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Stabilize `if let` guards (`feature(if_let_guard)`) ## Summary This proposes the stabilization of `if let` guards (tracking issue: rust-lang/rust#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: ```rust 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: ```rust // 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`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/scope.rs) - Verifies that bindings created in `if let` guards are properly scoped and available in match arms - [`shadowing.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/shadowing.rs) - Tests that variable shadowing works correctly within guards - [`scoping-consistency.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/scoping-consistency.rs) - Ensures temporaries in guards remain valid for the duration of their match arms **Type system integration:** - [`type-inference.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/type-inference.rs) - Confirms type inference works correctly in `if let` guards - [`typeck.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/typeck.rs) - Verifies type mismatches are caught appropriately **Pattern matching semantics:** - [`exhaustive.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/exhaustive.rs) - Validates that `if let` guards are correctly handled in exhaustiveness analysis - [`move-guard-if-let.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/move-guard-if-let.rs) and [`move-guard-if-let-chain.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/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`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/warns.rs) - Tests warnings for irrefutable patterns and unreachable code in guards - [`parens.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/parens.rs) - Ensures parentheses around `let` expressions are properly rejected - [`macro-expanded.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/macro-expanded.rs) - Verifies macro expansions that produce invalid constructs are caught - [`guard-mutability-2.rs`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/guard-mutability-2.rs) - Tests mutability and ownership violations in guards - [`ast-validate-guards.rs`](5796073c13/tests/ui/rfcs/rfc-2497-if-let-chains/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`](5796073c13/tests/ui/rfcs/rfc-2294-if-let-guard/drop-order.rs) - Check drop order of temporaries create in match guards - [`compare-drop-order.rs`](aef3f5fdf0/tests/ui/rfcs/rfc-2294-if-let-guard/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 - rust-lang/rust#140981 - A complicated drop order test involved `let chain` was made by @est31 - [`drop-order-comparisons-let-chains.rs`](902b4d2878/tests/ui/drop/drop-order-comparisons-let-chains.rs) - Compares drop order between `let chains` in `if let guard` and regular `if` expressions - [`if-let-guards.rs`](5650d716e0/tests/ui/drop/if-let-guards.rs) - Test correctness of drop order for bindings and temporaries - [`if-let-guards-2`](3a6c8c8f3d/tests/ui/drop/if-let-guards-2.rs) - 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 - [x] - rust-lang/rust#140981 - [x] - added tests description by @jieyouxu request - [x] - Concers from @scottmcm about stabilizing this across all editions - [x] - check if drop order in all edition when using `let chains` inside `if let` guard is the same - [x] - interactions with guard patters - [x] - pattern bindings drops before guard bindings https://github.com/rust-lang/rust/pull/143376 - [x] - documentaion (https://github.com/rust-lang/reference/pull/1957) - [ ] (non-blocking) add tests for [this](https://github.com/rust-lang/rust/issues/145237) and [this](https://github.com/rust-lang/rust/pull/141295#issuecomment-3173059821) --- **Related:** - Tracking Issue: rust-lang/rust#51114 - RFC: rust-lang/rfcs#2294 - Documentation PR: https://github.com/rust-lang/reference/pull/1957
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