Fix rust-mode lazy loading

[jroimartin]

This PR fixes the following error that happends when opening a .rs
file:

File mode specification error: (void-function rust-mode)

It conditionally autoloads the proper rust-mode version depending on
the user environment.

Fixes #528

[psibi]

Thanks, @jroimartin can you see why the tests are failing ?

[jroimartin]

Sure, I'm working on it.

[jroimartin]

@psibi PTAL. I think this is the smallest required change. On the flip side, it is a bit tricky. But having two major modes with the same name and loading them conditionally seems problematic. So, I opted for allowing the stub function (called rust-mode) to be redefined when the actual mode gets loaded.

[psibi]

Thanks, LGTM. @condy0919 Can you see if it works in your setup too ? I will test drive this PR too.

[condy0919]

Thanks, LGTM. @condy0919 Can you see if it works in your setup too ? I will test drive this PR too.

Sorry, I'm in a touring🥺 can't help that

[jroimartin]

No rush. There is a valid workaround which is just adding (require 'rust-mode) to init.el. It is just that it is nice to support lazy loading 🙂 This change also make rust-mode just work for people using plain package.el.

[psibi]

No worries, I'm also going to be on travel for the next two days. I will test this out more properly after that.

[psibi]

Doesn't this function conflict with the rust-mode function that is exposed by the rust-prog-mode / rust-treesitter-mode ?

[psibi]

Infact, give that you are calling rust-mode in the body of this function - it makes me more nervous. Do you think we can rename it to a different function instead, probably rust-mode-load or something like that ?

[jroimartin]

@psibi yeah, it is a bit hacky...

The thing is that we want to autoload rust-mode.el when the user calls (rust-mode) and then select the actual major mode. However, both versions of the major mode (rust-prog-mode and rust-treesitter-mode) are called rust-mode and which one to choose depends on the environment (the emacs version and the value of rust-mode-treesitter-derive). So, I don't know how to autoload one or the other conditionally.

Also, we cannot change the name of the major modes (and autoload both) because the hooks are named after them and other extensions expect these modes to be called rust-mode.

So, this PR autoloads a stub called rust-mode that is redefined by the specific call to define-derived-mode. Thus, the following calls to rust-mode execute the rust-mode function corresponding to the actual major mode.

[condy0919]

probably rust-mode-load or something like that ?

Or rust-mode-maybe, it's mentioned in the corresponding issue. But using another name may break users' config as many people manually do

(add-to-list 'auto-mode-alist '("\\.rs\\'" . rust-mode))

[jroimartin]

@psibi @condy0919 what do you think of db7d086? This option does not require messing up with the rust-mode function definition.

[condy0919]

@psibi @condy0919 what do you think of db7d086? This option does not require messing up with the rust-mode function definition.

This solution comes to mind at first, and the trick is used in the evil-exchange package.

[psibi]

I did some testing and this seems to work fine. @jroimartin Thanks for the PR. @condy0919 Thanks for testing and review.