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Formally implement let chains #88642

Merged
merged 1 commit into from
Jan 19, 2022
Merged

Formally implement let chains #88642

merged 1 commit into from
Jan 19, 2022

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c410-f3r
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@c410-f3r c410-f3r commented Sep 4, 2021 •
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Let chains

Thanks to @Centril for creating the RFC and special thanks to @matthewjasper for helping me since the beginning of this journey. In fact, @matthewjasper did much of the complicated MIR stuff so it's true to say that this feature wouldn't be possible without him. Thanks again @matthewjasper!

With the changes proposed in this PR, it will be possible to chain let expressions along side local variable declarations or ordinary conditional expressions. In other words, do much of what the if_chain crate already does.

fn u2_beautiful_day(lyrics: &str) {
    let mut lines = lyrics.trim().lines();

    if let Some(see_the_world) = lines.next().map(|el| el.trim())
        && let Some(see_china) = lines.next().map(|el| el.trim())
        && let Some(see_the_canyons) = lines.next().map(|el| el.trim())
        && let Some(see_the_tuna_fleets) = lines.next().map(|el| el.trim())
        && let Some(see_the_bedouin_fires) = lines.next().map(|el| el.trim())
        && let Some(see_the_oil_fields) = lines.next().map(|el| el.trim())
        && let Some(and_see_the_bird) = lines.next().map(|el| el.trim())
        && let Some(after_the_flood) = lines.next().map(|el| el.trim())

        && see_the_world == "See the world in green and blue"
        && see_china == "See China right in front of you"
        && see_the_canyons == "See the canyons broken by cloud"
        && see_the_tuna_fleets == "See the tuna fleets clearing the sea out"
        && see_the_bedouin_fires == "See the bedouin fires at night"
        && see_the_oil_fields == "See the oil fields at first light"
        && and_see_the_bird == "And see the bird with a leaf in her mouth"
        && after_the_flood == "After the flood all the colors came out"
    {
        println!("It was a beautiful day, don't let it get away");
    }
}

Other considerations

  • if let guard and let ... else features need special care and should be handled in a following PR.

  • Irrefutable patterns are allowed within a let chain context

  • Three Clippy lints were already converted to start dogfooding and help detect possible corner cases

cc #53667

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Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

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r? @oli-obk

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

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 4, 2021
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r? @matthewjasper

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ping from triage:
@c410-f3r
Returning to you to address build failures and after that, please switch back to S-waiting-on-review.
thanks.

@rustbot label: +S-waiting-on-author -S-waiting-on-review

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 27, 2021
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bors commented Sep 29, 2021

☔ The latest upstream changes (presumably #88950) made this pull request unmergeable. Please resolve the merge conflicts.

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BubbaSheen commented Sep 29, 2021 via email

@apiraino apiraino added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Oct 14, 2021
@JohnCSimon JohnCSimon added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 31, 2021
@c410-f3r c410-f3r closed this Nov 14, 2021
@c410-f3r c410-f3r reopened this Nov 14, 2021
@c410-f3r c410-f3r changed the title [WIP] Formally implement let chains Formally implement let chains Nov 14, 2021
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bors commented Nov 17, 2021

☔ The latest upstream changes (presumably #90966) made this pull request unmergeable. Please resolve the merge conflicts.

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What's the status of this PR?

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Chains with the && operator are working as expected but chains with the || operator still need to be figured out.

Believe it or not, I look into this PR quite regularly trying to solve stuff but I guess I am not good enough :)

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@c410-f3r: I'd be happy to work with you on Zulip if you'd like some help

@c410-f3r c410-f3r mentioned this pull request Jan 18, 2022
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@bors r+

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bors commented Jan 18, 2022

📌 Commit 5f74ef4 has been approved by matthewjasper

@bors 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 18, 2022
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@bors rollup=iffy

@matthiaskrgr matthiaskrgr mentioned this pull request Jan 19, 2022
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bors added a commit to rust-lang-ci/rust that referenced this pull request Jan 19, 2022
@bors
Auto merge of rust-lang#93069 - matthiaskrgr:rollup-gx1vkp7, r=matthi…
5e57faa 
…askrgr

Rollup of 10 pull requests

Successful merges:

 - rust-lang#88642 (Formally implement let chains)
 - rust-lang#89621 (doc: guarantee call order for sort_by_cached_key)
 - rust-lang#91278 (Use iterator instead of recursion in `codegen_place`)
 - rust-lang#92124 (Little improves in CString `new` when creating from slice)
 - rust-lang#92783 (Annotate dead code lint with notes about ignored derived impls)
 - rust-lang#92797 (Remove horizontal lines at top of page)
 - rust-lang#92920 (Move expr- and item-related pretty printing functions to modules)
 - rust-lang#93041 (Remove some unused ordering derivations based on `DefId`)
 - rust-lang#93051 (Add Option::is_some_with and Result::is_{ok,err}_with)
 - rust-lang#93062 (Update books)

Failed merges:

r? `@ghost`
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@bors bors merged commit 5d2928f into rust-lang:master Jan 19, 2022
@rustbot rustbot added this to the 1.60.0 milestone Jan 19, 2022
This was referenced Jan 19, 2022
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@HTGAzureX1212 HTGAzureX1212 mentioned this pull request Jan 20, 2022
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@HTGAzureX1212 HTGAzureX1212 mentioned this pull request Mar 3, 2022
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@c410-f3r c410-f3r mentioned this pull request Mar 14, 2022
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JohnTitor added a commit to JohnTitor/rust that referenced this pull request Jul 17, 2022
@JohnTitor
Rollup merge of rust-lang#94927 - c410-f3r:stabilize-let-chains, r=jo…
353d018 
…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
workingjubilee pushed a commit to tcdi/postgrestd that referenced this pull request Sep 15, 2022
@JohnTitor
Rollup merge of #94927 - c410-f3r:stabilize-let-chains, r=joshtriplett
ad27453 
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
@LeSeulArtichaut LeSeulArtichaut mentioned this pull request Nov 21, 2022
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@tdejager tdejager mentioned this pull request May 14, 2024
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