Improve error message when a directory is not a flake

So you now get

  $ nix build
  error: path '.' is not a flake (because it does not reference a Git repository)

rather than

  $ nix build
  error: unsupported argument '.'

1 files changed, 35 insertions, 17 deletions

diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index dbbf58861..a4726a59e 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -417,25 +417,43 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
                         Strings{"legacyPackages." + std::string(s, 8)}));
             }
 
-            else if (auto flakeRef = parseFlakeRef(s, true))
-                result.push_back(std::make_shared<InstallableFlake>(*this, std::move(*flakeRef),
-                        getDefaultFlakeAttrPaths()));
-
-            else if ((colon = s.rfind(':')) != std::string::npos) {
-                auto flakeRef = std::string(s, 0, colon);
-                auto attrPath = std::string(s, colon + 1);
-                result.push_back(std::make_shared<InstallableFlake>(
-                        *this,
-                        FlakeRef(flakeRef, true),
-                        attrPath,
-                        getDefaultFlakeAttrPathPrefixes()));
-            }
+            else {
+
+                std::exception_ptr flakeEx;
+
+                try {
+                    auto flakeRef = FlakeRef(s, true);
+                    result.push_back(std::make_shared<InstallableFlake>(
+                            *this, std::move(flakeRef), getDefaultFlakeAttrPaths()));
+                    continue;
+                } catch (MissingFlake &) {
+                    /* 's' could be parsed as a flakeref, but it
+                       references a local path that is not a flake. So
+                       take note of that. */
+                    flakeEx = std::current_exception();
+                } catch (BadFlakeRef &) {
+                }
+
+                if ((colon = s.rfind(':')) != std::string::npos) {
+                    auto flakeRef = std::string(s, 0, colon);
+                    auto attrPath = std::string(s, colon + 1);
+                    result.push_back(std::make_shared<InstallableFlake>(
+                            *this,
+                            FlakeRef(flakeRef, true),
+                            attrPath,
+                            getDefaultFlakeAttrPathPrefixes()));
+                }
 
-            else if (s.find('/') != std::string::npos && (storePath = follow(s)))
-                result.push_back(std::make_shared<InstallableStorePath>(*storePath));
+                else if (s.find('/') != std::string::npos && (storePath = follow(s)))
+                    result.push_back(std::make_shared<InstallableStorePath>(*storePath));
 
-            else
-                throw Error("unsupported argument '%s'", s);
+                else {
+                    if (flakeEx)
+                        std::rethrow_exception(flakeEx);
+                    else
+                        throw Error("unsupported argument '%s'", s);
+                }
+            }
         }
     }