added review on if let guard

Author: Kivooeo

if-let-guard-review.md

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+# `if let` guard in-depth review
+
+Rust’s *`if let` guards* feature allows match arms to include an `if let` condition, enabling a pattern match and a conditional check
+in one guard.  In other words, a match arm can be written as:
+
+```rust
+match value {
+    PATTERN if let GUARD_PAT = GUARD_EXPR => { /* body */ },
+    _ => { /* fallback */ },
+}
+```
+
+This arm is selected **only if** `value` matches `PATTERN` *and* additionally `GUARD_EXPR` matches `GUARD_PAT`.  Crucially, the
+variables bound by `PATTERN` are in scope when evaluating `GUARD_EXPR`, and the variables bound by both patterns
+(`PATTERN` and `GUARD_PAT`) are in scope in the arm’s body.  For example:
+
+```rust
+match foo {
+    Some(x) if let Ok(y) = compute(x) => {
+        // `x` from `Some(x)` and `y` from `Ok(y)` are both available here
+        println!("{}, {}", x, y);
+    }
+    _ => {}
+}
+```
+
+As the original RFC explains, the semantics are that the guard is chosen *if* the main pattern matches and the let-pattern
+in the guard also matches.  (Per design, a match arm may have *either* a normal `if` guard or an `if let` guard, but not both simultaneously.)
+
+Currently the feature is **unstable** and gated by `#![feature(if_let_guard)]`.  If used without the feature, the compiler
+emits error E0658: “`if let` guards are experimental” (with a suggestion to use `matches!(…)` instead).
+Tests in the Rust repository (e.g. `feature-gate.rs`) verify that using `if let` in a guard without the feature flag
+indeed produces this error.
+
+## Syntax and Examples
+
+The syntax for an `if let` guard follows the existing match-guard form, except using `if let` after the pattern:
+
+```text
+match EXPR {
+    PAT if let P = E => BODY,
+    // ...
+}
+```
+
+Here `PAT` is an arbitrary pattern for the match arm, and `if let P = E` is the guard.  You can also combine multiple conditions
+with `&&`.  In fact, because of the related “let chains” feature, you can write multiple `let`-bindings chained by `&&` in the
+same guard.  For example:
+
+```rust
+match value {
+    // Two let-conditions chained with `&&`
+    (Some(a), Some(b)) if let Ok(x) = f(a) && let Ok(y) = g(b) => {
+        // use a, b, x, y here
+    }
+    _ => {}
+}
+```
+
+Examples of valid `if let` guards (with the feature enabled) include:
+
+```rust
+match x {
+    (n, m) if let (0, Some(color)) = (n/10, color_for_code(m)) => { /* ... */ }
+    y if let Some(z) = helper(y) => { /* ... */ }
+    _ => { /* ... */ }
+}
+```
+
+If the syntax is used incorrectly, the compiler gives an appropriate error. For instance, writing `(let PAT = EXPR)` parenthesized
+or using `if (let PAT = EXPR)` (i.e. wrapping a `let` in extra parentheses) is not accepted as a valid guard and instead produces
+a parse error “expected expression, found `let` statement”. This is tested in the Rust UI tests (e.g. `parens.rs` and `feature-gate.rs`). In short, `if let` must appear exactly as a guard after an `if`, not inside extra parentheses.
+
+## Semantics and Variable Scope
+
+When a match arm has an `if let` guard, the evaluation proceeds as follows:
+
+1. The match scrutinee is matched against the arm’s main pattern `PAT`.  Any variables bound by `PAT` become available.
+2. If the main pattern matches, then the guard expression is evaluated.  In that expression, the bindings from `PAT` can be used.  The guard expression is of the form `let GUARD_PAT = GUARD_EXPR`.
+3. The result of `GUARD_EXPR` is matched against `GUARD_PAT`.  If this succeeds, then execution enters the arm’s body.  Otherwise the arm is skipped (and later arms are tried).
+
+Therefore, variables bound in the main pattern `PAT` are “live” during the evaluation of the guard, but any variables bound
+by `GUARD_PAT` only come into existence in the arm body (not in earlier code).  This corresponds directly to the RFC’s reference
+explanation: “the variables of `pat` are bound in `guard_expr`, and the variables of `pat` and `guard_pat` are bound in `body_expr`”.
+
+As an example, consider:
+
+```rust
+match (opt, val) {
+    (Some(x), _) if let Ok(y) = convert(x) => {
+        // Here `x` and `y` are in scope
+        println!("Converted {} into {}", x, y);
+    }
+    _ => {}
+}
+```
+
+Here the pattern `(Some(x), _)` binds `x`. Then `convert(x)` is called, and its result is matched to `Ok(y)`.  If both steps
+succeed, the body can use both `x` and `y`.  If either fails (pattern fails or guard fails), this arm is skipped.
+
+One important restriction is that a single match arm cannot have two `if`-guards.  That is, you cannot write something like
+`PAT if cond1 if let P = E => ...` with two separate `if`s.  You may combine a normal boolean condition with a `let`
+by chaining with `&&`, but only one `if` keyword is allowed.  The RFC explicitly states “An arm may not have both an
+`if` and an `if let` guard” (i.e. you can’t do `if cond && let ...` *and* then another `if`, etc.).
+(You *can* do something like `if let P = E && cond` by writing `if let P = E && cond =>`, treating the boolean as part
+of a let-chain, but that is a single `if` in syntax.)
+
+## Feature Gate and Errors
+
+As of now, `if let` guards are still unstable. The compiler requires the feature flag `#![feature(if_let_guard)]` to enable them.
+If one uses an `if let` guard without the feature, one gets an error similar to:
+
+```
+error[E0658]: `if let` guards are experimental
+   |
+LL |     _ if let true = true => {}
+   |        ^^^^^^^^^^^^^^^^
+   = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
+```
+
+This message is verified by the compiler’s test suite (e.g. `feature-gate.rs`) and comes from the feature-gate code in the parser.
+The tests also ensure the old (`let`-in-`if` without the feature) error is preserved. For example:
+
+```rust
+match () {
+    () if true && let 0 = 1 => {}      // error: `let` expressions are unstable (since no feature)
+    () if let 0 = 1 && true => {}      // error: `if let` guards are experimental
+    _ => {}
+}
+```
+
+The test suite checks that these errors mention both the unstable-let and the experimental guard exactly as above.
+Once the feature is stabilized, these errors will no longer appear.
+
+## Temporaries and Drop Order
+
+A subtle aspect of `if let` guards is the handling of temporaries (and destructor calls) within the guard expression.
+The Rust reference explains that a *match guard* creates its own temporary scope: any temporaries produced by `GUARD_EXPR`
+live only until the guard finishes evaluating. Concretely, this means:
+
+* The `guard_expr` is evaluated *after* matching `PAT` but *before* executing the arm’s body (if taken).
+* Any temporary values created during `guard_expr` are dropped immediately after the guard’s scope ends (i.e. before entering the arm body).
+* If the guard fails, those temporaries are dropped right then, and the compiler proceeds to the next arm.
+
+In effect, the drop semantics are the same as for an ordinary match guard or an `if let` in an `if` expression: no unexpected
+extension of lifetimes. (In Rust 2024 edition, there is a finer rule that even in `if let`
+expressions temporaries drop before the `else` block; but for match guards the effect is that temporaries from the
+guard are dropped before the arm body.)
+
+This behavior is exercised by the existing tests. For example, the `drop-order.rs` UI test uses `Drop`-implementing
+values in nested `if let` guards to verify the precise drop order. Those tests confirm that the values from the inner
+guards are dropped *first*, before values from outer contexts and before finally moving on to other arms. In short, the
+feature does not introduce any new irregularity in drop order: guard expressions are evaluated left-to-right
+(following let-chains semantics) and their temporaries die as soon as the guard completes.
+
+## Lifetimes and Variable Scope
+
+Aside from drop timing, lifetimes of references in the guard work as expected. Because the pattern variables (`PAT` bindings)
+are in scope during `GUARD_EXPR`, one can take references to them or otherwise use them. Any reference or borrow introduced
+by the guard is scoped to the guard and arm body. For example:
+
+```rust
+match &vec {
+    v if let [first, ref rest @ ..] = v[..] => {
+        // `first` and `rest` borrowed from `v` are valid here
+        println!("{}", first);
+    }
+    _ => {}
+}
+```
+
+Here `v` is `&Vec`, and the guard borrows parts of it; those references are valid in the arm body. If a guard binds by value
+(e.g. `if let x = some_moveable`), the usual move/borrow rules apply (see below), but in all cases the scopes follow the match-arm rules.
+
+Moreover, an `if let` guard cannot break exhaustiveness: each arm is either taken or skipped in the usual way.
+A guard cannot cause a pattern to match something it wouldn’t normally match, it only *restricts* a match further.
+Tests like `exhaustive.rs` ensure that match exhaustiveness is checked as usual (you still need a wildcard arm if needed).
+No special exhaustiveness rules are introduced.
+
+## Mutability and Moves
+
+Patterns inside guards obey the normal mutability and move semantics. You can use `mut`, `ref`, or `ref mut`
+in the guard pattern just like in a `let` or match pattern. For example, `if let Some(ref mut x) = foo()` will mutably
+borrow from `foo()`. The borrow-checker treats moves in a guard pattern exactly as it would in a regular pattern: a move of a
+binding only occurs if that branch is actually taken, and subsequent code cannot use a moved value.
+
+This is tested by the **move-guard-if-let** suite.  For instance, consider:
+
+```rust
+fn same_pattern(c: bool) {
+    let mut x: Box<_> = Box::new(1);
+    let v = (1, 2);
+    match v {
+        (1, _) if let y = *x && c => (),
+        (_, 2) if let z = *x => (),     // uses x after move
+        _ => {}
+    }
+}
+```
+
+With `#![feature(if_let_guard)]`, the compiler correctly reports that `x` is moved by the first guard and then used again by
+the second pattern, which is an error. In the test output one sees messages like “`value moved here`”
+and “`value used here after move`” exactly pointing to the `if let` bindings. (These errors match the compiler’s normal behavior,
+confirming that `if let` guards do not bypass the borrow rules.) In contrast, if the pattern had used `ref` (e.g. `if let ref y = x`),
+no move would occur. The test suite also covers using `&&` with or-patterns and ensures borrowck handles those correctly.
+
+In summary, moving or borrowing in an `if let` guard is just like doing so in a regular `if let` or match: the borrow checker
+ensures no use-after-move, and moves only happen if the pattern actually matches. The existing UI tests for moves and mutability
+all pass under the current implementation, so there is no unsoundness here.
+
+## Shadowing and Macros
+
+The usual Rust rules for shadowing and macros apply. An `if let` guard can introduce a new variable that *shadows* an existing one:
+
+```rust
+let x = 10;
+match v {
+    (true, _) if let x = compute() => {
+        // Here the `x` from the guard shadows the outer `x`.
+        println!("{}", x);
+    }
+    _ => {}
+}
+```
+
+This is allowed (just as in ordinary `if let` expressions) and works as expected; the tests (`shadowing.rs`) verify that the
+scoping is consistent.
+
+Macro expansion also works naturally. You can write macros that produce part of the guard. For example:
+
+```rust
+macro_rules! m { () => { Some(5) } }
+match opt {
+    Some(v) if let Some(w) = m!() => { /*...*/ }
+    _ => {}
+}
+```
+
+Since the parser sees the expanded code, `if let` guards inside macros are supported. The Rust tests include cases where macros
+expand to an `if let` guard (fully or partially) to ensure the feature handles macro hygiene correctly. In short, `if let` guards
+are not disabled or altered in macro contexts; they simply follow the normal macro expansion rules of Rust.
+
+## Soundness and Pitfalls
+
+No inherent unsoundness has been found in `if let` guards. They are purely syntactic sugar for nested pattern matching and condition
+checks. All borrow and move checks are done conservatively and correctly. The feature interacts with other parts of the language in
+predictable ways. For example:
+
+* **Refutability:**  An `if let` guard’s pattern is allowed to be refutable (since a failed match simply means skipping the arm). The tests ensure that irrefutable-let warnings do not occur (or can be allowed).
+* **Matching order:**  Guards are evaluated in sequence per arm; if the first part of a let-chain fails, later parts aren’t evaluated (preventing needless moves or panics).
+* **No new invariants:** Guard patterns do not introduce new lifetime or aliasing invariants beyond normal patterns. Temporaries and borrows expire normally.
+
+All of the edge cases are covered by the existing UI tests. For example, the `exhaustive.rs` test confirms that match exhaustiveness
+remains correct when using `if let` guards (i.e. a wildcard arm is needed if not all cases are covered).
+The `typeck.rs` and `type-inference.rs` tests verify that type inference and generic code work through `if let` guards as expected.
+The compiler’s own test suite includes dozens of `if let` guard tests under `src/test/ui/rfcs/rfc-2294-if-let-guard/`,
+and all of them pass with the current implementation.
+
+## Conclusion
+
+The feature is fully implemented in the compiler and exercised by many tests. Its syntax and semantics are clear and consistent with
+existing Rust rules: pattern bindings from the arm are in scope in the guard, and guard bindings are in scope in the arm body.
+The compiler enforces the usual ownership rules (as seen in the move tests) and handles temporaries in a straightforward way.
+
+**Status:** implemented and well-tested, awaiting only formal documentation [(I've also made one)](https://github.com/rust-lang/reference/pull/1823) to be fully ready for a stable release.
+
+**References:** details from RFC 2294 and the current compiler behavior are used above. Each cited source shows the design or
+diagnostics of `if let` guards in action.