Best Practices for Creating a Reusable Plugin with Rust Code & Handling Build Paths

[huanli233]

Hi everyone,

First, thank you for creating and maintaining this amazing library!

I'm trying to create a reusable Flutter plugin that contains a Rust core, with the goal of using it as a dependency in other Flutter applications. However, I'm running into persistent path-related issues when the plugin is compiled as a dependency, specifically on Windows.

My Goal

To create a Flutter plugin (let's call it my_rust_plugin) that can be reliably included in a main application (e.g., my_app) via pubspec.yaml (using a path or git dependency).

Project Structure (Simplified)

My plugin my_rust_plugin:

my_rust_plugin/
├── rust/
│   ├── Cargo.toml
│   └── src/
│       └── api.rs
├── windows/
│   └── CMakeLists.txt  <-- The source of the issue
├── lib/
│   └── main.dart
└── pubspec.yaml

My main app my_app:

# pubspec.yaml of my_app
dependencies:
  my_rust_plugin:
    path: ../my_rust_plugin

The Problem

When I build my_app, Flutter creates a symlinked directory for the plugin at my_app/build/windows/flutter/ephemeral/.plugin_symlinks/my_rust_plugin/.

The CMakeLists.txt inside this symlinked directory then fails to locate the rust source folder because simple relative paths like ../rust are no longer valid from this temporary build location.

Original Error Log

Before I started modifying the CMakeLists.txt to try and fix the path, this was the original error from the build system. It clearly shows the build tool trying to find Cargo.toml via a long and incorrect relative path (../../../../../../rust).

[+1428 ms]  SEVERE: ================================================================================
[      ] SEVERE : Cargokit BuildTool failed with error :
[D:\Projects\FlutterProjects\hard_root\build\windows\x64\plugins\rust_lib_adb_client\rust_lib_adb_client_cargokit.vcxproj]
[      ]  SEVERE: --------------------------------------------------------------------------------
[      ]  SEVERE: PathNotFoundException: Cannot open file, path =
'D:/Projects/FlutterProjects/hard_root/windows/flutter/ephemeral/.plugin_symlinks/rust_lib_adb_client/windows/../../../../../../rust\Cargo.toml' 
(OS Error: The system cannot find the path specified.
[      ]  SEVERE: , errno = 3)
[      ]  SEVERE: --------------------------------------------------------------------------------
[      ]  SEVERE: #0       _File.throwIfError (dart:io/file_impl.dart:675:7)
[      ]  SEVERE: #1       _File.openSync (dart:io/file_impl.dart:490:5)
[      ]  SEVERE: #2       _File.readAsBytesSync (dart:io/file_impl.dart:574:18)
[      ]  SEVERE: #3       _File.readAsStringSync (dart:io/file_impl.dart:624:18)
[      ]  SEVERE: #4       CrateInfo.load (package:build_tool/src/cargo.dart:45:35)
[      ]  SEVERE: #5       BuildEnvironment.fromEnvironment (package:build_tool/src/builder.dart:92:33)
[      ]  SEVERE: #6       BuildCMake.build (package:build_tool/src/build_cmake.dart:26:42)
[      ]  SEVERE: <asynchronous suspension>
[      ]  SEVERE: --------------------------------------------------------------------------------

My Questions

1. What is the canonical, officially recommended way to structure the CMakeLists.txt (and by extension, the iOS podspec and Android build.gradle) for a reusable plugin to ensure the Rust source is always found?
2. Is there an official template or example project for creating a distributable plugin that you could point me to?

Environment:

OS: Windows 11

Flutter Version: 3.6.2

Rust Version: 1.88.0

flutter_rust_bridge Version: 2.11.1

Thank you so much for any guidance you can provide!

[fzyzcjy]

hmm, frb has a auto generated plugin template. Does that work for your case?

[huanli233]

No, my adb_client library was created with it

[fzyzcjy]

Hmm, I am wondering why that package template does work in the default CI case. Also, does this https://matejknopp.com/post/flutter_plugin_in_rust_with_no_prebuilt_binaries/ work for you (that cargokit blog discusses how to make a rust plugin, and frb's auto-generate template just follows that approach)

[huanli233]

I've solved it. The issue was a mistake on my end with an incorrect directory structure in my plugin. My apologies for the noise.

After fixing my local setup to match the standard template, everything works perfectly as described in the blog post you shared.

Thank you again for your time and for pointing me to that resource – it was key to finding my error. I'm closing this discussion now.

[fzyzcjy]

You are welcome and happy to see it is solved!