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

@@ -528,26 +528,45 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     auto cache = openEvalCache(*state, lockedFlake);
     auto root = cache->getRoot();
 
-    for (auto & attrPath : getActualAttrPaths()) {
+    try {
-        auto attr = root->findAlongAttrPath(
+        for (auto & attrPath : getActualAttrPaths()) {
-            parseAttrPath(*state, attrPath),
+            auto attr = root->findAlongAttrPath(
-            true
+                parseAttrPath(*state, attrPath),
-        );
+                true
+            );
 
-        if (!attr) continue;
+            if (!attr) continue;
 
-        if (!attr->isDerivation())
+            if (!attr->isDerivation())
-            throw Error("flake output attribute '%s' is not a derivation", attrPath);
+                throw Error("flake output attribute '%s' is not a derivation", attrPath);
 
-        auto drvPath = attr->forceDerivation();
+            auto drvPath = attr->forceDerivation();
 
-        auto drvInfo = DerivationInfo{
+            auto drvInfo = DerivationInfo{
-            std::move(drvPath),
+                std::move(drvPath),
-            state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
+                    state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
-            attr->getAttr(state->sOutputName)->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",

tests/flake-missing-file.sh

@@ -0,0 +1,39 @@
+source common.sh
+
+if [[ -z $(type -p git) ]]; then
+    echo "Git not installed; skipping flake tests"
+    exit 99
+fi
+
+clearStore
+rm -rf $TEST_HOME/.cache $TEST_HOME/.config
+
+repo=$TEST_ROOT/flake
+
+rm -rf $repo $repo.tmp
+mkdir $repo
+git -C $repo init
+git -C $repo config user.email "foobar@example.com"
+git -C $repo config user.name "Foobar"
+
+cat > $repo/flake.nix <<EOF
+{
+  description = "Bla bla";
+
+  outputs = inputs: rec {
+    packages.$system.foo = import ./simple.nix;
+    defaultPackage.$system = packages.$system.foo;
+  };
+}
+EOF
+
+git -C $repo add flake.nix
+git -C $repo commit -m 'Initial'
+
+cp ./simple.nix $repo/
+nix build $repo |& grep -q "Did you forget to track it in Git" || \
+    fail "Trying to access a non git-tracked file should suggest that it is probably the issue"
+
+rm $repo/simple.nix
+nix build $repo |& (! grep -q "Did you forget to track it in Git") || \
+    fail "Trying to use an absent file shouldn’t suggest to git add it"

tests/local.mk

@@ -47,6 +47,7 @@ nix_tests = \
   describe-stores.sh \
   flakes.sh \
   flake-local-settings.sh \
+  flake-missing-file.sh \
   build.sh \
   compute-levels.sh \
   repl.sh ca/repl.sh \