[rust] try Rust 2021

[BohuTANG]

Summary

https://blog.rust-lang.org/2021/07/21/Rust-2021-public-testing.html

[BohuTANG]

@PsiACE

Hmm, Would you like a try?

[PsiACE]

Hmm, Would you like a try?

Ok

[zhang2014]

Very cool~