rust-mode: default to conventional indentation variable `tab-width` value

[mcandre]

To configure indentation width in rust-mode, the Emacs user modifies rust-indent-unit, as in (setq rust-indent-unit 4). This works, but it's nicer, more idiomatic, and more compatible with other Emacs modes to base the rust-indent-unit variable off of tab-width, and use a rust-mode-hook to customize tab-width rather than set rust-indent-unit globally.

[emberian]

@mcandre the rust-mode has been rewritten, and should respect this. Closing (pipe up if you disagree)

[pnkfelix]

The current rust-mode.el is using default-tab-width, but I think tab-width would be better. (In particular, one can assign a buffer-local value totab-width, which has been useful when I want to use 4-space tabs for Rust code, but 8-space tabs for Makefiles.)

[brson]

Can't we use 4 spaces by default, since that is the Rust coding convention? The current situation forces everybody to override the tab width.

[pnkfelix]

part of #8793

[andersk]

Yeah, this seems wrong. Emacs has separate tab-width and indent-level variables for a reason. Tabs are always displayed as 8 spaces by a variety of programs, but most programmers and style guides prefer 4 spaces or 2 spaces for indentation levels, and one should not have to override the width of a Tab character to a nonstandard value to comply with this preference. No other major mode uses tab-width to control indentation levels—I don’t know where the reporter heard otherwise. (You can make tab-width control indentation levels if you really want to, e.g. (defvaralias 'c-basic-offset 'tab-width), but this does not happen by default.)

[pnkfelix]

At this point, the emacs mode was revised (after I reopened this ticket, and maybe even after I wrote pnkfelix/rust@b6848f5 ); see SHA: 3596f66 which adds the customizable varaible rust-indent-offset, which should be customizable and so one does not need to modify default-tab-width

So I will just close this, I think customizing rust-indent-offset should be fine for this.

[andersk]

Alright, the customizability is there. Should I open another issue for changing the default to 4, like I expect most programmers will want? (Or is that more controversial than I expect?)

[pnkfelix]

@andersk: I won't object.

[andersk]

#8830.

[adrianbrink]

Is there a way to set rust-mode to always use tabs instead of spaces?