add coding_style.rst

Explain when using auto is acceptable

Author: soheilshahrouz

doc/src/dev/coding_style.rst

@@ -0,0 +1,46 @@
+
+Use of `auto`
+=============
+
+Use `auto` only when the type is long, complex, or hard to write explicitly.
+
+Examples where `auto` is appropriate:
+
+.. code-block:: cpp
+
+    auto it = container.begin();  // Iterator type is long and not helpful to spell out
+
+    // The return type is std::vector<std::vector<std::pair<int, float>>>
+    auto matrix = generate_adjacency_matrix();
+
+    // Lambdas have unreadable and compiler-generated types — use auto for them
+    auto add = [](int x, int y) { return x + y; };
+    
+
+Avoid `auto` when the type is simple and clear:
+
+.. code-block:: cpp
+
+    // Use type names when they are short and readable.
+    for (RRNodeId node_id : device_ctx.rr_graph.nodes()) {
+        t_rr_node_route_inf& node_inf = route_ctx.rr_node_route_inf[rr_id];
+    }
+
+    int count = 0;
+    std::string name = "example";
+    std::vector<int> numbers = {1, 2, 3};
+
+Avoid:
+
+.. code-block:: cpp
+
+    auto count = 0;                // Simple and obvious type
+    auto name = std::string("x");  // Hides a short, clear type
+
+    for (auto node_id : device_ctx.rr_graph.nodes()) {
+        // node_id is RRNodeId. Write it out for clarity.
+        auto& node_inf = route_ctx.rr_node_route_inf[rr_id];
+        // node_inf is t_rr_node_route_inf. Use the explicit type since it's simple and meaningful.
+    }
+
+Rationale: clear, explicit types help with readability and understanding. Avoid hiding simple types behind `auto`.

doc/src/dev/index.rst

@@ -11,5 +11,6 @@ Developer Guide
     c_api_doc
     code_documentation
     tutorials/index
+    coding_style
     ../SUPPORT
     ../LICENSE