != { }) ''
           echo ""
           echo "Available services:"
           ${lib.concatStringsSep "\n" (
-            lib.mapAttrsToList (name: _: "  echo \"  - ${name}\"") enabledServices
+            lib.mapAttrsToList (svcName: _: "  echo \"  - ${svcName}\"") enabledServices
           )}
           echo ""
           echo "Service management commands:"
@@ -248,6 +302,9 @@ in
           trap _pc_cleanup EXIT
         ''}
 
+        # Shell hooks from inputsFrom derivations
+        ${inputsFromShellHook}
+
         # User's custom shellHook
         ${shellHook}
       '';
@@ -256,8 +313,18 @@ in
     mkShell (
       shellArgs
       // {
+        inherit name;
+
         buildInputs =
-          buildInputs ++ packages ++ langPackages ++ [ processComposePackage ] ++ (lib.attrValues utilities);
+          mergedBuildInputs
+          ++ packages
+          ++ langPackages
+          ++ [ processComposePackage ]
+          ++ (lib.attrValues utilities);
+
+        nativeBuildInputs = mergedNativeBuildInputs;
+        propagatedBuildInputs = mergedPropagatedBuildInputs;
+        propagatedNativeBuildInputs = mergedPropagatedNativeBuildInputs;
 
         shellHook = enhancedShellHook;
 

docs/major-differences-nixpkgs.md

@@ -58,3 +58,27 @@ changes differ significantly from whath one would expct with Nixpkgs.
     run its tests.
   - This decouples test execution from the main build, allowing test failures
     or test-only dependency churn to avoid invalidating downstream consumers.
+
+## Development shells
+
+- Every `mkDerivation` output exposes a `toDevShell` passthru attribute that
+  converts the derivation into a development shell via `mkDevShell`.
+  - `toDevShell` preserves the original derivation's build environment
+    (dependencies, compiler, flags, environment variables) and forwards them
+    to `mkDevShell`, so the resulting shell automatically gains ekaos service
+    modules, language modules, and process-compose integration.
+  - Usage: `myPkg.toDevShell { }` returns a shell derivation directly.
+    Additional `mkDevShell` options (`modules`, `packages`, `shellHook`, etc.)
+    can be passed in the attrset argument.
+  - Accepts a function form for conditional inputs:
+    `myPkg.toDevShell (stdenv: { packages = lib.optionals stdenv.isLinux [ pkgs.strace ]; })`.
+- `mkDevShell` replaces `mkShell` as the primary shell builder.
+  - Accepts `modules` for ekaos service and language configuration, with
+    automatic process-compose integration for running services in the
+    background.
+  - Supports `inputsFrom` for propagating inputs from other derivations,
+    `env` for arbitrary environment variables, and the full set of dependency
+    list parameters (`nativeBuildInputs`, `propagatedBuildInputs`,
+    `propagatedNativeBuildInputs`).
+  - Nixpkgs has no equivalent; `mkShell` there is a thin wrapper around
+    `mkDerivation` with no service or module integration.

stdenv/generic/make-derivation.nix

@@ -1060,6 +1060,20 @@ let
           }
         );
 
+        # Convert this derivation to a development shell, preserving its
+        # build environment and gaining mkDevShell features (services,
+        # language modules, process-compose).
+        #   myPkg.toDevShell { }
+        #   myPkg.toDevShell { modules = [ ... ]; packages = [ ... ]; }
+        # Accepts either an attrset or a function (stdenv -> attrset).
+        toDevShell =
+          let
+            originalArgs = removeAttrs derivationArg attrsToRemoveLast;
+            toShell = import ./to-dev-shell.nix lib originalArgs;
+            shellFunc = f: if builtins.isFunction f then f stdenv else f;
+          in
+          f: config.mkDevShell (toShell (shellFunc f));
+
         inherit passthru overrideAttrs;
         inherit meta;
       }

stdenv/generic/to-dev-shell.nix

@@ -0,0 +1,166 @@
+lib:
+
+originalArgs:
+
+# Extract shell-relevant information from a mkDerivation's derivationArg
+# and merge it with user-supplied overrides, producing an attrset
+# suitable for mkDevShell.
+{
+  name ? if originalArgs ? name then "${originalArgs.name}-dev-shell" else "dev-shell",
+  # a list of packages to add to the shell environment
+  packages ? [ ],
+  # propagate all the inputs from the given derivations
+  inputsFrom ? [ ],
+  buildInputs ? [ ],
+  nativeBuildInputs ? [ ],
+  propagatedBuildInputs ? [ ],
+  propagatedNativeBuildInputs ? [ ],
+  shellHook ? "",
+  modules ? [ ],
+  env ? { },
+  ...
+}@attrs:
+let
+  # Merge original derivation inputs with user-supplied overrides
+  mergeInputs =
+    attrName:
+    (originalArgs.${attrName} or [ ])
+    ++ (attrs.${attrName} or [ ])
+    ++ (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs attrName inputsFrom)));
+
+  # Attrs from derivationArg that are build infrastructure, not environment
+  # variables.  Anything not in this set AND string/path-valued will be
+  # forwarded as an env var to the dev shell.
+  infrastructureAttrs = [
+    "name"
+    "pname"
+    "version"
+    "builder"
+    "args"
+    "system"
+    "outputs"
+    "out"
+    "src"
+    "srcs"
+    "sourceRoot"
+    "setSourceRoot"
+    "stdenv"
+    "buildInputs"
+    "nativeBuildInputs"
+    "propagatedBuildInputs"
+    "propagatedNativeBuildInputs"
+    "depsBuildBuild"
+    "depsBuildBuildPropagated"
+    "depsBuildTarget"
+    "depsBuildTargetPropagated"
+    "depsHostHost"
+    "depsHostHostPropagated"
+    "depsTargetTarget"
+    "depsTargetTargetPropagated"
+    "shellHook"
+    "patches"
+    "patchFlags"
+    "doCheck"
+    "doInstallCheck"
+    "strictDeps"
+    "userHook"
+    "__ignoreNulls"
+    "__structuredAttrs"
+    "__contentAddressed"
+    "outputHashAlgo"
+    "outputHashMode"
+    "outputHash"
+    "preferLocalBuild"
+    "allowSubstitutes"
+    "enableParallelBuilding"
+    "enableParallelChecking"
+    "enableParallelInstalling"
+    "meta"
+    "passthru"
+    "pos"
+    "separateDebugInfo"
+    "hardeningEnable"
+    "hardeningDisable"
+    "NIX_HARDENING_ENABLE"
+    "requiredSystemFeatures"
+    "__darwinAllowLocalNetworking"
+    "__sandboxProfile"
+    "__propagatedSandboxProfile"
+    "__impureHostDeps"
+    "__propagatedImpureHostDeps"
+    "allowedImpureDLLs"
+    "outputChecks"
+    "disallowedReferences"
+    "disallowedRequisites"
+    "allowedReferences"
+    "allowedRequisites"
+    "cmakeFlags"
+    "mesonFlags"
+    "configureFlags"
+    "configurePlatforms"
+    "makeFlags"
+    "makefile"
+    "installFlags"
+    "installTargets"
+    "dontInstall"
+    "dontBuild"
+    "dontConfigure"
+    "dontFixup"
+    "dontPatchShebangs"
+    "dontPatchELF"
+    "dontStrip"
+    "forceShare"
+    "setupHook"
+    "setupHooks"
+    "passAsFile"
+  ];
+
+  # Phase hooks and scripts (pre/post hooks, phase definitions, configure
+  # scripts, etc.) are build-time concerns and should not leak as env vars.
+  isPhaseAttr =
+    name:
+    lib.hasPrefix "pre" name
+    || lib.hasPrefix "post" name
+    || lib.hasSuffix "Phase" name
+    || lib.hasSuffix "Phases" name
+    || lib.hasSuffix "Hook" name
+    || lib.hasSuffix "Script" name
+    || lib.hasSuffix "Flags" name;
+
+  # Environment variables from the original derivation (everything that's
+  # a string and not infrastructure — these are typically set via `env` or
+  # as top-level attrs in mkDerivation).
+  originalEnv = lib.filterAttrs (
+    n: v:
+    !(builtins.elem n infrastructureAttrs)
+    && !(isPhaseAttr n)
+    && (builtins.isString v || builtins.isPath v)
+  ) originalArgs;
+
+  rest = builtins.removeAttrs attrs [
+    "name"
+    "packages"
+    "inputsFrom"
+    "buildInputs"
+    "nativeBuildInputs"
+    "propagatedBuildInputs"
+    "propagatedNativeBuildInputs"
+    "shellHook"
+    "modules"
+    "env"
+  ];
+in
+{
+  inherit name modules;
+
+  buildInputs = mergeInputs "buildInputs";
+  nativeBuildInputs = packages ++ (mergeInputs "nativeBuildInputs");
+  propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
+  propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
+
+  inherit inputsFrom shellHook;
+
+  # Merge original derivation env vars with user-supplied env
+  env = originalEnv // env;
+}
+// rest