"Add `::<>`" should trigger signature help

[jonas-schievink]

fn f() {
    std::mem::size_of$0();
}

The "Add ::<>" assist turns this into std::mem::size_of::<$0>(), but does not trigger the signature help popup.

Unlike completions, assists don't currently have this functionality built in, so some extra work is needed.

[jonas-schievink]

It appears that our non-support here is intentional, but I'm not sure why this isn't a problem for completions:

rust-analyzer/crates/rust-analyzer/src/handlers.rs

Lines 1048 to 1053 in e3ec877

	if !snap.config.code_action_literals() {
	// We intentionally don't support command-based actions, as those either
	// require either custom client-code or server-initiated edits. Server
	// initiated edits break causality, so we avoid those.
	return Ok(None);
	}

cc @matklad

[matklad]

So, originally CodeActions didn't support edit property. To actually apply an edit, code action would ping a server with a command, and the server would than apply the edit. This is the bit we intentionally don't support -- the edit must be specified in the action directly.

But it should be fine to specify an auxiliary command to be applied after edit!