The extension supports formatting the document opened in the active text editor on saving.
If the extension is runnning in RLS Mode, formatting is performed via rustfmt integrated into RLS.
If the extension is running in Legacy Mode, formatting is performed via separate rustfmt.
Read more about rustfmt configuration for Legacy Mode.
In order to format the document, make a right click and choose "Format Document".
In Legacy Mode "Format Selection" is also available, although this feature of rustfmt is currently incomplete.
Visual Studio Code supports formatting a document on saving.
Set "editor.formatOnSave" to true to make Visual Studio Code do that.