style guide: add let-chain rules

[calebcartwright]

Reopens #110568

refs #53667 and I suppose #132833 as well

This reflects the style rules that the style team had already agreed upon back in 2023, with the addition of literals in the lhs being permissible for single line formatting, and the removal of unnecessary language/example snippets around non-&& operators that was a small hiccup in the original PR.

It also reflects current formatting behavior implemented in rustfmt (though note that the adjustment to include literals has been implemented & merged, but is still pending a sync to nightly)

[rustbot]

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[rustbot]

Some changes occurred in src/doc/style-guide

cc @rust-lang/style

[joshtriplett]

Shouldn't the = be on the second line here, given the explanation above of "prefer to break before the ="?

[calebcartwright]

Good catch

I think the "before operator" prescription may have been a snippet of text we missed in #113145 (and some more context linked in rust-lang/style-team#179) when we changed the text to match the existing behavior of always breaking after the operator

for example, rustfmt has long formatted chainless if let Some(....) .... with breaks after = - https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=9f656a763b2b09c559d39568c2d5079c

fn main() {
    if let Some(x) = a_long_long_long_long_long_long_really_realllyyyy_long_expressionddddddddddddddddd {
        // ..
    }

    if let Some(relatively_long_thing) =
    a_long_long_long_long_long_long_really_realllyyyy_long_expression
{
        // ..
    }
}

is formatted as:

fn main() {
    if let Some(x) =
        a_long_long_long_long_long_long_really_realllyyyy_long_expressionddddddddddddddddd
    {
        // ..
    }

    if let Some(relatively_long_thing) =
        a_long_long_long_long_long_long_really_realllyyyy_long_expression
    {
        // ..
    }
}

I'm going to change the text in the control flow section to indicate breaking after under the same rationale as we had previously of "bug fix" to the style guide text, but happy to discuss more if you think that's the wrong direction

[joshtriplett]

Good call. I agree that we should match the existing behavior.

[joshtriplett]

@calebcartwright One review comment; with that addressed, r=me.

[joshtriplett]

LGTM, r=me whenever you think it ready.

[calebcartwright]

@bors r=joshtriplett rollup

[bors]

📌 Commit ee07e3f has been approved by joshtriplett

It is now in the queue for this repository.