Improve missing flake.nix error message

src/libexpr/primops/flake.cc

@@ -48,7 +48,7 @@ LockFile::FlakeEntry readFlakeEntry(nlohmann::json json)
 {
     FlakeRef flakeRef(json["uri"]);
     if (!flakeRef.isImmutable())
-        throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef.to_string());
+        throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef);
 
     LockFile::FlakeEntry entry(flakeRef);
 
@@ -57,7 +57,7 @@ LockFile::FlakeEntry readFlakeEntry(nlohmann::json json)
     for (auto i = nonFlakeRequires.begin(); i != nonFlakeRequires.end(); ++i) {
         FlakeRef flakeRef(i->value("uri", ""));
         if (!flakeRef.isImmutable())
-            throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef.to_string());
+            throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef);
         entry.nonFlakeEntries.insert_or_assign(i.key(), flakeRef);
     }
 
@@ -87,7 +87,7 @@ LockFile readLockFile(const Path & path)
     for (auto i = nonFlakeRequires.begin(); i != nonFlakeRequires.end(); ++i) {
         FlakeRef flakeRef(i->value("uri", ""));
         if (!flakeRef.isImmutable())
-            throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef.to_string());
+            throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef);
         lockFile.nonFlakeEntries.insert_or_assign(i.key(), flakeRef);
     }
 
@@ -160,7 +160,7 @@ static FlakeRef lookupFlake(EvalState & state, const FlakeRef & flakeRef,
     std::vector<FlakeRef> pastSearches = {})
 {
     if (registries.empty() && !flakeRef.isDirect())
-        throw Error("indirect flake reference '%s' is not allowed", flakeRef.to_string());
+        throw Error("indirect flake reference '%s' is not allowed", flakeRef);
 
     for (std::shared_ptr<FlakeRegistry> registry : registries) {
         auto i = registry->entries.find(flakeRef);
@@ -183,7 +183,7 @@ static FlakeRef lookupFlake(EvalState & state, const FlakeRef & flakeRef,
     }
 
     if (!flakeRef.isDirect())
-        throw Error("could not resolve flake reference '%s'", flakeRef.to_string());
+        throw Error("could not resolve flake reference '%s'", flakeRef);
 
     return flakeRef;
 }
@@ -194,7 +194,7 @@ static FlakeSourceInfo fetchFlake(EvalState & state, const FlakeRef flakeRef, bo
         impureIsAllowed ? state.getFlakeRegistries() : std::vector<std::shared_ptr<FlakeRegistry>>());
 
     if (evalSettings.pureEval && !impureIsAllowed && !fRef.isImmutable())
-        throw Error("requested to fetch mutable flake '%s' in pure mode", fRef.to_string());
+        throw Error("requested to fetch mutable flake '%s' in pure mode", fRef);
 
     // This only downloads only one revision of the repo, not the entire history.
     if (auto refData = std::get_if<FlakeRef::IsGitHub>(&fRef.data)) {
@@ -277,8 +277,12 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowe
                 + "/" + flake.sourceInfo.rev->to_string(Base16, false));
     }
 
+    Path flakeFile = sourceInfo.storePath + "/flake.nix";
+    if (!pathExists(flakeFile))
+        throw Error("source tree referenced by '%s' does not contain a 'flake.nix' file", flakeRef);
+
     Value vInfo;
-    state.evalFile(sourceInfo.storePath + "/flake.nix", vInfo); // FIXME: symlink attack
+    state.evalFile(flakeFile, vInfo); // FIXME: symlink attack
 
     state.forceAttrs(vInfo);
 

src/libexpr/primops/flakeref.cc

@@ -142,6 +142,12 @@ std::string FlakeRef::to_string() const
     return string;
 }
 
+std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef)
+{
+    str << flakeRef.to_string();
+    return str;
+}
+
 bool FlakeRef::isImmutable() const
 {
     return (bool) rev;

src/libexpr/primops/flakeref.hh

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "types.hh"
 #include "hash.hh"
 
@@ -173,4 +175,7 @@ struct FlakeRef
 
     FlakeRef baseRef() const;
 };
+
+std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);
+
 }

src/nix/flake.cc

@@ -32,7 +32,7 @@ struct CmdFlakeList : StoreCommand, MixEvalArgs
 
         for (auto & registry : registries)
             for (auto & entry : registry->entries)
-                std::cout << entry.first.to_string() << " " << entry.second.to_string() << "\n";
+                std::cout << entry.first << " " << entry.second << "\n";
     }
 };
 
@@ -48,7 +48,7 @@ void printFlakeInfo(Flake & flake, bool json) {
         std::cout << j.dump(4) << std::endl;
     } else {
         std::cout << "ID:          " << flake.id << "\n";
-        std::cout << "URI:         " << flake.sourceInfo.flakeRef.to_string() << "\n";
+        std::cout << "URI:         " << flake.sourceInfo.flakeRef << "\n";
         std::cout << "Description: " << flake.description << "\n";
         if (flake.sourceInfo.rev)
             std::cout << "Revision:    " << flake.sourceInfo.rev->to_string(Base16, false) << "\n";