Why does tokio depends on mio on wasm32-unknown-unknown by no reason

[Officeyutong]

To reproduce:

git clone https://github.com/nervosnetwork/ckb-sdk-rust
cd ckb-sdk-rust
echo 1.85 > rust-toolchain
rustup target add wasm32-unknown-unknown
cargo check --target wasm32-unknown-unknown

A lot of errors produced by mio can be seen, such as:

error[E0432]: unresolved import `crate::sys::IoSourceState`
  --> /root/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-e791a3f93f26854f/mio-1.0.3/src/io_source.rs:14:5
   |
14 | use crate::sys::IoSourceState;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ no `IoSourceState` in `sys`

error[E0432]: unresolved import `crate::sys::tcp`
  --> /root/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-e791a3f93f26854f/mio-1.0.3/src/net/tcp/listener.rs:19:17
   |
19 | use crate::sys::tcp::{bind, listen, new_for_addr};
   |                 ^^^ could not find `tcp` in `sys`

error[E0432]: unresolved import `crate::sys::tcp`
  --> /root/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-e791a3f93f26854f/mio-1.0.3/src/net/tcp/stream.rs:17:17
   |
17 | use crate::sys::tcp::{connect, new_for_addr};
   |                 ^^^ could not find `tcp` in `sys`

   Compiling crc32fast v1.4.2
   Compiling ckb-merkle-mountain-range v0.5.2
error[E0433]: failed to resolve: could not find `Selector` in `sys`
   --> /root/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-e791a3f93f26854f/mio-1.0.3/src/poll.rs:322:18
    |
322 |             sys::Selector::new().map(|selector| Poll {
    |                  ^^^^^^^^ could not find `Selector` in `sys`

The full output was attached as error.txt.

At the same time, cargo tree -e features --target wasm32-unknown-unknown shows that tokio depends on mio without feature net. The full output was attached as tree.txt

│   │   ├── tokio feature "default"
│   │   │   └── tokio v1.45.0
│   │   │       ├── mio v1.0.3
│   │   │       ├── pin-project-lite feature "default"
│   │   │       │   └── pin-project-lite v0.2.16
│   │   │       ├── bytes feature "default" (*)
│   │   │       └── tokio-macros feature "default"
│   │   │           └── tokio-macros v2.5.0 (proc-macro)
│   │   │               ├── proc-macro2 feature "default" (*)
│   │   │               ├── quote feature "default" (*)
│   │   │               ├── syn feature "default" (*)
│   │   │               └── syn feature "full" (*)

tree.txt
error.txt

Other information:

root@mnfe-pve:~/ckb-sdk-rust# cargo --version
cargo 1.85.0 (d73d2caf9 2024-12-31)
root@mnfe-pve:~/ckb-sdk-rust# rustc --version
rustc 1.85.0 (4d91de4e4 2025-02-17)
root@mnfe-pve:~/ckb-sdk-rust# neofetch
         .://:`              `://:.
       `hMMMMMMd/          /dMMMMMMh`          root@mnfe-pve 
        `sMMMMMMMd:      :mMMMMMMMs`           ------------- 
`-/+oo+/:`.yMMMMMMMh-  -hMMMMMMMy.`:/+oo+/-`   OS: Proxmox VE 8.4.1 x86_64 
`:oooooooo/`-hMMMMMMMyyMMMMMMMh-`/oooooooo:`   Host: PowerEdge R740 
  `/oooooooo:`:mMMMMMMMMMMMMm:`:oooooooo/`     Kernel: 6.8.12-10-pve 
    ./ooooooo+- +NMMMMMMMMN+ -+ooooooo/.       Uptime: 1 day, 20 hours, 2 mins 
      .+ooooooo+-`oNMMMMNo`-+ooooooo+.         Packages: 3201 (dpkg) 
        -+ooooooo/.`sMMs`./ooooooo+-           Shell: bash 5.2.15 
          :oooooooo/`..`/oooooooo:             Theme: Adwaita [GTK3] 
          :oooooooo/`..`/oooooooo:             Icons: Adwaita [GTK3] 
        -+ooooooo/.`sMMs`./ooooooo+-           Terminal: node 
      .+ooooooo+-`oNMMMMNo`-+ooooooo+.         CPU: Intel Xeon Gold 6138 (80) @ 1.000GHz 
    ./ooooooo+- +NMMMMMMMMN+ -+ooooooo/.       GPU: NVIDIA Tesla P40 
  `/oooooooo:`:mMMMMMMMMMMMMm:`:oooooooo/`     Memory: 36242MiB / 257532MiB 
`:oooooooo/`-hMMMMMMMyyMMMMMMMh-`/oooooooo:`
`-/+oo+/:`.yMMMMMMMh-  -hMMMMMMMy.`:/+oo+/-`                           
        `sMMMMMMMm:      :dMMMMMMMs`                                   
       `hMMMMMMd/          /dMMMMMMh`
         `://:`              `://:`

[Darksonn]

Tokio only depends on mio if you enable one of the features that require it, such as net, process, or signal.

[Officeyutong]

These three features are not enabled, which can bee seen through cargo tree --target wasm32-unknown-unknown

[Officeyutong]

Solved:
feature unification merged features introduced by dependencies that were not enabled on wasm32-unknown-unknown when resolver = "1" was set. Set resolver = "2" solved this problem.