Only emit useless_vec suggestion if the macro does not contain code comments

[GuillaumeGomez]

Fixes #13692.

If the vec! macro call contains comments, we should not provide suggestions and let users handle it however they see fit.

changelog: Only emit useless_vec suggestion if the macro does not contain code comments

[rustbot]

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[y21]

IMO the help message/suggestion was a fairly important part of the lint because now it's not 100% clear what the lint wants the user to do from the diagnostic alone (use an array instead).

What about just changing the applicability to something else other than MachineApplicable so that --fix won't autofix it but is still shown to the user as a hint to what the fix roughly looks like and they can apply it manually by preserving the comment?

(I guess ideally the suggestion could just include the last comment directly and keep it as MachineApplicable since all other comments except for the one after the last element are already correctly handled, but I can only think of hacky ways to do it like extending the snippet's span to the full vec![] invocation minus 1 (just behind the ]), and am not sure if that will always work so 🤷)

[GuillaumeGomez]

Yeah makes sense. Gonna change the applicability instead.

[xFrednet]

I agree with @y21 that it's better to only change the applicability, like you did now. (Thank you for the good suggestion!)

I would be in favor of expanding the snippet span instead. I don't really see when this should fail. Modulo macro expansion, which will probably make this a lot harder ^^.

I checked the code and don't see a super obvious way to hack it in. I don't think it's worth it to spent too much time on this, since changing the applicability already prevents automatic deletion of the comments.

So I'd say we can merge this once the comment (from my other review comment) has been updated :D

[GuillaumeGomez]

@rustbot ready

[xFrednet]

This comment is outdated:

Suggested change

	// If the `vec!` macro contains comment, better not emit any suggestion as it
	// would remove them. Let's just lint that the `vec!` is useless and let the user
	// decides what they wanna do.
	// If the `vec!` macro contains comment, better not make the suggestion machine applicable as it
	// would remove them.

[xFrednet]

I agree with @y21 that it's better to only change the applicability, like you did now. (Thank you for the good suggestion!)

I would be in favor of expanding the snippet span instead. I don't really see when this should fail. Modulo macro expansion, which will probably make this a lot harder ^^.

I checked the code and don't see a super obvious way to hack it in. I don't think it's worth it to spent too much time on this, since changing the applicability already prevents automatic deletion of the comments.

So I'd say we can merge this once the comment (from my other review comment) has been updated :D

[GuillaumeGomez]

Updated comment.

@rustbot ready