Database forking / speculative execution

[mkurnikov]

I'm implementing completion for my language server, which is based on salsa / rust-analyzer architecture.

To determine the correct context, usual technique is to amend the current file with the "fake identifier" at the cursor position.

fn main() {
    /*caret*/ 
}

->

fn main() {
    fake_ident/*caret*/
}

which requires cloning the database and calling set_file_text() on it.
There is a db.fork_db().as_view::<MyDb>() but I can't get mutable reference out of it.

It was discussed multiple times both in rust-analyzer repo and in Salsa Zulip channel, under the name "speculative execution".
Is there any timeline on this?