Firstly rust must be installed on your system.
For making rust documentation run the following command on your terminal.
rustup component add rust-docs
The above command will download and install the rust-doc on your system, this require some internet but after that you are good to use rust-doc offline.
Now run the following command in your terminal to view rust-documentation offline on your browser. It will automatically open in your default browser on running the following command.
rustup doc