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I am very for this change. This syntax feels natural and intuitive, and it is something I've pined for. My remarks are:
|
Thank you for the data point and the remarks. I think the primary motivation for also changing That said, if we consider iterating over multiple sources that produce elements in a zip-like fashion (but not strictly
I'm personally undecided on adding |
@aturon has asked me to clarify what exactly is being proposed to be accepted with this RFC. So here goes... The precedent set by closing RFCs #2443, #2441, and #2429 is that we don't accept RFCs reserving syntax without accepting a feature along with it. Therefore, to be consistent with this policy, and for the purposes of making So the idea with this RFC is that we accept the full feature now (the idea of chaining...), but leave finalizing the syntax itself unresolved-ish as we did with #2071 ( After the Rust 2018 has shipped, and when we have time, we can then implement the proposed syntax and experiment with it on nightly. The syntax will still have to be finalized by some other decision, such as with another RFC or on the tracking issue. |
My opinion hasn't changed since #2260 (comment). We should aim for supporting a convenient non-exhaustive pattern-matching expressions in general (
That's just not true, for expressions in If "slow to develop" refers to my promise to write an RFC for pattern-matching expressions |
@petrochenkov Totally understood -- did you see @Centril's clarifying comment? My understanding is that this RFC is meant to commit us to (1) some solution to this problem and (2) carving out a bit of space in the grammar so that the solution could be as discussed in the RFC. FWIW, I also have significant misgivings about solving this problem by essentially extending the special-cased treatment of @Centril I wonder if this should be made formally an "eRFC" to signify more clearly the issues above? |
@aturon |
These operators all have lower precedence than
Which means we also have the following ambiguity to consider: if let Range { start: _, end: false } = true..true && false {
println!("(current behavior)");
} else {
panic!();
} let result = loop {
if let Some(0u32) = break true && false {
panic!();
}
panic!();
};
assert_eq!(result, false); And then const F: fn() -> bool = || true;
if let Range { start: F, end } = F..|| false {
println!("(current behavior)");
assert!(!end());
} else {
panic!();
} let t = &&true;
if let Range { start: true, end } = t..&&false {
println!("(current behavior)");
assert_eq!(**end, false);
} else {
panic!();
} Please also ensure the parser accepts the following: if a || b {} // <-- no parenthesis, it is ok.
if a && b || c && d {}
// interpret as: `(a && b) || (c && d)` and rejects the following before if let Some(a) = b && c || d {}
// interpret as: `((let Some(a) = b) && c) || d` |
@aturon eRFC it is ;) @petrochenkov Yep, it's about making the proposed solution possible and recognizing that some solution (which could be I would personally be fine with experimenting with
I see; my impression was that it was more complicated from the discussions at #2260. This is the "bindings only live until the end of a full expression" rule?
Not at all :) It is referring to my perception of the support for
I thank you for it ❤️ |
I can't say I'm a huge fan of the specific proposed syntax, but I'm inclined to take this step regardless, so that even if we don't end up going with this particular syntax we can detect potentially ambiguous uses of it. I'm honestly tempted to argue that either grouping should require parentheses. Minor nit: for consistency, can you please always put Also, in the example of how |
The precedence change is to bind
I agree; it wasn't my finest phrasing. These days I think it's better to think of this in terms of what happens to |
@Centril Thanks for the explanation! |
@H2CO3 Welcome :) |
Argh, I didn’t see that RFC and I wish I did… I’m not a huge fan of the proposal, it seems too niche and introduces too much sugar (mixes two very different constructs — |
This is definitely not niche; in particular, this pattern arises frequently in the compiler (similar code like proc macros, clippy, anything that deals with AST-looking trees) and will pay for itself many times over. |
Yes, but
Which is definitely not part of the language (and shouldn’t, as introducing bindings in expressions seems completely out of question). So, yeah, maybe you’re introducing “conceptually simpler” sugar for very specific cases (again, I truly think it’s niche, and you put on a name on that niche: compilers and ASTs), but you’re also introducing confusion pretty much everywhere else where that “conceptually simpler” concept would feel applicable (while it’s not). |
I would also like to mention that |
Just yesterday I wanted this feature while changing the MPSC channels in |
In the code you showed, you could have used a guarded match. That’s already part of the language and it will do the job for most cases. Having lazy pattern-matching mixed with booleans makes me incomfortable. I’m not saying I’m against lazy pattern-matching (it’s actually handy). I’m just saying that using the |
…shtriplett Stabilize `let_chains` in Rust 1.64 # Stabilization proposal This PR proposes the stabilization of `#![feature(let_chains)]` in a future-compatibility way that will allow the **possible** addition of the `EXPR is PAT` syntax. Tracking issue: rust-lang#53667 Version: 1.64 (beta => 2022-08-11, stable => 2022-10-22). ## What is stabilized The ability to chain let expressions along side local variable declarations or ordinary conditional expressions. For example: ```rust pub enum Color { Blue, Red, Violet, } pub enum Flower { Rose, Tulip, Violet, } pub fn roses_are_red_violets_are_blue_printer( (first_flower, first_flower_color): (Flower, Color), (second_flower, second_flower_color): (Flower, Color), pick_up_lines: &[&str], ) { if let Flower::Rose = first_flower && let Color::Red = first_flower_color && let Flower::Violet = second_flower && let Color::Blue = second_flower_color && let &[first_pick_up_line, ..] = pick_up_lines { println!("Roses are red, violets are blue, {}", first_pick_up_line); } } fn main() { roses_are_red_violets_are_blue_printer( (Flower::Rose, Color::Red), (Flower::Violet, Color::Blue), &["sugar is sweet and so are you"], ); } ``` ## Motivation The main motivation for this feature is improving readability, ergonomics and reducing paper cuts. For more examples, see the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md). ## What isn't stabilized * Let chains in match guards (`if_let_guard`) * Resolution of divergent non-terminal matchers * The `EXPR is PAT` syntax ## History * On 2017-12-24, [RFC: if- and while-let-chains](rust-lang/rfcs#2260) * On 2018-07-12, [eRFC: if- and while-let-chains, take 2](rust-lang/rfcs#2497) * On 2018-08-24, [Tracking issue for eRFC 2497, "if- and while-let-chains, take 2](rust-lang#53667) * On 2019-03-19, [Run branch cleanup after copy prop](rust-lang#59290) * On 2019-03-26, [Generalize diagnostic for x = y where bool is the expected type](rust-lang#59439) * On 2019-04-24, [Introduce hir::ExprKind::Use and employ in for loop desugaring](rust-lang#60225) * On 2019-03-19, [[let_chains, 1/6] Remove hir::ExprKind::If](rust-lang#59288) * On 2019-05-15, [[let_chains, 2/6] Introduce Let(..) in AST, remove IfLet + WhileLet and parse let chains](rust-lang#60861) * On 2019-06-20, [[let_chains, 3/6] And then there was only Loop](rust-lang#61988) * On 2020-11-22, [Reintroduce hir::ExprKind::If](rust-lang#79328) * On 2020-12-24, [Introduce hir::ExprKind::Let - Take 2](rust-lang#80357) * On 2021-02-19, [Lower condition of if expression before it's "then" block](rust-lang#82308) * On 2021-09-01, [Fix drop handling for `if let` expressions](rust-lang#88572) * On 2021-09-04, [Formally implement let chains](rust-lang#88642) * On 2022-01-19, [Add tests to ensure that let_chains works with if_let_guard](rust-lang#93086) * On 2022-01-18, [Introduce `enhanced_binary_op` feature](rust-lang#93049) * On 2022-01-22, [Fix `let_chains` and `if_let_guard` feature flags](rust-lang#93213) * On 2022-02-25, [Initiate the inner usage of `let_chains`](rust-lang#94376) * On 2022-01-28, [[WIP] Introduce ast::StmtKind::LetElse to allow the usage of `let_else` with `let_chains`](rust-lang#93437) * On 2022-02-26, [1 - Make more use of `let_chains`](rust-lang#94396) * On 2022-02-26, [2 - Make more use of `let_chains`](rust-lang#94400) * On 2022-02-27, [3 - Make more use of `let_chains`](rust-lang#94420) * On 2022-02-28, [4 - Make more use of `let_chains`](rust-lang#94445) * On 2022-02-28, [5 - Make more use of `let_chains`](rust-lang#94448) * On 2022-02-28, [6 - Make more use of `let_chains`](rust-lang#94465) * On 2022-03-01, [7 - Make more use of `let_chains`](rust-lang#94476) * On 2022-03-01, [8 - Make more use of `let_chains`](rust-lang#94484) * On 2022-03-01, [9 - Make more use of `let_chains`](rust-lang#94498) * On 2022-03-08, [Warn users about `||` in let chain expressions](rust-lang#94754) From the first RFC (2017-12-24) to the theoretical future stabilization day (2022-10-22), it can be said that this feature took 4 years, 9 months and 28 days of research, development, discussions, agreements and headaches to be settled. ## Divergent non-terminal matchers More specifically, rust-lang#86730. ```rust macro_rules! mac { ($e:expr) => { if $e { true } else { false } }; } fn main() { // OK! assert_eq!(mac!(true && let 1 = 1), true); // ERROR! Anything starting with `let` is not considered an expression assert_eq!(mac!(let 1 = 1 && true), true); } ``` To the best of my knowledge, such error or divergence is orthogonal, does not prevent stabilization and can be tackled independently in the near future or effectively in the next Rust 2024 edition. If not, then https://github.com/c410-f3r/rust/tree/let-macro-blah contains a set of changes that will consider `let` an expression. It is possible that none of the solutions above satisfies all applicable constraints but I personally don't know of any other plausible answers. ## Alternative syntax Taking into account the usefulness of this feature and the overwhelming desire to use both now and in the past, `let PAT = EXPR` will be utilized for stabilization but it doesn't or shall create any obstacle for a **possible** future addition of `EXPR is PAT`. The introductory snippet would then be written as the following. ```rust if first_flower is Flower::Rose && first_flower_color is Color::Red && second_flower is Flower::Violet && second_flower_color is Color::Blue && pick_up_lines is &[first_pick_up_line, ..] { println!("Roses are red, violets are blue, {}", first_pick_up_line); } ``` Just to reinforce, this PR only unblocks a **possible** future road for `EXPR is PAT` and does emphasize what is better or what is worse. ## Tests * [Verifies the drop order of let chains and ensures it won't change in the future in an unpredictable way](https://github.com/rust-lang/rust/blob/master/src/test/ui/mir/mir_let_chains_drop_order.rs) * [AST lowering does not wrap let chains in an `DropTemps` expression](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs) * [Checks pretty printing output](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs) * [Verifies uninitialized variables due to MIR modifications](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/chains-without-let.rs) * [A collection of statements where `let` expressions are forbidden](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs) * [All or at least most of the places where let chains are allowed](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs) * [Ensures that irrefutable lets are allowed in let chains](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/irrefutable-lets.rs) * [issue-88498.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-88498.rs), [issue-90722.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-90722.rs), [issue-92145.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-92145.rs) and [issue-93150.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-93150.rs) were bugs found by third parties and fixed overtime. * [Indexing was triggering a ICE due to a wrongly constructed MIR graph](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/no-double-assigments.rs) * [Protects the precedence of `&&` in relation to other things](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs) * [`let_chains`, as well as `if_let_guard`, has a valid MIR graph that evaluates conditional expressions correctly](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/then-else-blocks.rs) Most of the infra-structure used by let chains is also used by `if` expressions in stable compiler versions since rust-lang#80357 and rust-lang#88572. As a result, no bugs were found since the integration of rust-lang#88642. ## Possible future work * Let chains in match guards is implemented and working but stabilization is blocked by `if_let_guard`. * The usage of `let_chains` with `let_else` is possible but not implemented. Regardless, one attempt was introduced and closed in rust-lang#93437. Thanks `@Centril` for creating the RFC and huge thanks (again) to `@matthewjasper` for all the reviews, mentoring and MIR implementations. Fixes rust-lang#53667
Stabilize `let_chains` in Rust 1.64 # Stabilization proposal This PR proposes the stabilization of `#![feature(let_chains)]` in a future-compatibility way that will allow the **possible** addition of the `EXPR is PAT` syntax. Tracking issue: #53667 Version: 1.64 (beta => 2022-08-11, stable => 2022-10-22). ## What is stabilized The ability to chain let expressions along side local variable declarations or ordinary conditional expressions. For example: ```rust pub enum Color { Blue, Red, Violet, } pub enum Flower { Rose, Tulip, Violet, } pub fn roses_are_red_violets_are_blue_printer( (first_flower, first_flower_color): (Flower, Color), (second_flower, second_flower_color): (Flower, Color), pick_up_lines: &[&str], ) { if let Flower::Rose = first_flower && let Color::Red = first_flower_color && let Flower::Violet = second_flower && let Color::Blue = second_flower_color && let &[first_pick_up_line, ..] = pick_up_lines { println!("Roses are red, violets are blue, {}", first_pick_up_line); } } fn main() { roses_are_red_violets_are_blue_printer( (Flower::Rose, Color::Red), (Flower::Violet, Color::Blue), &["sugar is sweet and so are you"], ); } ``` ## Motivation The main motivation for this feature is improving readability, ergonomics and reducing paper cuts. For more examples, see the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md). ## What isn't stabilized * Let chains in match guards (`if_let_guard`) * Resolution of divergent non-terminal matchers * The `EXPR is PAT` syntax ## History * On 2017-12-24, [RFC: if- and while-let-chains](rust-lang/rfcs#2260) * On 2018-07-12, [eRFC: if- and while-let-chains, take 2](rust-lang/rfcs#2497) * On 2018-08-24, [Tracking issue for eRFC 2497, "if- and while-let-chains, take 2](rust-lang/rust#53667) * On 2019-03-19, [Run branch cleanup after copy prop](rust-lang/rust#59290) * On 2019-03-26, [Generalize diagnostic for x = y where bool is the expected type](rust-lang/rust#59439) * On 2019-04-24, [Introduce hir::ExprKind::Use and employ in for loop desugaring](rust-lang/rust#60225) * On 2019-03-19, [[let_chains, 1/6] Remove hir::ExprKind::If](rust-lang/rust#59288) * On 2019-05-15, [[let_chains, 2/6] Introduce Let(..) in AST, remove IfLet + WhileLet and parse let chains](rust-lang/rust#60861) * On 2019-06-20, [[let_chains, 3/6] And then there was only Loop](rust-lang/rust#61988) * On 2020-11-22, [Reintroduce hir::ExprKind::If](rust-lang/rust#79328) * On 2020-12-24, [Introduce hir::ExprKind::Let - Take 2](rust-lang/rust#80357) * On 2021-02-19, [Lower condition of if expression before it's "then" block](rust-lang/rust#82308) * On 2021-09-01, [Fix drop handling for `if let` expressions](rust-lang/rust#88572) * On 2021-09-04, [Formally implement let chains](rust-lang/rust#88642) * On 2022-01-19, [Add tests to ensure that let_chains works with if_let_guard](rust-lang/rust#93086) * On 2022-01-18, [Introduce `enhanced_binary_op` feature](rust-lang/rust#93049) * On 2022-01-22, [Fix `let_chains` and `if_let_guard` feature flags](rust-lang/rust#93213) * On 2022-02-25, [Initiate the inner usage of `let_chains`](rust-lang/rust#94376) * On 2022-01-28, [[WIP] Introduce ast::StmtKind::LetElse to allow the usage of `let_else` with `let_chains`](rust-lang/rust#93437) * On 2022-02-26, [1 - Make more use of `let_chains`](rust-lang/rust#94396) * On 2022-02-26, [2 - Make more use of `let_chains`](rust-lang/rust#94400) * On 2022-02-27, [3 - Make more use of `let_chains`](rust-lang/rust#94420) * On 2022-02-28, [4 - Make more use of `let_chains`](rust-lang/rust#94445) * On 2022-02-28, [5 - Make more use of `let_chains`](rust-lang/rust#94448) * On 2022-02-28, [6 - Make more use of `let_chains`](rust-lang/rust#94465) * On 2022-03-01, [7 - Make more use of `let_chains`](rust-lang/rust#94476) * On 2022-03-01, [8 - Make more use of `let_chains`](rust-lang/rust#94484) * On 2022-03-01, [9 - Make more use of `let_chains`](rust-lang/rust#94498) * On 2022-03-08, [Warn users about `||` in let chain expressions](rust-lang/rust#94754) From the first RFC (2017-12-24) to the theoretical future stabilization day (2022-10-22), it can be said that this feature took 4 years, 9 months and 28 days of research, development, discussions, agreements and headaches to be settled. ## Divergent non-terminal matchers More specifically, rust-lang/rust#86730. ```rust macro_rules! mac { ($e:expr) => { if $e { true } else { false } }; } fn main() { // OK! assert_eq!(mac!(true && let 1 = 1), true); // ERROR! Anything starting with `let` is not considered an expression assert_eq!(mac!(let 1 = 1 && true), true); } ``` To the best of my knowledge, such error or divergence is orthogonal, does not prevent stabilization and can be tackled independently in the near future or effectively in the next Rust 2024 edition. If not, then https://github.com/c410-f3r/rust/tree/let-macro-blah contains a set of changes that will consider `let` an expression. It is possible that none of the solutions above satisfies all applicable constraints but I personally don't know of any other plausible answers. ## Alternative syntax Taking into account the usefulness of this feature and the overwhelming desire to use both now and in the past, `let PAT = EXPR` will be utilized for stabilization but it doesn't or shall create any obstacle for a **possible** future addition of `EXPR is PAT`. The introductory snippet would then be written as the following. ```rust if first_flower is Flower::Rose && first_flower_color is Color::Red && second_flower is Flower::Violet && second_flower_color is Color::Blue && pick_up_lines is &[first_pick_up_line, ..] { println!("Roses are red, violets are blue, {}", first_pick_up_line); } ``` Just to reinforce, this PR only unblocks a **possible** future road for `EXPR is PAT` and does emphasize what is better or what is worse. ## Tests * [Verifies the drop order of let chains and ensures it won't change in the future in an unpredictable way](https://github.com/rust-lang/rust/blob/master/src/test/ui/mir/mir_let_chains_drop_order.rs) * [AST lowering does not wrap let chains in an `DropTemps` expression](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs) * [Checks pretty printing output](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs) * [Verifies uninitialized variables due to MIR modifications](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/chains-without-let.rs) * [A collection of statements where `let` expressions are forbidden](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs) * [All or at least most of the places where let chains are allowed](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs) * [Ensures that irrefutable lets are allowed in let chains](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/irrefutable-lets.rs) * [issue-88498.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-88498.rs), [issue-90722.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-90722.rs), [issue-92145.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-92145.rs) and [issue-93150.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/issue-93150.rs) were bugs found by third parties and fixed overtime. * [Indexing was triggering a ICE due to a wrongly constructed MIR graph](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/no-double-assigments.rs) * [Protects the precedence of `&&` in relation to other things](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs) * [`let_chains`, as well as `if_let_guard`, has a valid MIR graph that evaluates conditional expressions correctly](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2497-if-let-chains/then-else-blocks.rs) Most of the infra-structure used by let chains is also used by `if` expressions in stable compiler versions since rust-lang/rust#80357 and rust-lang/rust#88572. As a result, no bugs were found since the integration of rust-lang/rust#88642. ## Possible future work * Let chains in match guards is implemented and working but stabilization is blocked by `if_let_guard`. * The usage of `let_chains` with `let_else` is possible but not implemented. Regardless, one attempt was introduced and closed in rust-lang/rust#93437. Thanks `@Centril` for creating the RFC and huge thanks (again) to `@matthewjasper` for all the reviews, mentoring and MIR implementations. Fixes #53667
Can somebody explain to me why the Swift |
@alper It was considered and the community highly disliked it. There is literally a survey report inside the RFC document. |
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
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
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
🖼️ Rendered
⏭ Tracking issue - Main
⏭ Tracking issue - Edition transitioning
📝 Summary
Extends
if let
andwhile let
-expressions with chaining, allowing you to combine multiplelet
s andbool
-typed conditions together naturally. After implementing this RFC, you'll be able to write, among other things:The main aim of this RFC is to decide that this is a problem worth solving as well as discussing a few available options. Most importantly, we want to make
let PAT = EXPR && ..
a possible option for Rust 2018.💖 Thanks
To everyone who participated in RFC #2260 and to the survey participants.
To @scottmcm for collaborating with me on the original RFC.
To @aturon and @nikomatsakis for taking the time to discuss this with me.
To @SergioBenitez for checking the feasibility of this in Rust 2018.
To @oli-obk for providing me with the useful example in the summary ;)
To @joshtriplett for the consultation on the style.
To @kennytm for improving clarity on the Rust 2018 migration technical changes.