nix dev-shell: Use 'provides.devShell' by default

Thus

  $ nix dev-shell

will now build the 'provides.devShell' attribute from the flake in the
current directory. If it doesn't exist, it falls back to
'provides.defaultPackage'.

flake.nix

@@ -17,5 +17,10 @@
     packages.nix = hydraJobs.build.x86_64-linux;
 
     defaultPackage = packages.nix;
+
+    devShell = import ./shell.nix {
+      nixpkgs = deps.nixpkgs;
+    };
+
   };
 }

shell.nix

@@ -1,6 +1,8 @@
-{ useClang ? false }:
+{ useClang ? false
+, nixpkgs ? builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.03.tar.gz
+}:
 
-with import (builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.03.tar.gz) {};
+with import nixpkgs { system = builtins.currentSystem or "x86_64-linux"; };
 
 with import ./release-common.nix { inherit pkgs; };
 

src/nix/command.hh

@@ -88,6 +88,11 @@ struct SourceExprCommand : virtual Args, StoreCommand, MixEvalArgs
     std::shared_ptr<Installable> parseInstallable(
         ref<Store> store, const std::string & installable);
 
+    virtual Strings getDefaultFlakeAttrPaths()
+    {
+        return {"defaultPackage"};
+    }
+
 private:
 
     std::shared_ptr<EvalState> evalState;

src/nix/installables.cc

@@ -142,13 +142,18 @@ struct InstallableAttrPath : InstallableValue
 struct InstallableFlake : InstallableValue
 {
     FlakeRef flakeRef;
-    std::string attrPath;
+    Strings attrPaths;
+    bool searchPackages = false;
 
-    InstallableFlake(SourceExprCommand & cmd, FlakeRef && flakeRef, const std::string & attrPath)
-        : InstallableValue(cmd), flakeRef(flakeRef), attrPath(attrPath)
+    InstallableFlake(SourceExprCommand & cmd, FlakeRef && flakeRef, Strings attrPaths)
+        : InstallableValue(cmd), flakeRef(flakeRef), attrPaths(std::move(attrPaths))
     { }
 
-    std::string what() override { return flakeRef.to_string() + ":" + attrPath; }
+    InstallableFlake(SourceExprCommand & cmd, FlakeRef && flakeRef, std::string attrPath)
+        : InstallableValue(cmd), flakeRef(flakeRef), attrPaths{attrPath}, searchPackages(true)
+    { }
+
+    std::string what() override { return flakeRef.to_string() + ":" + *attrPaths.begin(); }
 
     Value * toValue(EvalState & state) override
     {
@@ -166,18 +171,31 @@ struct InstallableFlake : InstallableValue
 
         auto emptyArgs = state.allocBindings(0);
 
+        // As a convenience, look for the attribute in
+        // 'provides.packages'.
+        if (searchPackages) {
             if (auto aPackages = *vProvides->attrs->get(state.symbols.create("packages"))) {
                 try {
-                auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *aPackages->value);
+                    auto * v = findAlongAttrPath(state, *attrPaths.begin(), *emptyArgs, *aPackages->value);
+                    state.forceValue(*v);
+                    return v;
+                } catch (AttrPathNotFound & e) {
+                }
+            }
+        }
+
+        // Otherwise, look for it in 'provides'.
+        for (auto & attrPath : attrPaths) {
+            try {
+                auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *vProvides);
                 state.forceValue(*v);
                 return v;
             } catch (AttrPathNotFound & e) {
             }
         }
 
-        auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *vProvides);
-        state.forceValue(*v);
-        return v;
+        throw Error("flake '%s' does not provide attribute %s",
+            flakeRef, concatStringsSep(", ", quoteStrings(attrPaths)));
     }
 };
 
@@ -216,7 +234,8 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             else if (hasPrefix(s, "nixpkgs.")) {
                 bool static warned;
                 warnOnce(warned, "the syntax 'nixpkgs.<attr>' is deprecated; use 'nixpkgs:<attr>' instead");
-                result.push_back(std::make_shared<InstallableFlake>(*this, FlakeRef("nixpkgs"), std::string(s, 8)));
+                result.push_back(std::make_shared<InstallableFlake>(*this, FlakeRef("nixpkgs"),
+                        Strings{"packages." + std::string(s, 8)}));
             }
 
             else if ((colon = s.rfind(':')) != std::string::npos) {
@@ -233,7 +252,8 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
                 if (storePath != "")
                     result.push_back(std::make_shared<InstallableStorePath>(storePath));
                 else
-                    result.push_back(std::make_shared<InstallableFlake>(*this, FlakeRef(s, true), "defaultPackage"));
+                    result.push_back(std::make_shared<InstallableFlake>(*this, FlakeRef(s, true),
+                            getDefaultFlakeAttrPaths()));
             }
 
             else

src/nix/shell.cc

@@ -125,6 +125,7 @@ struct Common : InstallableCommand
         "BASHOPTS",
         "EUID",
         "NIX_BUILD_TOP",
+        "NIX_ENFORCE_PURITY",
         "PPID",
         "PWD",
         "SHELLOPTS",
@@ -156,6 +157,11 @@ struct Common : InstallableCommand
         for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
             out << fmt("export %s=\"$NIX_BUILD_TOP\"\n", i);
     }
+
+    Strings getDefaultFlakeAttrPaths() override
+    {
+        return {"devShell", "defaultPackage"};
+    }
 };
 
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix")