Stabilize if let guards (feature(if_let_guard))

[Kivooeo]

Summary

This proposes the stabilization of if let guards (tracking issue: #51114, RFC: rust-lang/rfcs#2294). This feature allows if let expressions to be used directly within match arm guards, enabling conditional pattern matching within guard clauses.

What is being stabilized

The ability to use if let expressions within match arm guards.

Example:

enum Command {
    Run(String),
    Stop,
    Pause,
}

fn process_command(cmd: Command, state: &mut String) {
    match cmd {
        Command::Run(name) if let Some(first_char) = name.chars().next() && first_char.is_ascii_alphabetic() => {
            // Both `name` and `first_char` are available here
            println!("Running command: {} (starts with '{}')", name, first_char);
            state.push_str(&format!("Running {}", name));
        }
        Command::Run(name) => {
            println!("Cannot run command '{}'. Invalid name.", name);
        }
        Command::Stop if state.contains("running") => {
            println!("Stopping current process.");
            state.clear();
        }
        _ => {
            println!("Unhandled command or state.");
        }
    }
}

Motivation

The primary motivation for if let guards is to reduce nesting and improve readability when conditional logic depends on pattern matching. Without this feature, such logic requires nested if let statements within match arms:

// Without if let guards
match value {
    Some(x) => {
        if let Ok(y) = compute(x) {
            // Both `x` and `y` are available here
            println!("{}, {}", x, y);
        }
    }
    _ => {}
}

// With if let guards
match value {
    Some(x) if let Ok(y) = compute(x) => {
        // Both `x` and `y` are available here
        println!("{}, {}", x, y);
    }
    _ => {}
}

Implementation and Testing

The feature has been implemented and tested comprehensively across different scenarios:

Core Functionality Tests

Scoping and variable binding:

• scope.rs - Verifies that bindings created in if let guards are properly scoped and available in match arms
• shadowing.rs - Tests that variable shadowing works correctly within guards
• scoping-consistency.rs - Ensures temporaries in guards remain valid for the duration of their match arms

Type system integration:

• type-inference.rs - Confirms type inference works correctly in if let guards
• typeck.rs - Verifies type mismatches are caught appropriately

Pattern matching semantics:

• exhaustive.rs - Validates that if let guards are correctly handled in exhaustiveness analysis
• move-guard-if-let.rs and move-guard-if-let-chain.rs - Test that conditional moves in guards are tracked correctly by the borrow checker

Error Handling and Diagnostics

• warns.rs - Tests warnings for irrefutable patterns and unreachable code in guards
• parens.rs - Ensures parentheses around let expressions are properly rejected
• macro-expanded.rs - Verifies macro expansions that produce invalid constructs are caught
• guard-mutability-2.rs - Tests mutability and ownership violations in guards
• ast-validate-guards.rs - Validates AST-level syntax restrictions

Drop Order and Temporaries

Key insight: Unlike let_chains in regular if expressions, if let guards do not have drop order inconsistencies because:

1. Match guards are clearly scoped to their arms
2. There is no "else block" equivalent that could cause temporal confusion

• drop-order.rs - Tests that temporaries in guards are dropped at the correct time
• compare-drop-order.rs - Compares drop order between if let guards and nested if let in match arms, confirming they behave identically across all editions
• Add match guard let chain drop order and scoping tests #140981 - A complicated drop order test involved let chain was made by @est31

Edition Compatibility

This feature stabilizes on all editions, unlike let_chains which was limited to edition 2024. This is safe because:

1. if let guards don't suffer from the drop order issues that affected let_chains in regular if expressions
2. The scoping is unambiguous - guards are clearly tied to their match arms
3. Extensive testing confirms identical behavior across all editions

Interactions with Future Features

The lang team has reviewed potential interactions with planned "guard patterns" and determined that stabilizing if let guards now does not create obstacles for future work. The scoping and evaluation semantics established here align with what guard patterns will need.

Unresolved Issues

All blocking issues have been resolved:

• - Add match guard let chain drop order and scoping tests #140981
• - added tests description by @jieyouxu request
• - Concers from @scottmcm about stabilizing this across all editions
• - check if drop order in all edition when using let chains inside if let guard is the same
• - interactions with guard patters
• - pattern bindings drops before guard bindings
• - documentaion

---

Related:

• Tracking Issue: Tracking issue for RFC 2294, "if let guard" #51114
• RFC: propose if let guard rfcs#2294
• Documentation PR: if let guards documentation reference#1823

[rustbot]

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Some changes occurred to the CTFE machinery

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Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

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[traviscross]

cc @est31 @ehuss

[traviscross]

cc @Nadrieril

[SparrowLii]

This needs a fcp so I'd like to roll this to someone more familiar with this feature
r? compiler

[oli-obk]

r? @est31

[Kivooeo]

@est31 huge thanks for everything! From the moment I jumped into this feature, you've been there guiding me through all of this. You patiently answered all my questions, helped me understand the complex parts, and when I wanted to try something new and asked to handle the stabilization PR, you trusted me with it even though I was just getting started

Honestly, most of the work and insights here are yours. You didn't just help technically - you made it possible for someone new like me to actually contribute something meaningful. That kind of mentorship means a lot, really appreciate it!

[traviscross]

@bors r-

I appreciate and share the enthusiasm, but this shouldn't merge until the Reference PR is approved, which it is not yet.

[BoxyUwU]

The reference PR was (before this r-) marked S-waiting-on-stabilization so I would have assumed that means that this was not blocked on the reference PR and that it was approved

[traviscross]

Yes, we on lang-docs could probably use to better document the system here.

In this case, the intended signals were that:

• It was still marked S-waiting-on-review.
• It lacks an approving review.
• This issue is marked S-waiting-on-documentation.

(That the Reference PR was also marked S-waiting-on-stabilization was additive, and is our signal that even after we remove S-waiting-on-review, we still should not merge it until the stabilization is finalized. Sometimes, even then, we hesitate to leave an approving review if the stabilization has not yet finished FCP, as sometimes on the lang side, we change things about the stabilization, and so on the Reference side, we like to take a last look before approving to be sure the Reference PR is still correct at the end of the FCP.)

[bors]

☔ The latest upstream changes (presumably #142220) made this pull request unmergeable. Please resolve the merge conflicts.

[dianne]

I'm sorry to bring this up after the FCP, but having worked some more on implementing guard patterns, I've noticed the drop order here actually isn't quite what I'd expect, and there are in fact still some missing drop order tests. All the tests I can find in #140981 for if let guards are for the temporary drop order among guards in let chains; that order does look good. But I can't find any tests comparing the drop order of if let guard temporaries to the pattern bindings for the match arm the guard is on. Here's an example (playground):

assert_drop_order(1..=3, |o| {
    // `if let` guards' drops are *after* pattern bindings'
    match [LogDrop(o, 2), LogDrop(o, 1)] {
        [x, y] if let z = LogDrop(o, 3) => {}
        _ => unreachable!(),
    }
});
    
assert_drop_order(1..=3, |o| {
    // I would expect this drop order.
    match [LogDrop(o, 3), LogDrop(o, 2)] {
        [x, y] => {
            let z = LogDrop(o, 1);
        }
    }
});

This also means dropck problems (playground):

// `x` is dropped first, resulting in a dropck failure.
match SignificantDrop(0) {
    x if let y = SignificantDrop(&x) => {}
    _ => unreachable!(),
}
// `y` is dropped first; everything's fine.
match SignificantDrop(0) {
    x => {
        let y = SignificantDrop(&x);
    }
}

y still wouldn't be usable in the match arm in that first example even if everything was fixed, due to the binding of x moving out of the scrutinee (iiuc, that first y = SignificantDrop(&x) is relative to the scrutinee, not the binding of x used in the arm). But y should still be usable later in the guard. Binding x by reference would normally also lead to a dropck failure in this case (while still leaving y usable in the arm), but by-ref bindings are dropped last when there's a guard (#142057), so dropck doesn't complain.

[traviscross]

Great catch. That does violate my expectations. Thanks @dianne for bringing that to our attention.

@rfcbot concern drop-order-bug-found-by-dianne

[traviscross]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[traviscross]

Given the near miss -- despite the care we took to avoid it -- it now feels like, for this and these sort of things, maybe we should expect proof that the testing is exhaustive against the code that's expected to be equivalent, e.g. by some automated means to generate tests for all possible cases.

[Kivooeo]

@dianne, this is a valuable catch, there is nothing to sorry about, it's better to find this before we stabilize the feature, rather than after

about your test, the reason i was confident about this part because we had this test compare-if-let-guard which shows that there is not differences between if let inside match's arm and if let guard, but, it doesnt show exactly the problem with dropping match bindings (which i was not knowing about)

i analyzed the MIR and can confirm the problem

    match Some(1) {
        Some(_x) if let Some(_y) = Some(2) => {},
        _ => {}
    }

for some reasons compiler creates guard bindings before pattern bindings

    bb2: {
        _5 = copy ((_6 as Some).0: i32); // _6 = Option::<i32>::Some(const 2_i32);
        _3 = copy ((_1 as Some).0: i32); // _1 = Option::<i32>::Some(const 1_i32);
        goto -> bb3;
    }

this means pattern bindings would be dropped first (reverse order), causing the dropck issues you identified

[dianne]

I believe creating guard bindings before pattern bindings is intentional: it's needed for https://doc.rust-lang.org/reference/expressions/match-expr.html#r-expr.match.guard.value. I think the drops will need manual rescheduling to untwist them¹; some amount of manual rescheduling is already done for or-patterns, so there's precedent. Ideally I'd like to implement this by specifying the drop order explicitly, but that's soft-blocked on #142163² (and it might just be easier anyway to use the drop order we get from lowering the guard, but shuffled to happen after the arm's bindings are dropped).

Footnotes

1. Unless we delay doing by-value bindings for the guard until after by-value bindings for the pattern, but that would require some refactors and also would restrict moving out of a let guard's bindings later in the guard. I don't think that'd be worth avoiding rescheduling drops. ↩

2. let guards with or-patterns in their pattern have a strange drop order that's currently hard to specify, which makes it harder to schedule drops for the arm body all at once. ↩

[Kivooeo]

I'm not sure if the team has already seen #142163, @traviscross, but I wanted to bring it to your attention. It's a fairly significant issue, and if it wasn't brought up at the last meeting a couple of days ago, I think it would be worth putting on the agenda soon (though I'm not entirely familiar with how these meetings are planned)

As for the issue itself, I currently see two possible approaches:

1. Document the current behavior and treat it as non-blocking — as far as I understand, this behavior isn’t exclusive to if let guards but is part of a more general issue. Also, from what I can tell, it's not something that can be used to break soundness — for example, the borrow checker already prevents misuses in the if let context. If we fix this issue after stabilization, as I understand it, this probably won’t break backwards compatibility (please correct me or share your opinion on this point, @dianne)

2. Wait until the issue is resolved, and proceed with stabilization only after that

I’m not saying this isn’t a real issue — it definitely is — but it seems to be more of an external problem rather than one specific to this feature. I'm not sure what the best path forward for this stabilization PR is, so I just wanted to share the two directions which are the most obvious to me

[dianne]

as far as I understand, this behavior isn’t exclusive to if let guards but is part of a more general issue.

That's also my understanding. The only thing that makes if let guards special is that their scrutinee/bindings are created/declared in the match arm's scope. Thus, explicitly specifying the drop order for a match arm as a whole requires explicitly specifying that drop order (which makes it less convenient). That's not the only way to resolve the issue with let guards, though, so I don't think it would need to block let guard stabilization. I think the easiest solution to let guard drop order currently would be to use the drop order we cget from lowering guards, but move them to happen before the arm's pattern's drops, keeping the ordering the same as it is now besides that. This wouldn't require solving the or-patterns issue.

There might also be reason to specify and stabilize let guards, as-is, with guards' bindings and scrutinees being dropped after the arm's pattern's. I've been trying to think in terms of what's needed for guard patterns to find reasons to keep/untwist the drop order, but so far I haven't come up with anything in those terms beyond "implementation convenience". Even for hypothetical if let guard patterns, all I've found so far is that they're more complicated than I previously had thought ^^;

If we fix this issue after stabilization, as I understand it, this probably won’t break backwards compatibility

Fixing #142163 is a breaking change regardless, but I'd consider it mostly orthogonal to let guards. let guards do increase its surface area in a way, by adding more ways to write patterns, but in many cases they'd be written as patterns regardless of whether the language has let guards. I have been in a situation where let guards would have let me write things as patterns that I otherwise wouldn't have been able to, but I also didn't depend on drop order or or-patterns. I'm not sure it's possible to get a perfect picture of how the impact of fixing #142163 changes based on stabilizing let guards, but I think it's minimal. I just happened to find a lot of drop order inconsistencies in a short timeframe because they affect the implementation of guard patterns.

[traviscross]

I'm curious to hear from @est31 here as well. Even if it's part of a more general issue, it'd be better if possible if we could do something here to avoid increasing the surface area of it, and I'm curious to hear from everyone what the options might be there.

[traviscross]

Separately, I'm curious to hear if there might be some structured way that we could test this to assure ourselves that we really have covered all of the possible drop order cases before the next attempt at stabilization.