Render the box pattern removal diagnostic more actionable & fully remove box expression recovery

[fmease]

Follow-up to #156749. This way, users don't have to comb through (probably outdated) tracking issues trying to piece together how to fix their code (in the most idiomatic way). Lemme know if you think this is unnecessary.

Re. box expression removal, see #162008 (comment) (TL;DR: it's been 3 years).

cc @cyrgani

_{(No LLM was or will be used by me during the entire creation process of this PR)}

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[cyrgani]

Maybe it's also time to just remove the suggestion for box_syntax? The removal PR (#108471) was merged in 1.70, three years ago, so everyone should have had plenty of time to move away from it.

[fmease]

Modifying can_begin* usually affects stable(!) macro rules matching behavior. In this case, it leads to code like the following to go from fail to pass:

macro_rules! mk {
    ($e:expr) => { compile_error!("expr"); };
    (box $e:expr) => {};
}

mk!(box 0);

Strictly speaking that's a new guarantee (in the sense that re-adding box exprs would be breaking). It's unclear if lang FCP #108471 (comment) covers this. If you don't want to involve T-lang at this time, we could also keep kw::Box in can_begin_expr for now and add a FIXME to revisit it in the future.

View changes since the review

[fee1-dead]

I would prefer us to split this out of the suggestion / expression recovery remove, and nominate lang for it, this PR should just add a FIXME.

However, let's document that changing this function can change stable macro rules behavior too.

[fee1-dead]

LGTM, r=me

View changes since this review

[fee1-dead]

I would prefer us to split this out of the suggestion / expression recovery remove, and nominate lang for it, this PR should just add a FIXME.

However, let's document that changing this function can change stable macro rules behavior too.