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bors 8bf5a8d12f Auto merge of #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]: https://github.com/rust-lang/rust/issues/53667
[RFC 2497]: https://github.com/rust-lang/rfcs/pull/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 #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]: https://github.com/rust-lang/rust/issues/104843
[stay consistent with `if let`]: https://github.com/rust-lang/rust/pull/103293#issuecomment-1293408574
[drop order changes]: https://github.com/rust-lang/rust/issues/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.
* https://github.com/rust-lang/rust/pull/94927
* https://github.com/rust-lang/rust/pull/94951
* https://github.com/rust-lang/rust/pull/94974
* https://github.com/rust-lang/rust/pull/95008
* https://github.com/rust-lang/rust/pull/97295
* https://github.com/rust-lang/rust/pull/98633
* https://github.com/rust-lang/rust/pull/99731
* https://github.com/rust-lang/rust/pull/102394
* https://github.com/rust-lang/rust/pull/100526
* https://github.com/rust-lang/rust/pull/100538
* https://github.com/rust-lang/rust/pull/102998
* https://github.com/rust-lang/rust/pull/103405
* https://github.com/rust-lang/rust/pull/103293
* https://github.com/rust-lang/rust/pull/107251
* https://github.com/rust-lang/rust/pull/110568
* https://github.com/rust-lang/rust/pull/115677
* https://github.com/rust-lang/rust/pull/117743
* https://github.com/rust-lang/rust/pull/117770
* https://github.com/rust-lang/rust/pull/118191
* https://github.com/rust-lang/rust/pull/119554
* https://github.com/rust-lang/rust/pull/129394
* https://github.com/rust-lang/rust/pull/132828
* https://github.com/rust-lang/reference/pull/1179
* https://github.com/rust-lang/reference/pull/1251
* https://github.com/rust-lang/rustfmt/pull/5910

[original stabilization PR]: https://github.com/rust-lang/rust/pull/94927

## Adoption history

### In the compiler

* History before March 14, 2022 can be found in the [original stabilization PR].
* https://github.com/rust-lang/rust/pull/115983
* https://github.com/rust-lang/rust/pull/116549
* https://github.com/rust-lang/rust/pull/116688

### Outside of the compiler

* https://github.com/rust-lang/rust-clippy/pull/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`](4adafcf40a/tests/ui/rfcs/rfc-2294-if-let-guard/partially-macro-expanded.rs), [`macro-expanded.rs`](4adafcf40a/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`](4adafcf40a/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`](4adafcf40a/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`](4adafcf40a/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`](4adafcf40a/tests/ui/rfcs/rfc-2294-if-let-guard/shadowing.rs): shadowing in if let guards works as expected
[`ast-validate-guards.rs`](4adafcf40a/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 #88642 has added some tests with very simple usages of `let else`, mostly as regression tests to early bugs.

[`then-else-blocks.rs`](4adafcf40a/tests/ui/rfcs/rfc-2497-if-let-chains/then-else-blocks.rs)
[`ast-lowering-does-not-wrap-let-chains.rs`](4adafcf40a/tests/ui/rfcs/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs)
[`issue-90722.rs`](4adafcf40a/tests/ui/rfcs/rfc-2497-if-let-chains/issue-90722.rs)
[`issue-92145.rs`](4adafcf40a/tests/ui/rfcs/rfc-2497-if-let-chains/issue-92145.rs)

### Drop order/MIR scoping tests

[`issue-100276.rs`](4adafcf40a/tests/ui/drop/issue-100276.rs): let expressions on RHS aren't terminating scopes
[`drop_order.rs`](4adafcf40a/tests/ui/drop/drop_order.rs): exhaustive temporary drop order test for various Rust constructs, including let chains
[`scope.rs`](4adafcf40a/tests/ui/rfcs/rfc-2294-if-let-guard/scope.rs): match guard scoping test
[`drop-scope.rs`](4adafcf40a/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`](4adafcf40a/tests/ui/drop/drop_order_if_let_rescope.rs): if let rescoping on edition 2024, including chains
[`mir_let_chains_drop_order.rs`](4adafcf40a/tests/ui/mir/mir_let_chains_drop_order.rs): comprehensive drop order test for let chains, distinguishes editions 2021 and 2024.
[`issue-99938.rs`](4adafcf40a/tests/ui/rfcs/rfc-2497-if-let-chains/issue-99938.rs), [`issue-99852.rs`](4adafcf40a/tests/ui/mir/issue-99852.rs) both bad MIR ICEs fixed by #102394

### Linting

[`irrefutable-lets.rs`](4adafcf40a/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`](4adafcf40a/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`](2128d8df0e/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`](4adafcf40a/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`](4adafcf40a/tests/ui/parser/issues/issue-103381.rs): Graceful recovery of incorrect chaining of `if` and `if let`
[`semi-in-let-chain.rs`](4adafcf40a/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`](4adafcf40a/tests/ui/parser/deli-ident-issue-1.rs), [`brace-in-let-chain.rs`](4adafcf40a/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`](4adafcf40a/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`](4adafcf40a/tests/ui/expr/if/attrs/let-chains-attr.rs): attributes work on let chains

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

[`if-let.rs`](4adafcf40a/tests/coverage/branch/if-let.rs): MC/DC coverage tests for let chains
[`logical_or_in_conditional.rs`](4adafcf40a/tests/mir-opt/building/logical_or_in_conditional.rs): not really about let chains, more about dropping/scoping behaviour of `||`
[`stringify.rs`](4adafcf40a/tests/ui/macros/stringify.rs): exhaustive test of the `stringify` macro
[`expanded-interpolation.rs`](4adafcf40a/tests/ui/unpretty/expanded-interpolation.rs), [`expanded-exhaustive.rs`](4adafcf40a/tests/ui/unpretty/expanded-exhaustive.rs): Exhaustive test of `-Zunpretty`
[`diverges-not.rs`](4adafcf40a/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.
  * https://github.com/rust-lang/lang-team/issues/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]: https://github.com/rust-lang/rfcs/pull/3573
[`if_let_guard` tracking issue]: https://github.com/rust-lang/rust/issues/51114

## Open questions / blockers

- [ ] bad recovery if you don't put a `let` (I don't think this is a blocker): [#117977](https://github.com/rust-lang/rust/issues/117977)
- [x] An instance where a temporary lives shorter than with nested ifs, breaking compilation: [#103476](https://github.com/rust-lang/rust/issues/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 #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 https://github.com/rust-lang/rust/pull/132828
- [x] [Style](https://rust-lang.zulipchat.com/#narrow/channel/346005-t-style/topic/let.20chains.20stabilization.20and.20formatting): https://github.com/rust-lang/rust/pull/139456
- [x] https://github.com/rust-lang/rust/issues/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: https://github.com/rust-lang/reference/pull/1740
- [x] Add chapter to the Rust 2024 [edition guide]: https://github.com/rust-lang/edition-guide/pull/337
- [x] Resolve open questions on desired drop order.

[original reference PR]: https://github.com/rust-lang/reference/pull/1179
[edition guide]: https://github.com/rust-lang/edition-guide
2025-04-22 07:54:10 +00:00
..
rustc
rustc_abi Initial UnsafePinned/UnsafeUnpin impl [Part 1: Libs] 2025-04-13 01:11:04 -04:00
rustc_arena
rustc_ast Auto merge of #139897 - nnethercote:rm-OpenDelim-CloseDelim, r=petrochenkov 2025-04-22 01:15:06 +00:00
rustc_ast_ir
rustc_ast_lowering Rollup merge of #139615 - nnethercote:rm-name_or_empty, r=jdonszelmann 2025-04-18 05:16:29 +02:00
rustc_ast_passes Rollup merge of #139615 - nnethercote:rm-name_or_empty, r=jdonszelmann 2025-04-18 05:16:29 +02:00
rustc_ast_pretty Remove token::{Open,Close}Delim. 2025-04-21 07:35:56 +10:00
rustc_attr_data_structures Remove #[rustc_macro_edition_2021]. 2025-04-20 11:15:46 +02:00
rustc_attr_parsing Auto merge of #139897 - nnethercote:rm-OpenDelim-CloseDelim, r=petrochenkov 2025-04-22 01:15:06 +00:00
rustc_baked_icu_data
rustc_borrowck Auto merge of #139114 - m-ou-se:super-let-pin, r=davidtwco 2025-04-19 08:01:53 +00:00
rustc_builtin_macros remove a couple clones 2025-04-20 18:58:46 +02:00
rustc_codegen_cranelift stabilize naked_functions 2025-04-20 11:18:38 +02:00
rustc_codegen_gcc Rollup merge of #137953 - RalfJung:simd-intrinsic-masks, r=WaffleLapkin 2025-04-20 13:02:48 +00:00
rustc_codegen_llvm Rollup merge of #137953 - RalfJung:simd-intrinsic-masks, r=WaffleLapkin 2025-04-20 13:02:48 +00:00
rustc_codegen_ssa Rollup merge of #137953 - RalfJung:simd-intrinsic-masks, r=WaffleLapkin 2025-04-20 13:02:48 +00:00
rustc_const_eval Rollup merge of #139974 - Patrick-6:change-visibility, r=RalfJung 2025-04-17 21:53:27 +02:00
rustc_data_structures Switch to diagnostic::on_unimplemented 2025-04-14 01:38:18 +02:00
rustc_driver Remove recursion_limit increases. 2025-04-02 16:25:27 +11:00
rustc_driver_impl Auto merge of #124141 - nnethercote:rm-Nonterminal-and-TokenKind-Interpolated, r=petrochenkov 2025-04-14 03:56:55 +00:00
rustc_error_codes stabilize naked_functions 2025-04-20 11:18:38 +02:00
rustc_error_messages update cfgs 2025-04-09 12:29:59 +01:00
rustc_errors Move eager translation to a method on Diag 2025-04-16 21:38:59 -04:00
rustc_expand Auto merge of #139897 - nnethercote:rm-OpenDelim-CloseDelim, r=petrochenkov 2025-04-22 01:15:06 +00:00
rustc_feature Rollup merge of #134213 - folkertdev:stabilize-naked-functions, r=tgross35,Amanieu,traviscross 2025-04-21 18:53:15 +00:00
rustc_fluent_macro Replace proc_macro::SourceFile by Span::{file, local_file}. 2025-04-11 15:07:08 +02:00
rustc_fs_util
rustc_graphviz
rustc_hashes
rustc_hir Rollup merge of #139615 - nnethercote:rm-name_or_empty, r=jdonszelmann 2025-04-18 05:16:29 +02:00
rustc_hir_analysis Don't compute name of associated item if it's an RPITIT 2025-04-20 16:08:39 +00:00
rustc_hir_pretty Fix HIR pretty-printing of fns with just a variadic arg. 2025-04-15 10:41:10 +10:00
rustc_hir_typeck Rollup merge of #140094 - Kivooeo:raw-pointer-assignment-suggestion, r=compiler-errors 2025-04-22 01:22:13 +00:00
rustc_incremental Replace infallible name_or_empty methods with fallible name methods. 2025-04-17 09:50:52 +10:00
rustc_index Add copy_within to IndexSlice 2025-04-15 10:44:28 -04:00
rustc_index_macros In rustc_mir_tranform, iterate over index newtypes instead of ints 2025-04-12 11:53:07 +00:00
rustc_infer Auto merge of #139768 - compiler-errors:split-fold, r=lcnr 2025-04-16 01:46:01 +00:00
rustc_interface Rollup merge of #139042 - compiler-errors:do-not-optimize-switchint, r=saethlin 2025-04-19 19:30:46 +00:00
rustc_lexer Replace rustc_lexer/unescape with rustc-literal-escaper crate 2025-04-04 14:44:45 +02:00
rustc_lint Auto merge of #139949 - matthiaskrgr:rollup-pxc5tsx, r=matthiaskrgr 2025-04-17 11:21:54 +00:00
rustc_lint_defs Replace infallible name_or_empty methods with fallible name methods. 2025-04-17 09:50:52 +10:00
rustc_llvm Update the minimum external LLVM to 19 2025-04-05 11:44:38 -07:00
rustc_log
rustc_macros Move eager translation to a method on Diag 2025-04-16 21:38:59 -04:00
rustc_metadata Rollup merge of #139834 - ChrisDenton:spf, r=WaffleLapkin 2025-04-19 15:09:35 +00:00
rustc_middle Rollup merge of #140052 - GuillaumeGomez:fix-140026, r=nnethercote 2025-04-21 18:53:18 +00:00
rustc_mir_build remove a couple clones 2025-04-20 18:58:46 +02:00
rustc_mir_dataflow Avoid an unwrap in RustcMirAttrs::set_field. 2025-04-17 09:51:32 +10:00
rustc_mir_transform Rollup merge of #140024 - cjgillot:continue-jumping, r=compiler-errors 2025-04-19 19:30:49 +00:00
rustc_monomorphize Only consider MonoItem::Fn when preventing inlining for autodiff source functions 2025-04-19 03:36:02 -04:00
rustc_next_trait_solver Don't assemble non-env/bound candidates if projection is rigid 2025-04-18 01:44:06 +00:00
rustc_parse Auto merge of #132833 - est31:stabilize_let_chains, r=fee1-dead 2025-04-22 07:54:10 +00:00
rustc_parse_format Update rustc-literal-escaper version to 0.0.2 2025-04-04 22:26:10 +02:00
rustc_passes stabilize naked_functions 2025-04-20 11:18:38 +02:00
rustc_pattern_analysis Move eager translation to a method on Diag 2025-04-16 21:38:59 -04:00
rustc_privacy Remove recursion_limit increases. 2025-04-02 16:25:27 +11:00
rustc_query_impl Auto merge of #124141 - nnethercote:rm-Nonterminal-and-TokenKind-Interpolated, r=petrochenkov 2025-04-14 03:56:55 +00:00
rustc_query_system Rollup merge of #139236 - Zoxc:anon-counter, r=davidtwco 2025-04-17 00:14:24 +02:00
rustc_resolve Remove #[rustc_macro_edition_2021]. 2025-04-20 11:15:46 +02:00
rustc_sanitizers Rollup merge of #139669 - nnethercote:overhaul-AssocItem, r=oli-obk 2025-04-15 15:47:27 +10:00
rustc_serialize Bump FileEncoder buffer size to 64 kB 2025-04-10 18:52:03 +02:00
rustc_session Rollup merge of #140077 - xizheyin:issue-139805, r=jieyouxu 2025-04-22 01:22:12 +00:00
rustc_smir Rollup merge of #139669 - nnethercote:overhaul-AssocItem, r=oli-obk 2025-04-15 15:47:27 +10:00
rustc_span Remove #[rustc_macro_edition_2021]. 2025-04-20 11:15:46 +02:00
rustc_symbol_mangling Rollup merge of #139848 - nnethercote:kw-Empty-5, r=compiler-errors 2025-04-15 21:16:05 +02:00
rustc_target Auto merge of #140043 - ChrisDenton:rollup-vwf0s9j, r=ChrisDenton 2025-04-20 02:08:02 +00:00
rustc_trait_selection Rollup merge of #140021 - compiler-errors:no-deep-norm-ice, r=lcnr 2025-04-21 15:55:58 +00:00
rustc_traits re-use sized fast path 2025-04-09 10:42:26 +00:00
rustc_transmute cleanup redundant pattern instances 2025-04-21 14:15:32 +02:00
rustc_ty_utils Rollup merge of #139669 - nnethercote:overhaul-AssocItem, r=oli-obk 2025-04-15 15:47:27 +10:00
rustc_type_ir Fix replacing supertrait aliases in ReplaceProjectionWith 2025-04-16 20:05:55 +00:00
rustc_type_ir_macros Split TypeFolder and FallibleTypeFolder 2025-04-15 18:30:35 +00:00
stable_mir let rustc_smir host stable_mir for refactoring 2025-04-05 18:23:07 +08:00