mkDerivation: add toDevShell

generic/make-derivation.nix

@@ -635,6 +635,12 @@ extendDerivation
        disallowedRequisites = [ ];
      });
 
+     toDevShell = let
+       originalArgs = deleteFixedOutputRelatedAttrs derivationArg;
+       toShell = import ./to-dev-shell.nix lib originalArgs;
+       shellFunc = f: if builtins.isFunction f then f stdenv else f;
+     in f: derivation (toShell (shellFunc f));
+
      inherit passthru overrideAttrs;
      inherit meta;
    } //

generic/to-dev-shell.nix

@@ -0,0 +1,64 @@
+lib:
+
+originalArgs:
+
+# A special kind of derivation that is only meant to be consumed by the
+# nix-shell.
+{ name ? if originalArgs ? name then "${originalArgs.name}-dev-shell" else "nix-shell"
+, # a list of packages to add to the shell environment
+  packages ? [ ]
+, # propagate all the inputs from the given derivations
+  inputsFrom ? [ ]
+, buildInputs ? [ ]
+, nativeBuildInputs ? [ ]
+, propagatedBuildInputs ? [ ]
+, propagatedNativeBuildInputs ? [ ]
+, ...
+}@attrs:
+let
+  mergeInputs = name:
+    (originalArgs.${name} or [ ]) ++
+    (attrs.${name} or [ ]) ++
+    # 1. get all `{build,nativeBuild,...}Inputs` from the elements of `inputsFrom`
+    # 2. since that is a list of lists, `flatten` that into a regular list
+    # 3. filter out of the result everything that's in `inputsFrom` itself
+    # this leaves actual dependencies of the derivations in `inputsFrom`, but never the derivations themselves
+    (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs name inputsFrom)));
+
+  rest = builtins.removeAttrs attrs [
+    "name"
+    "packages"
+    "inputsFrom"
+    "buildInputs"
+    "nativeBuildInputs"
+    "propagatedBuildInputs"
+    "propagatedNativeBuildInputs"
+    "shellHook"
+  ];
+in {
+  inherit name;
+
+  buildInputs = mergeInputs "buildInputs";
+  nativeBuildInputs = packages ++ (mergeInputs "nativeBuildInputs");
+  propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
+  propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
+
+  shellHook = lib.concatStringsSep "\n" (lib.catAttrs "shellHook"
+    (lib.reverseList inputsFrom ++ [ attrs ]));
+
+  phases = [ "buildPhase" ];
+
+  buildPhase = ''
+    { echo "------------------------------------------------------------";
+      echo " WARNING: the existence of this path is not guaranteed.";
+      echo " It is an internal implementation detail for pkgs.mkShell.";
+      echo "------------------------------------------------------------";
+      echo;
+      # Record all build inputs as runtime dependencies
+      export;
+    } >> "$out"
+  '';
+
+  preferLocalBuild = true;
+} // rest
+