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

---

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]

r? @SparrowLii

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[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

[bors]

☔ The latest upstream changes (presumably #119899) made this pull request unmergeable. Please resolve the merge conflicts.

[est31]

Just as a note, I'll be doing a trip from June 7 until June 16, and I don't think I will be bringing my laptop so I won't be able to review this right after the FCP finishes. This might put the stabilization into a later release due to the cutoff being June 20. Feel free to look for other reviewers or wait until I'm back again.

[Kivooeo]

@est31
since you'll be away when the FCP ends, maybe it would make sense to share some of your thoughts on the final review a bit earlier? It doesn't have to be anything formal — even a rough idea of what you'd be looking for could be helpful for another reviewer stepping in while you're offline
given that it's unlikely any new blockers will show up before the FCP finishes, I think it would make sense to share your thoughts now if you have the time
i really appreciate your insights and want to make sure the process goes as smoothly as possible before the cutoff
(also hope you have a great trip!)

[est31]

r=me if FCP passes, PR is squashed, and the nits are addressed

[fee1-dead]

please squash the commits, see https://rustc-dev-guide.rust-lang.org/git.html#squash-your-commits

[fee1-dead]

Nice work, thank you for following through with the reviews! r=me and est31 once the FCP finishes and this last nit is applied