Introduce `enhanced_binary_op` feature #93049

c410-f3r · 2022-01-18T21:17:04Z

Addresses the requests of @rust-lang/lang in #88642 (comment).
Direct usage could be applied through a FCP but everything was extracted into a feature flag as inferred by previous comments.

r? @matthewjasper
cc #53667

leonardo-m · 2022-01-18T21:21:12Z

if false || { z = 3; false} || z == 3 {}

That's ugly code to find in the wild

c410-f3r · 2022-01-29T10:24:32Z

@matthewjasper Looks like this feature is more useful with || operators and since they aren't currently supported, I will close this PR in the meanwhile.

c410-f3r · 2022-03-11T13:22:48Z

@matthewjasper I am reopening this PR based on our previous discussion about ||

apiraino · 2022-03-24T11:01:22Z

By reading the latest comment and linked PR, I infer that there is some lang design work to do, so I'll tentatively flip the review switch (but please update it's not correct).

@rustbot author

c410-f3r · 2022-03-24T11:39:02Z

Hum... I not aware of any remaining designing work although I could be wrong. FWICT, this feature is mostly about relaxing the existing rules.

For let chains, it doesn't make much different because the following snippet:

fn and_chain() {
    let z;
    if let true = true && { z = 3; true } && z == 3 {}
}

Could be rewritten as:

fn and_chain() {
    if let true = true && let z = 3 && true && z == 3 {
         println!("z is in scope");
    }
}

c410-f3r · 2022-03-24T11:47:06Z

Looks like ppl are not very interest in this feature so you or anyone else can close this PR if desired :)

pnkfelix · 2022-03-24T14:46:32Z

nominating for lang team to confirm that this generalization of existing let_chains behavior is still desired. (It would be good to double-check what RFC itself said, and then confirm with lang team that that is desirable.)

c410-f3r · 2022-03-24T18:24:45Z

Actually this feature was introduced by mistake.

My MIR-FU is not that good so I initially thought that ExprKind::LogicalOp should also be modified in order to introduce let chains but no, it wasn't necessary.

This situation was in fact quite a surprise to me and I didn't know stuff until #88642 (review) 🤷

Let chains and || operators are independent things that happen to interact with this feature but aren't coupled at all. Therefore, it is possible to say that this PR is something that could be added but doesn't look very attractive.

c410-f3r · 2022-04-19T14:44:21Z

I will go ahead and close this PR again. It is clear that such feature is not very appealing (at least for now).

The code is fairly simple so a future possible re-addition shouldn't be a problem for anyone interested enough.

…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 matthewjasper Jan 18, 2022

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Introduce enhanced_binary_op feature

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c410-f3r force-pushed the foo-bar-baz branch from cc0b969 to 83bbd18 Compare January 19, 2022 18:27

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Stabilize let_chains in Rust 1.64 #94927

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Introduce `enhanced_binary_op` feature #93049

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Introduce enhanced_binary_op feature #93049

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Introduce `enhanced_binary_op` feature #93049

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