Auto merge of rust-lang#132833 - est31:stabilize_let_chains, r=fee1-dead

Stabilize let chains in the 2024 edition

# Stabilization report

This proposes the stabilization of `let_chains` ([tracking issue], [RFC 2497]) in the [2024 edition] of Rust.

[tracking issue]: rust-lang#53667
[RFC 2497]: rust-lang/rfcs#2497
[2024 edition]: https://doc.rust-lang.org/nightly/edition-guide/rust-2024/index.html

## What is being stabilized

The ability to `&&`-chain `let` statements inside `if` and `while` is being stabilized, allowing intermixture with boolean expressions. The patterns inside the `let` sub-expressions can be irrefutable or refutable.

```Rust
struct FnCall<'a> {
    fn_name: &'a str,
    args: Vec<i32>,
}

fn is_legal_ident(s: &str) -> bool {
    s.chars()
        .all(|c| ('a'..='z').contains(&c) || ('A'..='Z').contains(&c))
}

impl<'a> FnCall<'a> {
    fn parse(s: &'a str) -> Option<Self> {
        if let Some((fn_name, after_name)) = s.split_once("(")
            && !fn_name.is_empty()
            && is_legal_ident(fn_name)
            && let Some((args_str, "")) = after_name.rsplit_once(")")
        {
            let args = args_str
                .split(',')
                .map(|arg| arg.parse())
                .collect::<Result<Vec<_>, _>>();
            args.ok().map(|args| FnCall { fn_name, args })
        } else {
            None
        }
    }
    fn exec(&self) -> Option<i32> {
        let iter = self.args.iter().copied();
        match self.fn_name {
            "sum" => Some(iter.sum()),
            "max" => iter.max(),
            "min" => iter.min(),
            _ => None,
        }
    }
}

fn main() {
    println!("{:?}", FnCall::parse("sum(1,2,3)").unwrap().exec());
    println!("{:?}", FnCall::parse("max(4,5)").unwrap().exec());
}
```

The feature will only be stabilized for the 2024 edition and future editions. Users of past editions will get an error with a hint to update the edition.

closes rust-lang#53667

## Why 2024 edition?

Rust generally tries to ship new features to all editions. So even the oldest editions receive the newest features. However, sometimes a feature requires a breaking change so much that offering the feature without the breaking change makes no sense. This occurs rarely, but has happened in the 2018 edition already with `async` and `await` syntax. It required an edition boundary in order for `async`/`await` to become keywords, and the entire feature foots on those keywords.

In the instance of let chains, the issue is the drop order of `if let` chains. If we want `if let` chains to be compatible with `if let`, drop order makes it hard for us to [generate correct MIR]. It would be strange to have different behaviour for `if let ... {}` and `if true && let ... {}`. So it's better to [stay consistent with `if let`].

In edition 2024, [drop order changes] have been introduced to make `if let` temporaries be lived more shortly. These changes also affected `if let` chains. These changes make sense even if you don't take the `if let` chains MIR generation problem into account. But if we want to use them as the solution to the MIR generation problem, we need to restrict let chains to edition 2024 and beyond: for let chains, it's not just a change towards more sensible behaviour, but one required for correct function.

[generate correct MIR]: rust-lang#104843
[stay consistent with `if let`]: rust-lang#103293 (comment)
[drop order changes]: rust-lang#124085

## Introduction considerations

As edition 2024 is very new, this stabilization PR only makes it possible to use let chains on 2024 without that feature gate, it doesn't mark that feature gate as stable/removed. I would propose to continue offering the `let_chains` feature (behind a feature gate) for a limited time (maybe 3 months after stabilization?) on older editions to allow nightly users to adopt edition 2024 at their own pace. After that, the feature gate shall be marked as *stabilized*, not removed, and replaced by an error on editions 2021 and below.

## Implementation history

* History from before March 14, 2022 can be found in the [original stabilization PR] that was reverted.
* rust-lang#94927
* rust-lang#94951
* rust-lang#94974
* rust-lang#95008
* rust-lang#97295
* rust-lang#98633
* rust-lang#99731
* rust-lang#102394
* rust-lang#100526
* rust-lang#100538
* rust-lang#102998
* rust-lang#103405
* rust-lang#103293
* rust-lang#107251
* rust-lang#110568
* rust-lang#115677
* rust-lang#117743
* rust-lang#117770
* rust-lang#118191
* rust-lang#119554
* rust-lang#129394
* rust-lang#132828
* rust-lang/reference#1179
* rust-lang/reference#1251
* rust-lang/rustfmt#5910

[original stabilization PR]: rust-lang#94927

## Adoption history

### In the compiler

* History before March 14, 2022 can be found in the [original stabilization PR].
* rust-lang#115983
* rust-lang#116549
* rust-lang#116688

### Outside of the compiler

* rust-lang/rust-clippy#11750
* [rspack](https://github.com/web-infra-dev/rspack)
* [risingwave](https://github.com/risingwavelabs/risingwave)
* [dylint](https://github.com/trailofbits/dylint)
* [convex-backend](https://github.com/get-convex/convex-backend)
* [tikv](https://github.com/tikv/tikv)
* [Daft](https://github.com/Eventual-Inc/Daft)
* [greptimedb](https://github.com/GreptimeTeam/greptimedb)

## Tests

<details>

### Intentional restrictions

[`partially-macro-expanded.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/partially-macro-expanded.rs), [`macro-expanded.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/macro-expanded.rs): it is possible to use macros to expand to both the pattern and the expression inside a let chain, but not to the entire `let pat = expr` operand.
[`parens.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/parens.rs): `if (let pat = expr)` is not allowed in chains
[`ensure-that-let-else-does-not-interact-with-let-chains.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs): `let...else` doesn't support chaining.

### Overlap with match guards

[`move-guard-if-let-chain.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/move-guard-if-let-chain.rs): test for the `use moved value` error working well in match guards. could maybe be extended with let chains that have more than one `let`
[`shadowing.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/shadowing.rs): shadowing in if let guards works as expected
[`ast-validate-guards.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/ast-validate-guards.rs): let chains in match guards require the match guards feature gate

### Simple cases from the early days

PR rust-lang#88642 has added some tests with very simple usages of `let else`, mostly as regression tests to early bugs.

[`then-else-blocks.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/then-else-blocks.rs)
[`ast-lowering-does-not-wrap-let-chains.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs)
[`issue-90722.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/issue-90722.rs)
[`issue-92145.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/issue-92145.rs)

### Drop order/MIR scoping tests

[`issue-100276.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/drop/issue-100276.rs): let expressions on RHS aren't terminating scopes
[`drop_order.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/drop/drop_order.rs): exhaustive temporary drop order test for various Rust constructs, including let chains
[`scope.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/scope.rs): match guard scoping test
[`drop-scope.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2294-if-let-guard/drop-scope.rs): another match guard scoping test, ensuring that temporaries in if-let guards live for the arm
[`drop_order_if_let_rescope.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/drop/drop_order_if_let_rescope.rs): if let rescoping on edition 2024, including chains
[`mir_let_chains_drop_order.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/mir/mir_let_chains_drop_order.rs): comprehensive drop order test for let chains, distinguishes editions 2021 and 2024.
[`issue-99938.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/issue-99938.rs), [`issue-99852.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/mir/issue-99852.rs) both bad MIR ICEs fixed by rust-lang#102394

### Linting

[`irrefutable-lets.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/irrefutable-lets.rs): trailing and leading irrefutable let patterns get linted for, others don't. The lint is turned off for `else if`.
[`issue-121070-let-range.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/lint/issue-121070-let-range.rs): regression test for false positive of the unused parens lint, precedence requires the `()`s here

### Parser: intentional restrictions

[`disallowed-positions.rs`](https://github.com/rust-lang/rust/blob/2128d8df0e858edcbe6a0861bac948b88b7fabc3/tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.rs): `let` in expression context is rejected everywhere except at the top level
[`invalid-let-in-a-valid-let-context.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-2497-if-let-chains/invalid-let-in-a-valid-let-context.rs): nested `let` is not allowed (let's are no legal expressions just because they are allowed in `if` and `while`).

### Parser: recovery

[`issue-103381.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/parser/issues/issue-103381.rs): Graceful recovery of incorrect chaining of `if` and `if let`
[`semi-in-let-chain.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/parser/semi-in-let-chain.rs): Ensure that stray `;`s in let chains give nice errors (`if_chain!` users might be accustomed to `;`s)
[`deli-ident-issue-1.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/parser/deli-ident-issue-1.rs), [`brace-in-let-chain.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/parser/brace-in-let-chain.rs): Ensure that stray unclosed `{`s in let chains give nice errors and hints

### Misc

[`conflicting_bindings.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/pattern/usefulness/conflicting_bindings.rs): the conflicting bindings check also works in let chains. Personally, I'd extend it to chains with multiple let's as well.
[`let-chains-attr.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/expr/if/attrs/let-chains-attr.rs): attributes work on let chains

### Tangential tests with `#![feature(let_chains)]`

[`if-let.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/coverage/branch/if-let.rs): MC/DC coverage tests for let chains
[`logical_or_in_conditional.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/mir-opt/building/logical_or_in_conditional.rs): not really about let chains, more about dropping/scoping behaviour of `||`
[`stringify.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/macros/stringify.rs): exhaustive test of the `stringify` macro
[`expanded-interpolation.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/unpretty/expanded-interpolation.rs), [`expanded-exhaustive.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/unpretty/expanded-exhaustive.rs): Exhaustive test of `-Zunpretty`
[`diverges-not.rs`](https://github.com/rust-lang/rust/blob/4adafcf40aa6064d2bbcb44bc1a50b3b1e86e5e0/tests/ui/rfcs/rfc-0000-never_patterns/diverges-not.rs): Never type, mostly tangential to let chains

</details>

## Possible future work

* There is proposals to allow `if let Pat(bindings) = expr {}` to be written as `if expr is Pat(bindings) {}` ([RFC 3573]). `if let` chains are a natural extension of the already existing `if let` syntax, and I'd argue orthogonal towards `is` syntax.
  * rust-lang/lang-team#297
* One could have similar chaining inside `let ... else` statements. There is no proposed RFC for this however, nor is it implemented on nightly.
* Match guards have the `if` keyword as well, but on stable Rust, they don't support `let`. The functionality is available via an unstable feature ([`if_let_guard` tracking issue]). Stabilization of let chains affects this feature in so far as match guards containing let chains now only need the `if_let_guard` feature gate be present instead of also the `let_chains` feature (NOTE: this PR doesn't implement this simplification, it's left for future work).

[RFC 3573]: rust-lang/rfcs#3573
[`if_let_guard` tracking issue]: rust-lang#51114

## Open questions / blockers

- [ ] bad recovery if you don't put a `let` (I don't think this is a blocker): [rust-lang#117977](rust-lang#117977)
- [x] An instance where a temporary lives shorter than with nested ifs, breaking compilation: [rust-lang#103476](rust-lang#103476). Personally I don't think this is a blocker either, as it's an edge case. Edit: turns out to not reproduce in edition 2025 any more, due to let rescoping. regression test added in rust-lang#133093
- [x] One should probably extend the tests for `move-guard-if-let-chain.rs` and `conflicting_bindings.rs` to have chains with multiple let's: done in 133093
- [x] Parsing rejection tests: addressed by rust-lang#132828
- [x] [Style](https://rust-lang.zulipchat.com/#narrow/channel/346005-t-style/topic/let.20chains.20stabilization.20and.20formatting): rust-lang#139456
- [x] rust-lang#86730 explicitly mentions `let_else`. I think we can live with `let pat = expr` not evaluating as `expr` for macro_rules macros, especially given that `let pat = expr` is not a legal expression anywhere except inside `if` and `while`.
- [x] Documentation in the reference: rust-lang/reference#1740
- [x] Add chapter to the Rust 2024 [edition guide]: rust-lang/edition-guide#337
- [x] Resolve open questions on desired drop order.

[original reference PR]: rust-lang/reference#1179
[edition guide]: https://github.com/rust-lang/edition-guide

Author: bors

compiler/rustc_parse/src/parser/expr.rs

@@ -26,6 +26,7 @@ use rustc_macros::Subdiagnostic;
 use rustc_session::errors::{ExprParenthesesNeeded, report_lit_error};
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
+use rustc_span::edition::Edition;
 use rustc_span::source_map::{self, Spanned};
 use rustc_span::{BytePos, ErrorGuaranteed, Ident, Pos, Span, Symbol, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
@@ -2602,7 +2603,10 @@ impl<'a> Parser<'a> {
     /// Parses an `if` expression (`if` token already eaten).
     fn parse_expr_if(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let cond = self.parse_expr_cond()?;
+        // Scoping code checks the top level edition of the `if`; let's match it here.
+        // The `CondChecker` also checks the edition of the `let` itself, just to make sure.
+        let let_chains_policy = LetChainsPolicy::EditionDependent { current_edition: lo.edition() };
+        let cond = self.parse_expr_cond(let_chains_policy)?;
         self.parse_if_after_cond(lo, cond)
     }
 
@@ -2711,18 +2715,17 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses the condition of a `if` or `while` expression.
+    ///
+    /// The specified `edition` in `let_chains_policy` should be that of the whole `if` construct,
+    /// i.e. the same span we use to later decide whether the drop behaviour should be that of
+    /// edition `..=2021` or that of `2024..`.
     // Public because it is used in rustfmt forks such as https://github.com/tucant/rustfmt/blob/30c83df9e1db10007bdd16dafce8a86b404329b2/src/parse/macros/html.rs#L57 for custom if expressions.
-    pub fn parse_expr_cond(&mut self) -> PResult<'a, P<Expr>> {
+    pub fn parse_expr_cond(&mut self, let_chains_policy: LetChainsPolicy) -> PResult<'a, P<Expr>> {
         let attrs = self.parse_outer_attributes()?;
         let (mut cond, _) =
             self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, attrs)?;
 
-        CondChecker::new(self).visit_expr(&mut cond);
-
-        if let ExprKind::Let(_, _, _, Recovered::No) = cond.kind {
-            // Remove the last feature gating of a `let` expression since it's stable.
-            self.psess.gated_spans.ungate_last(sym::let_chains, cond.span);
-        }
+        CondChecker::new(self, let_chains_policy).visit_expr(&mut cond);
 
         Ok(cond)
     }
@@ -3017,7 +3020,8 @@ impl<'a> Parser<'a> {
 
     /// Parses a `while` or `while let` expression (`while` token already eaten).
     fn parse_expr_while(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_expr_cond().map_err(|mut err| {
+        let policy = LetChainsPolicy::EditionDependent { current_edition: lo.edition() };
+        let cond = self.parse_expr_cond(policy).map_err(|mut err| {
             err.span_label(lo, "while parsing the condition of this `while` expression");
             err
         })?;
@@ -3401,17 +3405,17 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_match_arm_guard(&mut self) -> PResult<'a, Option<P<Expr>>> {
-        // Used to check the `let_chains` and `if_let_guard` features mostly by scanning
+        // Used to check the `if_let_guard` feature mostly by scanning
         // `&&` tokens.
-        fn check_let_expr(expr: &Expr) -> (bool, bool) {
+        fn has_let_expr(expr: &Expr) -> bool {
             match &expr.kind {
                 ExprKind::Binary(BinOp { node: BinOpKind::And, .. }, lhs, rhs) => {
-                    let lhs_rslt = check_let_expr(lhs);
-                    let rhs_rslt = check_let_expr(rhs);
-                    (lhs_rslt.0 || rhs_rslt.0, false)
+                    let lhs_rslt = has_let_expr(lhs);
+                    let rhs_rslt = has_let_expr(rhs);
+                    lhs_rslt || rhs_rslt
                 }
-                ExprKind::Let(..) => (true, true),
-                _ => (false, true),
+                ExprKind::Let(..) => true,
+                _ => false,
             }
         }
         if !self.eat_keyword(exp!(If)) {
@@ -3422,14 +3426,9 @@ impl<'a> Parser<'a> {
         let if_span = self.prev_token.span;
         let mut cond = self.parse_match_guard_condition()?;
 
-        CondChecker::new(self).visit_expr(&mut cond);
+        CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
 
-        let (has_let_expr, does_not_have_bin_op) = check_let_expr(&cond);
-        if has_let_expr {
-            if does_not_have_bin_op {
-                // Remove the last feature gating of a `let` expression since it's stable.
-                self.psess.gated_spans.ungate_last(sym::let_chains, cond.span);
-            }
+        if has_let_expr(&cond) {
             let span = if_span.to(cond.span);
             self.psess.gated_spans.gate(sym::if_let_guard, span);
         }
@@ -3456,7 +3455,7 @@ impl<'a> Parser<'a> {
                     unreachable!()
                 };
                 self.psess.gated_spans.ungate_last(sym::guard_patterns, cond.span);
-                CondChecker::new(self).visit_expr(&mut cond);
+                CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
                 let right = self.prev_token.span;
                 self.dcx().emit_err(errors::ParenthesesInMatchPat {
                     span: vec![left, right],
@@ -4027,7 +4026,14 @@ pub(crate) enum ForbiddenLetReason {
     NotSupportedParentheses(#[primary_span] Span),
 }
 
-/// Visitor to check for invalid/unstable use of `ExprKind::Let` that can't
+/// Whether let chains are allowed on all editions, or it's edition dependent (allowed only on
+/// 2024 and later). In case of edition dependence, specify the currently present edition.
+pub enum LetChainsPolicy {
+    AlwaysAllowed,
+    EditionDependent { current_edition: Edition },
+}
+
+/// Visitor to check for invalid use of `ExprKind::Let` that can't
 /// easily be caught in parsing. For example:
 ///
 /// ```rust,ignore (example)
@@ -4038,19 +4044,29 @@ pub(crate) enum ForbiddenLetReason {
 /// ```
 struct CondChecker<'a> {
     parser: &'a Parser<'a>,
+    let_chains_policy: LetChainsPolicy,
+    depth: u32,
     forbid_let_reason: Option<ForbiddenLetReason>,
     missing_let: Option<errors::MaybeMissingLet>,
     comparison: Option<errors::MaybeComparison>,
 }
 
 impl<'a> CondChecker<'a> {
-    fn new(parser: &'a Parser<'a>) -> Self {
-        CondChecker { parser, forbid_let_reason: None, missing_let: None, comparison: None }
+    fn new(parser: &'a Parser<'a>, let_chains_policy: LetChainsPolicy) -> Self {
+        CondChecker {
+            parser,
+            forbid_let_reason: None,
+            missing_let: None,
+            comparison: None,
+            let_chains_policy,
+            depth: 0,
+        }
     }
 }
 
 impl MutVisitor for CondChecker<'_> {
     fn visit_expr(&mut self, e: &mut P<Expr>) {
+        self.depth += 1;
         use ForbiddenLetReason::*;
 
         let span = e.span;
@@ -4065,8 +4081,16 @@ impl MutVisitor for CondChecker<'_> {
                             comparison: self.comparison,
                         },
                     ));
-                } else {
-                    self.parser.psess.gated_spans.gate(sym::let_chains, span);
+                } else if self.depth > 1 {
+                    // Top level `let` is always allowed; only gate chains
+                    match self.let_chains_policy {
+                        LetChainsPolicy::AlwaysAllowed => (),
+                        LetChainsPolicy::EditionDependent { current_edition } => {
+                            if !current_edition.at_least_rust_2024() || !span.at_least_rust_2024() {
+                                self.parser.psess.gated_spans.gate(sym::let_chains, span);
+                            }
+                        }
+                    }
                 }
             }
             ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, _, _) => {
@@ -4168,5 +4192,6 @@ impl MutVisitor for CondChecker<'_> {
                 // These would forbid any let expressions they contain already.
             }
         }
+        self.depth -= 1;
     }
 }

tests/mir-opt/building/logical_or_in_conditional.rs

@@ -1,6 +1,6 @@
 // skip-filecheck
 //@ compile-flags: -Z validate-mir
-#![feature(let_chains)]
+//@ edition: 2024
 struct Droppy(u8);
 impl Drop for Droppy {
     fn drop(&mut self) {

tests/mir-opt/building/logical_or_in_conditional.test_complex.built.after.mir

@@ -19,7 +19,7 @@ fn test_complex() -> () {
     bb0: {
         StorageLive(_1);
         StorageLive(_2);
-        _2 = E::f() -> [return: bb1, unwind: bb34];
+        _2 = E::f() -> [return: bb1, unwind: bb35];
     }
 
     bb1: {
@@ -42,7 +42,7 @@ fn test_complex() -> () {
 
     bb5: {
         StorageLive(_4);
-        _4 = always_true() -> [return: bb6, unwind: bb34];
+        _4 = always_true() -> [return: bb6, unwind: bb35];
     }
 
     bb6: {
@@ -64,7 +64,7 @@ fn test_complex() -> () {
     }
 
     bb9: {
-        drop(_7) -> [return: bb11, unwind: bb34];
+        drop(_7) -> [return: bb11, unwind: bb35];
     }
 
     bb10: {
@@ -78,7 +78,7 @@ fn test_complex() -> () {
     }
 
     bb12: {
-        drop(_7) -> [return: bb13, unwind: bb34];
+        drop(_7) -> [return: bb13, unwind: bb35];
     }
 
     bb13: {
@@ -98,7 +98,7 @@ fn test_complex() -> () {
     }
 
     bb15: {
-        drop(_10) -> [return: bb17, unwind: bb34];
+        drop(_10) -> [return: bb17, unwind: bb35];
     }
 
     bb16: {
@@ -113,11 +113,12 @@ fn test_complex() -> () {
 
     bb18: {
         _1 = const ();
+        StorageDead(_2);
         goto -> bb22;
     }
 
     bb19: {
-        drop(_10) -> [return: bb20, unwind: bb34];
+        drop(_10) -> [return: bb20, unwind: bb35];
     }
 
     bb20: {
@@ -127,6 +128,7 @@ fn test_complex() -> () {
     }
 
     bb21: {
+        StorageDead(_2);
         _1 = const ();
         goto -> bb22;
     }
@@ -135,18 +137,17 @@ fn test_complex() -> () {
         StorageDead(_8);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_2);
         StorageDead(_1);
         StorageLive(_11);
-        _11 = always_true() -> [return: bb23, unwind: bb34];
+        _11 = always_true() -> [return: bb23, unwind: bb35];
     }
 
     bb23: {
         switchInt(move _11) -> [0: bb25, otherwise: bb24];
     }
 
     bb24: {
-        goto -> bb32;
+        goto -> bb33;
     }
 
     bb25: {
@@ -155,7 +156,7 @@ fn test_complex() -> () {
 
     bb26: {
         StorageLive(_12);
-        _12 = E::f() -> [return: bb27, unwind: bb34];
+        _12 = E::f() -> [return: bb27, unwind: bb35];
     }
 
     bb27: {
@@ -178,21 +179,26 @@ fn test_complex() -> () {
 
     bb31: {
         _0 = const ();
-        goto -> bb33;
+        StorageDead(_12);
+        goto -> bb34;
     }
 
     bb32: {
-        _0 = const ();
+        StorageDead(_12);
         goto -> bb33;
     }
 
     bb33: {
+        _0 = const ();
+        goto -> bb34;
+    }
+
+    bb34: {
         StorageDead(_11);
-        StorageDead(_12);
         return;
     }
 
-    bb34 (cleanup): {
+    bb35 (cleanup): {
         resume;
     }
 }

tests/ui/deriving/auxiliary/malicious-macro.rs

@@ -1,4 +1,4 @@
-#![feature(let_chains)]
+//@ edition: 2024
 
 extern crate proc_macro;
 

tests/ui/drop/drop-order-comparisons.e2021.fixed

@@ -24,7 +24,7 @@
 //@ [e2024] edition: 2024
 //@ run-pass
 
-#![feature(let_chains)]
+#![cfg_attr(e2021, feature(let_chains))]
 #![cfg_attr(e2021, warn(rust_2024_compatibility))]
 
 fn t_bindings() {

tests/ui/drop/drop-order-comparisons.rs

@@ -24,7 +24,7 @@
 //@ [e2024] edition: 2024
 //@ run-pass
 
-#![feature(let_chains)]
+#![cfg_attr(e2021, feature(let_chains))]
 #![cfg_attr(e2021, warn(rust_2024_compatibility))]
 
 fn t_bindings() {

tests/ui/drop/drop_order.rs

@@ -2,9 +2,10 @@
 //@ compile-flags: -Z validate-mir
 //@ revisions: edition2021 edition2024
 //@ [edition2021] edition: 2021
+//@ [edition2024] compile-flags: -Z lint-mir
 //@ [edition2024] edition: 2024
 
-#![feature(let_chains)]
+#![cfg_attr(edition2021, feature(let_chains))]
 
 use std::cell::RefCell;
 use std::convert::TryInto;

tests/ui/drop/drop_order_if_let_rescope.rs

@@ -1,8 +1,6 @@
 //@ run-pass
 //@ edition:2024
-//@ compile-flags: -Z validate-mir
-
-#![feature(let_chains)]
+//@ compile-flags: -Z validate-mir -Z lint-mir
 
 use std::cell::RefCell;
 use std::convert::TryInto;

tests/ui/drop/issue-100276.rs

@@ -1,6 +1,11 @@
 //@ check-pass
 //@ compile-flags: -Z validate-mir
-#![feature(let_chains)]
+//@ revisions: edition2021 edition2024
+//@ [edition2021] edition: 2021
+//@ [edition2024] compile-flags: -Z lint-mir
+//@ [edition2024] edition: 2024
+
+#![cfg_attr(edition2021, feature(let_chains))]
 
 fn let_chains(entry: std::io::Result<std::fs::DirEntry>) {
     if let Ok(entry) = entry

tests/ui/expr/if/attrs/let-chains-attr.rs

@@ -1,6 +1,5 @@
 //@ check-pass
-
-#![feature(let_chains)]
+//@ edition:2024
 
 #[cfg(false)]
 fn foo() {