Revert let chains reference docs #1251
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Is this definitely going to be reverted on the master branch? |
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Well yes, I think so, but regardless, it seems we can't revert on beta without reverting here apparently? rust-lang/rust#100538 (comment) |
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I can make a beta branch to revert here if it isn't going to be reverted on master. I'll assume it will be reverted and go ahead and merge here on master. There have been a few small changes to the reference since the beta branch, but I think they are fine to pull along (none of them are version-specific). |
ehuss
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Aug 14, 2022
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You should be able to update the submodule to the latest master on your beta branch. Let me know if you need any help. |
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Makes sense, thanks for clarifying. |
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Aug 17, 2022
Update books ## nomicon 2 commits in 8d1e4dccf71114ff56f328f671f2026d8e6b62a2..8e6aa3448515a0654e347b5e2510f1d4bc4d5a64 2022-07-18 18:12:35 -0400 to 2022-08-15 15:36:13 -0700 - Update the `repr(transparent)` section to reflect the current state (rust-lang/nomicon#376) - [typo] typo on limits of lifetime chapter (rust-lang/nomicon#377) ## reference 5 commits in f3d3953bf3b158d596c96d55ce5366f9f3f972e9..e647eb102890e8927f488bea12672b079eff8d9d 2022-08-01 17:17:37 -0700 to 2022-08-16 11:35:27 -0700 - #[non_exhaustive] on variant blocks cross-crate as casts (rust-lang/reference#1249) - Revert let chains reference docs (rust-lang/reference#1251) - Update subtyping.md (rust-lang/reference#1240) - a fix about .await (rust-lang/reference#1245) - Be less specific about the representation of `+bundle` (rust-lang/reference#1246) ## book 2 commits in 36383b4da21dbd0a0781473bc8ad7ef0ed1b6751..42ca0ef484fcc8437a0682cee23abe4b7c407d52 2022-07-19 21:03:20 -0400 to 2022-08-12 21:52:02 -0400 - Missing period at end of sentence - Fix grammar in ch06-02 ## rust-by-example 8 commits in ee342dc91e1ba1bb1e1f1318f84bbe3bfac04798..03301f8ae55fa6f20f7ea152a517598e6db2cdb7 2022-07-27 11:06:36 -0300 to 2022-08-14 08:51:44 -0300 - Update print.md (rust-lang/rust-by-example#1597) - in Meta, replace 'playpen' with 'playground' (rust-lang/rust-by-example#1596) - Update doc comment to link to name field without compilation warning (rust-lang/rust-by-example#1595) - add line numbers for playpen fixes rust-lang/rust-by-example#1593 (rust-lang/rust-by-example#1594) - clarify that the map-reduce example relies on static data (rust-lang/rust-by-example#1592) - Update flow_control.md (rust-lang/rust-by-example#1591) - Remove duplicate line in the hello/print.md file (rust-lang/rust-by-example#1590) - Make rust editable in chapter on defaults (rust-lang/rust-by-example#1589) ## rustc-dev-guide 15 commits in 04f3cf0..d3daa1f 2022-07-31 07:46:57 +0200 to 2022-08-13 10:00:38 +0900 - Improve the "Diagnostic items" chapter (rust-lang/rustc-dev-guide#1427) - date-check: crates-io - fix/improve compiler-debugging - Update src/compiler-debugging.md - add gdb tips for symbol-mangling-version - move references down to avoid clutter (rust-lang/rustc-dev-guide#1420) - update date-check format on github issue (rust-lang/rustc-dev-guide#1416) - Fix legend colors in dark mode - Add color for downloaded nodes - Add colors to diagram - Add bootstrapping diagram - date-check: rustc_codegen_ssa is still alive - note is now too old to be relevant - date-check: be more strict - make date-check more lightweight (rust-lang/rustc-dev-guide#1394) ## edition-guide 3 commits in c55611dd6c58bdeb52423b5c52fd0f3c93615ba8..6038be9d37d7251c966b486154af621d1794d7af 2022-02-21 14:21:39 +0100 to 2022-08-15 08:12:42 -0700 - use title "The Rust Edition Guide" everywhere (rust-lang/edition-guide#280) - "Creating a new project": add example using 'cargo new --edition YEAR' (rust-lang/edition-guide#279) - fixes rust-lang/edition-guide#277: mention rust 1.0 release month and year (rust-lang/edition-guide#278)
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Update books ## nomicon 2 commits in 8d1e4dccf71114ff56f328f671f2026d8e6b62a2..8e6aa3448515a0654e347b5e2510f1d4bc4d5a64 2022-07-18 18:12:35 -0400 to 2022-08-15 15:36:13 -0700 - Update the `repr(transparent)` section to reflect the current state (rust-lang/nomicon#376) - [typo] typo on limits of lifetime chapter (rust-lang/nomicon#377) ## reference 5 commits in f3d3953bf3b158d596c96d55ce5366f9f3f972e9..e647eb102890e8927f488bea12672b079eff8d9d 2022-08-01 17:17:37 -0700 to 2022-08-16 11:35:27 -0700 - #[non_exhaustive] on variant blocks cross-crate as casts (rust-lang/reference#1249) - Revert let chains reference docs (rust-lang/reference#1251) - Update subtyping.md (rust-lang/reference#1240) - a fix about .await (rust-lang/reference#1245) - Be less specific about the representation of `+bundle` (rust-lang/reference#1246) ## book 2 commits in 36383b4da21dbd0a0781473bc8ad7ef0ed1b6751..42ca0ef484fcc8437a0682cee23abe4b7c407d52 2022-07-19 21:03:20 -0400 to 2022-08-12 21:52:02 -0400 - Missing period at end of sentence - Fix grammar in ch06-02 ## rust-by-example 8 commits in ee342dc91e1ba1bb1e1f1318f84bbe3bfac04798..03301f8ae55fa6f20f7ea152a517598e6db2cdb7 2022-07-27 11:06:36 -0300 to 2022-08-14 08:51:44 -0300 - Update print.md (rust-lang/rust-by-example#1597) - in Meta, replace 'playpen' with 'playground' (rust-lang/rust-by-example#1596) - Update doc comment to link to name field without compilation warning (rust-lang/rust-by-example#1595) - add line numbers for playpen fixes rust-lang/rust-by-example#1593 (rust-lang/rust-by-example#1594) - clarify that the map-reduce example relies on static data (rust-lang/rust-by-example#1592) - Update flow_control.md (rust-lang/rust-by-example#1591) - Remove duplicate line in the hello/print.md file (rust-lang/rust-by-example#1590) - Make rust editable in chapter on defaults (rust-lang/rust-by-example#1589) ## rustc-dev-guide 15 commits in 04f3cf0..d3daa1f 2022-07-31 07:46:57 +0200 to 2022-08-13 10:00:38 +0900 - Improve the "Diagnostic items" chapter (rust-lang/rustc-dev-guide#1427) - date-check: crates-io - fix/improve compiler-debugging - Update src/compiler-debugging.md - add gdb tips for symbol-mangling-version - move references down to avoid clutter (rust-lang/rustc-dev-guide#1420) - update date-check format on github issue (rust-lang/rustc-dev-guide#1416) - Fix legend colors in dark mode - Add color for downloaded nodes - Add colors to diagram - Add bootstrapping diagram - date-check: rustc_codegen_ssa is still alive - note is now too old to be relevant - date-check: be more strict - make date-check more lightweight (rust-lang/rustc-dev-guide#1394) ## edition-guide 3 commits in c55611dd6c58bdeb52423b5c52fd0f3c93615ba8..6038be9d37d7251c966b486154af621d1794d7af 2022-02-21 14:21:39 +0100 to 2022-08-15 08:12:42 -0700 - use title "The Rust Edition Guide" everywhere (rust-lang/edition-guide#280) - "Creating a new project": add example using 'cargo new --edition YEAR' (rust-lang/edition-guide#279) - fixes rust-lang/edition-guide#277: mention rust 1.0 release month and year (rust-lang/edition-guide#278)
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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
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Apr 23, 2025
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/rust#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 #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/rust#104843 [stay consistent with `if let`]: rust-lang/rust#103293 (comment) [drop order changes]: rust-lang/rust#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/rust#94927 * rust-lang/rust#94951 * rust-lang/rust#94974 * rust-lang/rust#95008 * rust-lang/rust#97295 * rust-lang/rust#98633 * rust-lang/rust#99731 * rust-lang/rust#102394 * rust-lang/rust#100526 * rust-lang/rust#100538 * rust-lang/rust#102998 * rust-lang/rust#103405 * rust-lang/rust#103293 * rust-lang/rust#107251 * rust-lang/rust#110568 * rust-lang/rust#115677 * rust-lang/rust#117743 * rust-lang/rust#117770 * rust-lang/rust#118191 * rust-lang/rust#119554 * rust-lang/rust#129394 * rust-lang/rust#132828 * rust-lang/reference#1179 * rust-lang/reference#1251 * rust-lang/rustfmt#5910 [original stabilization PR]: rust-lang/rust#94927 ## Adoption history ### In the compiler * History before March 14, 2022 can be found in the [original stabilization PR]. * rust-lang/rust#115983 * rust-lang/rust#116549 * rust-lang/rust#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 #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 #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/rust#51114 ## Open questions / blockers - [ ] bad recovery if you don't put a `let` (I don't think this is a blocker): [#117977](rust-lang/rust#117977) - [x] An instance where a temporary lives shorter than with nested ifs, breaking compilation: [#103476](rust-lang/rust#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 rust-lang/rust#132828 - [x] [Style](https://rust-lang.zulipchat.com/#narrow/channel/346005-t-style/topic/let.20chains.20stabilization.20and.20formatting): rust-lang/rust#139456 - [x] rust-lang/rust#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
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Reverts #1179 because let-chains are getting unstabilized in rust-lang/rust/#100538