4 - Make more use of `let_chains` #94445

c410-f3r · 2022-02-28T10:49:30Z

Continuation of #94376.

cc #53667

rust-highfive · 2022-02-28T10:49:33Z

r? @cjgillot

(rust-highfive has picked a reviewer for you, use r? to override)

Continuation of rust-lang#94376. cc rust-lang#53667

cjgillot

Thanks. Just one nit. r=me once resolved.

cjgillot · 2022-02-28T18:37:43Z

compiler/rustc_parse/src/parser/mod.rs

+        if $allow_qpath_recovery
+                    && $self.look_ahead(1, |t| t == &token::ModSep)
+                    && let token::Interpolated(nt) = &$self.token.kind
+                    && let token::NtTy(ty) = &**nt


The formatting is a bit odd here.

Rustfmt stuff and for some reason, #[rustfmt::skip] breaks the build

I can remove this snippet if you want it

If rustfmt says so...

cjgillot · 2022-02-28T18:45:17Z

@bors r+ rollup

bors · 2022-02-28T18:45:19Z

📌 Commit e3e902b has been approved by cjgillot

…askrgr Rollup of 6 pull requests Successful merges: - rust-lang#92399 (fix typo in btree/vec doc: Self -> self) - rust-lang#92823 (Tweak diagnostics) - rust-lang#94248 (Fix ICE when passing block to while-loop condition) - rust-lang#94414 (Fix ICE when using Box<T, A> with large A) - rust-lang#94445 (4 - Make more use of `let_chains`) - rust-lang#94449 (Add long explanation for E0726) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

…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

rust-highfive assigned cjgillot Feb 28, 2022

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Feb 28, 2022

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 28, 2022

4 - Make more use of let_chains

e3e902b

Continuation of rust-lang#94376. cc rust-lang#53667

c410-f3r force-pushed the more-let-chains branch from 50787dd to e3e902b Compare February 28, 2022 10:50

cjgillot approved these changes Feb 28, 2022

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 28, 2022

matthiaskrgr mentioned this pull request Feb 28, 2022

Rollup of 6 pull requests #94453

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bors merged commit 5be38d2 into rust-lang:master Feb 28, 2022

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Stabilize let_chains in Rust 1.64 #94927

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4 - Make more use of `let_chains` #94445

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4 - Make more use of let_chains #94445

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4 - Make more use of `let_chains` #94445

4 - Make more use of `let_chains` #94445