Try and help if a file is missing at eval time

When evaluating a local git flake, if a file a missing at eval-time but
present in the source directory, tell the user that he probably forgot
to check it out

Fix #4507

src/libcmd/installables.cc

@@ -535,26 +535,45 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     auto cache = openEvalCache(*state, lockedFlake);
     auto root = cache->getRoot();
 
-    for (auto & attrPath : getActualAttrPaths()) {
-        auto attr = root->findAlongAttrPath(
-            parseAttrPath(*state, attrPath),
-            true
-        );
+        try {
+            for (auto & attrPath : getActualAttrPaths()) {
+                auto attr = root->findAlongAttrPath(
+                    parseAttrPath(*state, attrPath),
+                    true
+                );
 
-        if (!attr) continue;
+            if (!attr) continue;
 
-        if (!attr->isDerivation())
-            throw Error("flake output attribute '%s' is not a derivation", attrPath);
+            if (!attr->isDerivation())
+                throw Error("flake output attribute '%s' is not a derivation", attrPath);
 
-        auto drvPath = attr->forceDerivation();
+            auto drvPath = attr->forceDerivation();
 
-        auto drvInfo = DerivationInfo{
-            std::move(drvPath),
-            state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
-            attr->getAttr(state->sOutputName)->getString()
-        };
+            auto drvInfo = DerivationInfo{
+                std::move(drvPath),
+                state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
+                attr->getAttr(state->sOutputName)->getString()
+            };
 
-        return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+            return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+        }
+    } catch (FileError & e) {
+        if (e.errNo != ENOENT) throw;
+        if (!state->store->isInStore(e.path)) throw;
+        auto resolvedInput = lockedFlake->flake.resolvedRef.input;
+        auto [sourceStorePath, relPath] = state->store->toStorePath(e.path);
+        if (sourceStorePath != lockedFlake->flake.lockedRef.input.computeStorePath(*state->store)) throw;
+        if (fetchers::maybeGetStrAttr(resolvedInput.attrs, "type").value_or("") != "git") throw;
+        if (auto repositoryPath = resolvedInput.getSourcePath()) {
+            auto sourcePath = *repositoryPath + "/" + relPath;
+            if (pathExists(sourcePath))
+                e.addTrace(
+                    noPos,
+                    "The file is present in '%s'. "
+                    "Did you forget to track it in Git?",
+                    *repositoryPath);
+        }
+        throw;
     }
 
     throw Error("flake '%s' does not provide attribute %s",