vim: set Rust tab conventions

[lilyball]

No description provided.

[lilyball]

This supersedes #15416.

[bluss]

noo. Why change tabstop? This is generally discouraged.

You should use set expandtab shiftwidth=4 softtabstop=4

softtabstop is important for sane backspace.

[steveklabnik]

I have tabstop set, and my backspace works fine. What's the issue with it?

[lilyball]

softtabstop defaults to 0, meaning that tabstop controls the behavior of <Tab> by default. The reason to set softtabstop to 4 and leave tabstop alone is so files that use hard tabs still render those tabs at 8 spaces, but that assumes that Rust files with hard tabs should treat those tabs as 8 spaces. I have personally never seen a Rust file that had hard tabs that were intended to be interpreted as 8 spaces. I've certainly never seen the classically emacs-style 4-space indent 8-space hard tab, which is what setting softtabstop to 4 and leaving tabstop at 8 is really meant to support.

[bluss]

I guess leaving tabstop at 8 is overly conservative. But softtabstop=4 makes sure a backspace removes a full 4-space indent. If there isn't any other way to get that behaviour, we will need sts

[lilyball]

@blake2-ppc Is backspace deleting 4 spaces actually desired functionality? I don't generally consider that something I expect in Vim, but of course most language plugins don't try and change the default tab settings anyway. Perhaps it is something worth having.

[bluss]

@kballard I don't have any objective data points. IMO deleting an indent in one keystroke (backspace) when inserted by one keystroke (tab) is a requirement for using spaces for indents. I'm sure many editors do this, I don't have that many but Geany does this too.