libcmd/installables: implement completion for Nix expressions passed via -f

src/libcmd/installables.cc

@@ -171,14 +171,44 @@ Strings SourceExprCommand::getDefaultFlakeAttrPathPrefixes()
 
 void SourceExprCommand::completeInstallable(std::string_view prefix)
 {
-    if (file) return; // FIXME
+    if (file) {
+        evalSettings.pureEval = false;
+        auto state = getEvalState();
+        Expr *e = state->parseExprFromFile(
+            resolveExprPath(state->checkSourcePath(lookupFileArg(*state, *file)))
+        );
 
+        Value root;
+        state->eval(e, root);
+
+        auto autoArgs = getAutoArgs(*state);
+
+        std::string prefix_ = std::string(prefix);
+        auto sep = prefix_.rfind('.');
+        if (sep != std::string::npos) {
+            prefix_.erase(sep);
+        } else {
+            prefix_ = "";
+        }
+
+        Value &v1(*findAlongAttrPath(*state, prefix_, *autoArgs, root).first);
+        state->forceValue(v1);
+        Value v2;
+        state->autoCallFunction(*autoArgs, v1, v2);
+
+        if (v2.type() == nAttrs) {
+            for (auto & i : *v2.attrs) {
+                completions->add(i.name);
+            }
+        }
+    } else {
         completeFlakeRefWithFragment(
             getEvalState(),
             lockFlags,
             getDefaultFlakeAttrPathPrefixes(),
             getDefaultFlakeAttrPaths(),
             prefix);
+    }
 }
 
 void completeFlakeRefWithFragment(