[let_chains] Forbid let inside parentheses

[c410-f3r]

Parenthesizes are mostly a no-op in let chains, in other words, they are mostly ignored.

let opt = Some(Some(1i32));

if (let Some(a) = opt && (let Some(b) = a)) && b == 1 {
    println!("`b` is declared inside but used outside");
}

As seen above, such behavior can lead to confusion.

A proper fix or nested encapsulation would probably require research, time and a modified MIR graph so in this PR I simply denied any let inside parentheses. Non-let stuff are still allowed.

fn main() {
    let fun = || true;
    
    if let true = (true && fun()) && (true) {
        println!("Allowed");
    }
}

It is worth noting that let ... is not an expression and the RFC did not mention this specific situation.

cc @matthewjasper

[rust-highfive]

r? @wesleywiser

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[bors]

☔ The latest upstream changes (presumably #95403) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

@rfcbot fcp merge

Hi @rust-lang/lang -- I'd like to unblock let-chains. It seems to me that permitting parentheses to contain let introduces some lack of clarity and isn't really particularly important. I'd like to FCP just removing it from what is accepted, as proposed by this PR.

If I understand correctly, the result of this is that a let Pattern = Expr can only appear at the "top-level" of an if or as part of a && that is itself (transitively) in the "top-level". Things like the following are therefore illegal:

if (
    let Some(x) = option
) {
...
}

but this would work

if let Some(x) = option && something && let Some(z) = option { ... }

as would this

if let Some(x) = option && let Some(z) = option && (something || something_else) { ... }

[rfcbot]

Team member @nikomatsakis has proposed to merge this. The next step is review by the rest of the tagged team members:

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm

Concerns:

• currently resolved by [let_chains] Forbid let inside parentheses #95008 (comment)

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[scottmcm]

This sounds great; let's do it. We can always allow it in future, so personally I think we could even just merge the PR without waiting for an FCP.

@rfcbot reviewed

[joshtriplett]

@rfcbot reviewed

@rfcbot concern currently

Can we please not use the wording "currently" in this context? That wording seems to imply us doing so in the future.

(I'm entirely in favor of this change otherwise.)

[c410-f3r]

The "currently" word was removed as demanded

[joshtriplett]

@rfcbot resolved currently

Thank you!

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[c410-f3r]

Someone has to r+

[wesleywiser]

@bors r+

[bors]

📌 Commit 6ee3c47 has been approved by wesleywiser