这好像是vscode rust插件的问题。相关issue仍然是open的状态：
https://github.com/rust-lang/vscode-rust/issues/622
https://github.com/rust-lang/vscode-rust/issues/700
解决方案是在settings.json里加入

"rust-client.rustupPath": "$HOME/.cargo/bin/rustup",

然后重启vscode就好了。

参考文献：
https://*.com/questions/60816886/how-do-i-solve-couldnt-start-client-rust-language-server-with-the-rust-vs-cod