Stabilize let chains in the 2024 edition

[est31]

Stabilization report

This proposes the stabilization of let_chains (tracking issue, RFC 2497) in the 2024 edition of Rust.

What is being stabilized

The ability to &&-chain let statements inside if and while is being stabilized, allowing intermixture with boolean expressions. The patterns inside the let sub-expressions can be irrefutable or refutable.

struct FnCall<'a> {
    fn_name: &'a str,
    args: Vec<i32>,
}

fn is_legal_ident(s: &str) -> bool {
    s.chars()
        .all(|c| ('a'..='z').contains(&c) || ('A'..='Z').contains(&c))
}

impl<'a> FnCall<'a> {
    fn parse(s: &'a str) -> Option<Self> {
        if let Some((fn_name, after_name)) = s.split_once("(")
            && !fn_name.is_empty()
            && is_legal_ident(fn_name)
            && let Some((args_str, "")) = after_name.rsplit_once(")")
        {
            let args = args_str
                .split(',')
                .map(|arg| arg.parse())
                .collect::<Result<Vec<_>, _>>();
            args.ok().map(|args| FnCall { fn_name, args })
        } else {
            None
        }
    }
    fn exec(&self) -> Option<i32> {
        let iter = self.args.iter().copied();
        match self.fn_name {
            "sum" => Some(iter.sum()),
            "max" => iter.max(),
            "min" => iter.min(),
            _ => None,
        }
    }
}

fn main() {
    println!("{:?}", FnCall::parse("sum(1,2,3)").unwrap().exec());
    println!("{:?}", FnCall::parse("max(4,5)").unwrap().exec());
}

The feature will only be stabilized for the 2024 edition and future editions. Users of past editions will get an error with a hint to update the edition.

closes #53667

Why 2024 edition?

Rust generally tries to ship new features to all editions. So even the oldest editions receive the newest features. However, sometimes a feature requires a breaking change so much that offering the feature without the breaking change makes no sense. This occurs rarely, but has happened in the 2018 edition already with async and await syntax. It required an edition boundary in order for async/await to become keywords, and the entire feature foots on those keywords.

In the instance of let chains, the issue is the drop order of if let chains. If we want if let chains to be compatible with if let, drop order makes it hard for us to generate correct MIR. It would be strange to have different behaviour for if let ... {} and if true && let ... {}. So it's better to stay consistent with if let.

In edition 2024, drop order changes have been introduced to make if let temporaries be lived more shortly. These changes also affected if let chains. These changes make sense even if you don't take the if let chains MIR generation problem into account. But if we want to use them as the solution to the MIR generation problem, we need to restrict let chains to edition 2024 and beyond: for let chains, it's not just a change towards more sensible behaviour, but one required for correct function.

Introduction considerations

As edition 2024 is very new, this stabilization PR only makes it possible to use let chains on 2024 without that feature gate, it doesn't mark that feature gate as stable/removed. I would propose to continue offering the let_chains feature (behind a feature gate) for a limited time (maybe 3 months after stabilization?) on older editions to allow nightly users to adopt edition 2024 at their own pace. After that, the feature gate shall be marked as stabilized, not removed, and replaced by an error on editions 2021 and below.

Implementation history

• History from before March 14, 2022 can be found in the original stabilization PR that was reverted.
• Stabilize let_chains in Rust 1.64 #94927
• Extend the irrefutable_let_patterns lint to let chains #94951
• Ensure that let_else does not interact with let_chains #94974
• [let_chains] Forbid let inside parentheses #95008
• [rustc_parse] Forbid lets in certain places #97295
• Fix last let_chains blocker #98633
• Remove let-chain close brace check. #99731
• Fix unwind drop glue for if-then scopes #102394
• Add tests for the drop behavior of some control flow constructs #100526
• Revert let_chains stabilization #100538
• Drop temporaries created in a condition, even if it's a let chain #102998
• Detect incorrect chaining of if and if let conditions and recover #103405
• Remove drop order twist of && and || and make them associative #103293
• Rescope temp lifetime in if-let into IfElse with migration lint #107251
• docs(style): add let-chain style rules #110568
• Improve invalid let expression handling #115677
• Suggest removing ; for ; within let-chains #117743
• Catch stray { in let-chains #117770
• Suggest let or == on typo'd let-chain #118191
• Fix scoping for let chains in match guards #119554
• Don't lint irrefutable_let_patterns on leading patterns if else if let-chains #129394
• Additional tests to ensure let is rejected during parsing #132828
• Add let_chains references reference#1179
• Revert let chains reference docs reference#1251
• Support let-chains rustfmt#5910

Adoption history

In the compiler

• History before March 14, 2022 can be found in the original stabilization PR.
• fix confusing let chain indentation in rustc_resolve #115983
• Simplify some mir passes by using let chains #116549
• Format all the let-chains in compiler crates #116688

Outside of the compiler

• Replace if_chain with let chains rust-clippy#11750
• rspack
• risingwave
• dylint
• convex-backend
• tikv
• Daft
• greptimedb

Tests

Intentional restrictions

partially-macro-expanded.rs, macro-expanded.rs: it is possible to use macros to expand to both the pattern and the expression inside a let chain, but not to the entire let pat = expr operand.
parens.rs: if (let pat = expr) is not allowed in chains
ensure-that-let-else-does-not-interact-with-let-chains.rs: let...else doesn't support chaining.

Overlap with match guards

move-guard-if-let-chain.rs: test for the use moved value error working well in match guards. could maybe be extended with let chains that have more than one let
shadowing.rs: shadowing in if let guards works as expected
ast-validate-guards.rs: let chains in match guards require the match guards feature gate

Simple cases from the early days

PR #88642 has added some tests with very simple usages of let else, mostly as regression tests to early bugs.

then-else-blocks.rs
ast-lowering-does-not-wrap-let-chains.rs
issue-90722.rs
issue-92145.rs

Drop order/MIR scoping tests

issue-100276.rs: let expressions on RHS aren't terminating scopes
drop_order.rs: exhaustive temporary drop order test for various Rust constructs, including let chains
scope.rs: match guard scoping test
drop-scope.rs: another match guard scoping test, ensuring that temporaries in if-let guards live for the arm
drop_order_if_let_rescope.rs: if let rescoping on edition 2024, including chains
mir_let_chains_drop_order.rs: comprehensive drop order test for let chains, distinguishes editions 2021 and 2024.
issue-99938.rs, issue-99852.rs both bad MIR ICEs fixed by #102394

Linting

irrefutable-lets.rs: trailing and leading irrefutable let patterns get linted for, others don't. The lint is turned off for else if.
issue-121070-let-range.rs: regression test for false positive of the unused parens lint, precedence requires the ()s here

Parser: intentional restrictions

disallowed-positions.rs: let in expression context is rejected everywhere except at the top level
invalid-let-in-a-valid-let-context.rs: nested let is not allowed (let's are no legal expressions just because they are allowed in if and while).

Parser: recovery

issue-103381.rs: Graceful recovery of incorrect chaining of if and if let
semi-in-let-chain.rs: Ensure that stray ;s in let chains give nice errors (if_chain! users might be accustomed to ;s)
deli-ident-issue-1.rs, brace-in-let-chain.rs: Ensure that stray unclosed {s in let chains give nice errors and hints

Misc

conflicting_bindings.rs: the conflicting bindings check also works in let chains. Personally, I'd extend it to chains with multiple let's as well.
let-chains-attr.rs: attributes work on let chains

Tangential tests with #![feature(let_chains)]

if-let.rs: MC/DC coverage tests for let chains
logical_or_in_conditional.rs: not really about let chains, more about dropping/scoping behaviour of ||
stringify.rs: exhaustive test of the stringify macro
expanded-interpolation.rs, expanded-exhaustive.rs: Exhaustive test of -Zunpretty
diverges-not.rs: Never type, mostly tangential to let chains

Possible future work

• There is proposals to allow if let Pat(bindings) = expr {} to be written as if expr is Pat(bindings) {} (RFC 3573). if let chains are a natural extension of the already existing if let syntax, and I'd argue orthogonal towards is syntax.
  • Consensus check: let-chains and is are not mutually exclusive lang-team#297
• One could have similar chaining inside let ... else statements. There is no proposed RFC for this however, nor is it implemented on nightly.
• Match guards have the if keyword as well, but on stable Rust, they don't support let. The functionality is available via an unstable feature (if_let_guard tracking issue). Stabilization of let chains affects this feature in so far as match guards containing let chains now only need the if_let_guard feature gate be present instead of also the let_chains feature (NOTE: this PR doesn't implement this simplification, it's left for future work).

Open questions / blockers

• bad recovery if you don't put a let (I don't think this is a blocker): #117977
• An instance where a temporary lives shorter than with nested ifs, breaking compilation: #103476. Personally I don't think this is a blocker either, as it's an edge case. Edit: turns out to not reproduce in edition 2025 any more, due to let rescoping. regression test added in Let chains tests #133093
• One should probably extend the tests for move-guard-if-let-chain.rs and conflicting_bindings.rs to have chains with multiple let's: done in 133093
• Parsing rejection tests: addressed by Additional tests to ensure let is rejected during parsing #132828
• Formatting: nightly rustfmt can format let chains since already more than a year ago. However, the PR to add the respective policy document has not been merged.
• Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730 explicitly mentions let_else. I think we can live with let pat = expr not evaluating as expr for macro_rules macros, especially given that let pat = expr is not a legal expression anywhere except inside if and while.
• Documentation in the reference: Document let_chains again reference#1740
• Add chapter to the Rust 2024 edition guide: Document if/while let chains edition-guide#337
• Resolve open questions on desired drop order.

[traviscross]

Process-wise, we normally do the FCP on the stabilization PR, and I think that'd make the most sense here. Let us know if you need us on lang to weigh in on anything specifically, e.g. any of the open questions, to make such a stabilization PR possible.

cc @rust-lang/lang for visibility.

And cc @rust-lang/style given the open item on the style guide.

@rustbot labels +I-style-nominated

Putting on the edition hat, since this would be an edition item in some sense, we should talk about this in our next edition call.

@rustbot labels +I-edition-nominated

[rustbot]

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[est31]

Process-wise, we normally do the FCP on the stabilization PR, and I think that'd make the most sense here.

@traviscross understood, I've converted it to a pull request using the gh cli. Thanks for cc'ing the relevant teams.

[traviscross]

Beautiful, thanks. Let's nominate this for lang discussion too then.

@rustbot labels +I-lang-nominated

[traviscross]

@rustbot labels -I-edition-nominated

We talked this one through in the edition call. Since there's no breakage or migration here, we're happy for this one to land in Rust 2024 either ahead of or after the release of Rust 2024 itself as long as there is an edition guide chapter for it.

[traviscross]

Thanks for doing those things. On the lang side, this stabilization is a priority for us, and we're planning to discuss to see about doing what we can to clear the way here.

[nikomatsakis]

@rustbot nominate +I-lang-nominated

Here is my write-up of the drop order questions. Based on the tests from this PR, I believe the way you can think of it is this:

Given an if-statement if C0 && ... && Cn { B1 } else { B2 } where each condition Ci = $expr | let $pat = $expr is either regular boolean expressions or a let $pat = $expr match:

• Any temporaries introduced as part of the let conditions in the C will be dropped on exit from the then-block B1 or on entry to the else block B2.
• Any temporaries introduced in boolean expressions (which cannot possibly be referenced) will be dropped on entry to the then-block.

This test demonstrates temporaries being dropped after exiting B1 and this test demonstrates temporaries being dropped before entering B2.

This behavior is equivalent to what you get today with nested if-let chains, as demonstrated by this test (then-block). (This equivalence doesn't hold, but also doesn't make sense, in the else case.)

This order is somewhat different from what you get (test case) if you wrote a series of let-else statements with breaks like

'outer: {
    'else_blk: {
        if C1 else { break 'else_blk; };
        ...
        let Pat_i = Expr_i else { break 'else_blk; };  // Ci = let Pat_i = Expr_i
        ...
        if Cn else { break 'else_blk; };
        $then-block;
        break 'outer;
    }
    $else-block;
}

In that case, unbound values from the RHS are dropped as soon as you exit the statement let Ok(_) = $expr else { break 'else_blk; }. It's not clear that anybody would expect this equivalence to hold (or even think of it).

The TL;DR for me is -- the order is a bit crazy, but it aligns with nested if-let, and I think it's the best we can do right now that meets the various criteria. In general I think it is good to drop sooner vs later, and I would not (for example) want temporaries in boolean expressions to be dropped later just to align with temporaries from block patterns. We decided in Rust 2024 not to have "potentially referenced" temporaries in scenarios like match some_lock.lock().unwrap().do_something() drop sooner, and so that means that I would expect the temporaries for if let scenarios to cover the then-block, which means we are stuck with that inconsistency. This seems to be the best we can do.

[rustbot]

Error: The feature nominate is not enabled in this repository.
To enable it add its section in the triagebot.toml in the root of the repository.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[nikomatsakis]

@rustbot labels +I-lang-nominated

[joshtriplett]

@nikomatsakis Thanks for the summary! That drop order sounds reasonable to me. I can concoct scenarios where the different drop order for boolean expressions would cause an issue, but I don't think this will be a problem in practice.

So, no concerns with this moving forward.

[est31]

Thanks for the writeup @nikomatsakis. I have marked the "Resolve open questions on desired drop order" checkbox.

Now the only blocker is the fmt RFC. I will try to find some time for it on the weekend, let's see.

[joshtriplett]

@est31 As a clarification: style doesn't need an RFC, just a PR patching the style guide.

[camsteffen]

There's a discussion for style rules at rust-lang/style-team#169. I think there may have also been a style team meeting on it but I don't know if that's captured somewhere.

[bors]

☔ The latest upstream changes (presumably #138523) made this pull request unmergeable. Please resolve the merge conflicts.

[fee1-dead]

Has there been any precedent/writings elsewhere that new language features require style RFCs/modifications to the style guide before getting stabilized? I don't really think that that should block this, so I believe the resolve-open-items concern should be marked as resolved and this should go into fcp.

I'll take another look at the compiler implementation once fcp is completed.

[traviscross]

It's in the nightly style procedure document from the style team, which they added here and FCPed here, and then on lang we FCPed acceptance of this blocker here.

[VorpalBlade]

Given that nothing has happened for the style guide since 2023 regarding led chains if I'm reading this correctly it doesn't seem sensible to continue blocking on that. Is that really the up-to-date issue on this?

The style team should be asked to prioritise making a decision on this. In the 2024 survey, let chains is one of the top wanted unstable features (first if you look at "would improve my code", near the top if you look at "unblock use case").

[traviscross]

The style team knows this is a priority.

[bors]

☔ The latest upstream changes (presumably #138478) made this pull request unmergeable. Please resolve the merge conflicts.