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 - Check drop order of temporaries create in match guards
• 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
• drop-order-comparisons-let-chains.rs - Compares drop order between let chains in if let guard and regular if expressions
• if-let-guards.rs - Test correctness of drop order for bindings and temporaries
• if-let-guards-2 - The same test as above but more comprehensive and tests more interactions between different features and their drop order, checking that drop order is correct, created by @traviscross

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

• - 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 add a scope for if let guard temporaries and bindings #143376
• - documentaion (specify if let guards with updated scoping rules reference#1957)
• (non-blocking) add tests for this and this

---

Related:

• Tracking Issue: Tracking issue for RFC 2294, "if let guard" #51114
• RFC: propose if let guard rfcs#2294
• Documentation PR: specify if let guards with updated scoping rules reference#1957

[rustbot]

r? @SparrowLii

rustbot has assigned @SparrowLii.
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

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rustbot]

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

cc @rust-lang/rust-analyzer

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[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]

I tested this with regular let chains (outside of if-let guards) and observed the same behavior - pin!() changes the drop order

if let x = LoudDrop("0")
    && let y = pin!(LoudDrop("1"))
{}

This suggests the issue might be related to let chains in general, or specifically to the interaction between let chains and "super let". If I recall correctly, super let was implemented after let chains were stabilized, which could explain this interaction

[Kivooeo]

I did some additional testing and found that the drop order follows a predictable pattern (though this doesn't change the fact that pin!() changes the expected order). Should we consider documenting this behavior?

    if let x = LoudDrop("0")
        && let y = pin!(LoudDrop("1"))
        && let z = pin!(LoudDrop("2"))
        && let w = LoudDrop("3")
    {}
    // 3
    // 0
    // 2
    // 1

The pattern appears to be: drops occur from bottom to top, but any pinned values are moved to the end of the drop queue

[theemathas]

I've filed this as #145328

[Kivooeo]

Hi everyone,

It’s been a while since my last update - quite a lot has happened in the meantime, but the main thing is I'm still here

I’ve noticed that many fixes, including the main concern, have already been merged - big thanks to @dianne for the help, and to @theemathas for spotting some interesting edge cases

Also, the documentation (thanks again to dianne) has been created and marked as waiting-on-stabilization. Am I right in assuming that means it’s in good shape?

So from what I see, the major concerns like the drop order bug and the docs are resolved

I just wanted to check with the team: are there any other blockers before I start extending the test suite and working through the conflicts?

[traviscross]

Thanks for checking in and following up on this. Looking back over the issue, I agree that the primary blockers here have been resolved. I would indeed suggest extending the tests and rebasing the branch.

We haven't yet approved the Reference PR (rust-lang/reference#1957), and that will need to happen before we merge this PR, but on a skim, it looks to be in good shape; this won't hold things up unduly. We'll probably want to see the extended tests before approving this anyway, so as to compare the behavior with what we're documenting.

If you could leave a note here when the tests are extended and the branch is rebased, I'll plan to have a look, resolve the concern I had filed, restart the proposed FCP, and finish reviewing the Reference PR.

[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[Kivooeo]

Conflicts are resolved

For test part from what I see is dianne added a test for drop order which is covering the drop order fix, I updated description and specify 3 new tests in there (one of them I adopted from here #141295 (comment))

And as for tests for this and this I will add them later on if noone else will do this before, and point them in unresolved issues as non blocking

[bors]

☔ The latest upstream changes (presumably #148049) made this pull request unmergeable. Please resolve the merge conflicts.