Implement destructuring assignment #71156

fanzier · 2020-04-14T23:01:30Z

Implement destructuring assignment, as suggested in rust-lang/rfcs#372. The accompanying RFC is rust-lang/rfcs#2909.

Note: This PR has been split up into #78748, #78836, #79016, which have been merged.

Quick summary: This allows destructuring the LHS of an assignment if it's a (possibly nested) tuple, tuple struct, or slice.
It is implemented via a desugaring (AST -> HIR lowering) as follows:

(a,b) = (1,2)

... becomes ...

{
  let (lhs0,lhs1) = (1,2);
  a = lhs0;
  b = lhs1;
}

Parser changes:

In order to support _ in the LHS, it allows parsing _ as an expression (a change to the parser). It is still disallowed in all other places.
Struct expressions are not required to have a base expression, i.e. Struct { a: 1, .. } becomes legal (in order to act like a struct pattern).

Tracking issue: #71126.

rust-highfive · 2020-04-14T23:01:34Z

r? @estebank

(rust_highfive has picked a reviewer for you, use r? to override)

src/librustc_ast_lowering/expr.rs

src/test/ui/destructuring-assignment/nested_destructure.rs

src/test/ui/destructuring-assignment/struct_destructure.rs

src/librustc_ast_lowering/expr.rs

src/test/ui/destructuring-assignment/tuple_struct_destructure.rs

varkor · 2020-04-19T16:43:49Z

src/test/ui/destructuring-assignment/tuple_destructure.rs

@@ -16,4 +16,7 @@ fn main() {
    assert_eq!((a, b), (2, 2));
    (b, ..) = (5, 6, 7);
    assert_eq!(b, 5);
+
+    // Check the empty tuple.
+    () = ();


This is quite amusing.

since the assignment expression returns () this means the following is now valid Rust:

()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=()=();

A beautiful addition to the weird-exprs test.

Add it to the weird-exprs.rs test :)

goffrie · 2020-04-22T06:14:03Z

src/librustc_ast/mut_visit.rs

@@ -1225,7 +1226,9 @@ pub fn noop_visit_expr<T: MutVisitor>(Expr { kind, id, span, attrs }: &mut Expr,
        ExprKind::Struct(path, fields, expr) => {
            vis.visit_path(path);
            fields.flat_map_in_place(|field| vis.flat_map_field(field));
-            visit_opt(expr, |expr| vis.visit_expr(expr));
+            if let StructRest::Base(expr) = expr {


Would it be better to write this as a match with all the cases expanded?

This case is only ever going to apply to one variant, so I don't think there's danger of missing anything like this.

bors · 2020-05-02T23:37:33Z

☔ The latest upstream changes (presumably #69274) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2020-05-03T12:08:05Z

☔ The latest upstream changes (presumably #71828) made this pull request unmergeable. Please resolve the merge conflicts.

rust-highfive · 2020-05-03T12:40:40Z

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========================== Starting Command Output ===========================
[command]/bin/bash --noprofile --norc /home/vsts/work/_temp/5b0453d1-e90d-4699-89d8-8e332f64d40b.sh

##[section]Finishing: Disable git automatic line ending conversion
##[section]Starting: Checkout rust-lang/rust@refs/pull/71156/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
---
##[command]git remote add origin https://github.com/rust-lang/rust
##[command]git config gc.auto 0
##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
##[command]git config --get-all http.proxy
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/71156/merge:refs/remotes/pull/71156/merge
---
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Step 6/7 : ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
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Step 7/7 : ENV SCRIPT python3 ../x.py test src/tools/expand-yaml-anchors &&            python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu &&            python3 ../x.py build --stage 0 src/tools/build-manifest &&            python3 ../x.py test --stage 0 src/tools/compiletest &&            python3 ../x.py test src/tools/tidy &&            /scripts/validate-toolstate.sh
 ---> c5008ef7ae8e
Successfully built c5008ef7ae8e
Successfully tagged rust-ci:latest
Built container sha256:c5008ef7ae8e94d7ef502e3cef26e61208e14ebdb36913f3a8bb86291bd6430b
---
    Checking rustc_feature v0.0.0 (/checkout/src/librustc_feature)
    Checking fmt_macros v0.0.0 (/checkout/src/libfmt_macros)
    Checking rustc_ast_pretty v0.0.0 (/checkout/src/librustc_ast_pretty)
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    Checking rustc_query_system v0.0.0 (/checkout/src/librustc_query_system)
    Checking rustc_attr v0.0.0 (/checkout/src/librustc_attr)
    Checking rustc_parse v0.0.0 (/checkout/src/librustc_parse)
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    Checking rustc_ast_lowering v0.0.0 (/checkout/src/librustc_ast_lowering)
---
  local time: Sun May  3 10:56:20 UTC 2020
  network time: Sun, 03 May 2020 10:56:20 GMT
== end clock drift check ==

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Muirrum · 2020-06-13T13:22:26Z

@rustbot modify labels to +S-waiting-on-author -S-waiting-on-review

leonardo-m · 2020-07-22T11:40:42Z

I hope to see this ergonomy improvement in Rust

Muirrum · 2020-08-06T16:50:40Z

@fanzier Any updates on this PR?

kennytm · 2020-08-07T09:09:31Z

@Muirrum the RFC rust-lang/rfcs#2909 have to be merged first before this PR can be progressed.

leonardo-m · 2020-08-11T11:44:48Z

The most basic use case:
(a, b) = (b, a);

To replace:
swap(&mut a, &mut b);

Probably unrelated: with const generics I sometimes wish to do something like this:

const (A, B): (u32, u32) = (10, 20);

fanzier · 2020-11-04T16:10:39Z

I tried to rebase this branch first but there was a conflict with the PR #77283, which doesn't work anymore with the desugaring in this PR. As a consequence, the test src/test/ui/issues/issue-77218.rs fails. Since I don't understand @estebank's PR well, I'm not sure how to fix this. The problem is the check in compiler/rustc_typeck/src/check/expr.rs in the function check_expr_assign, which doesn't fire anymore.
@varkor any ideas?

Edit: To be clear, this is just a regression in the diagnostics: the help message is missing.

fanzier · 2020-11-04T19:18:30Z

As suggested by @petrochenkov, I've opened a new PR just for tuple destructuring as a first step: #78748.

…enkov Implement destructuring assignment for tuples This is the first step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the first part of rust-lang#71156, which was split up to allow for easier review. Quick summary: This change allows destructuring the LHS of an assignment if it's a (possibly nested) tuple. It is implemented via a desugaring (AST -> HIR lowering) as follows: ```rust (a,b) = (1,2) ``` ... becomes ... ```rust { let (lhs0,lhs1) = (1,2); a = lhs0; b = lhs1; } ``` Thanks to `@varkor` who helped with the implementation, particularly around default binding modes. r? `@petrochenkov`

bors · 2020-11-09T02:58:45Z

☔ The latest upstream changes (presumably #78889) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

arora-aman · 2020-11-11T04:46:25Z

Can someone confirm I'm understanding this correctly?

The following will be converted

let tup = (1, 2)
let (_, x) = tup

to

let tup = (1, 2)
let (lhs1, lhs2) = tup
let _ = lhs1;
let x = lhs2;

programmerjake · 2020-11-11T05:26:27Z

Can someone confirm I'm understanding this correctly?

yes, except it would work like this:
The following will be converted

let tup = (1, 2);
(_, x) = tup;

to

let tup = (1, 2);
let (_, lhs2) = tup;
x = lhs2;

…ing, r=petrochenkov Implement destructuring assignment for structs and slices This is the second step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the second part of rust-lang#71156, which was split up to allow for easier review. Note that the first PR (rust-lang#78748) is not merged yet, so it is included as the first commit in this one. I thought this would allow the review to start earlier because I have some time this weekend to respond to reviews. If `@petrochenkov` prefers to wait until the first PR is merged, I totally understand, of course. This PR implements destructuring assignment for (tuple) structs and slices. In order to do this, the following *parser change* was necessary: struct expressions are not required to have a base expression, i.e. `Struct { a: 1, .. }` becomes legal (in order to act like a struct pattern). Unfortunately, this PR slightly regresses the diagnostics implemented in rust-lang#77283. However, it is only a missing help message in `src/test/ui/issues/issue-77218.rs`. Other instances of this diagnostic are not affected. Since I don't exactly understand how this help message works and how to fix it yet, I was hoping it's OK to regress this temporarily and fix it in a follow-up PR. Thanks to `@varkor` who helped with the implementation, particularly around the struct rest changes. r? `@petrochenkov`

…ing, r=petrochenkov Implement destructuring assignment for structs and slices This is the second step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the second part of rust-lang#71156, which was split up to allow for easier review. Note that the first PR (rust-lang#78748) is not merged yet, so it is included as the first commit in this one. I thought this would allow the review to start earlier because I have some time this weekend to respond to reviews. If ``@petrochenkov`` prefers to wait until the first PR is merged, I totally understand, of course. This PR implements destructuring assignment for (tuple) structs and slices. In order to do this, the following *parser change* was necessary: struct expressions are not required to have a base expression, i.e. `Struct { a: 1, .. }` becomes legal (in order to act like a struct pattern). Unfortunately, this PR slightly regresses the diagnostics implemented in rust-lang#77283. However, it is only a missing help message in `src/test/ui/issues/issue-77218.rs`. Other instances of this diagnostic are not affected. Since I don't exactly understand how this help message works and how to fix it yet, I was hoping it's OK to regress this temporarily and fix it in a follow-up PR. Thanks to ``@varkor`` who helped with the implementation, particularly around the struct rest changes. r? ``@petrochenkov``

fanzier · 2020-11-14T19:08:05Z

As all three parts of this PR (#78748, #78836, #79016) have been merged (or at least approved), I'll close this PR.

A huge thank you to @petrochenkov for reviewing them so quickly and thoroughly, catching a few bugs in the process!

test: add `()=()=()=...` to weird-exprs.rs Idea from rust-lang#71156 (comment) 😄 Builds on nightly since rust-lang#78748 has been merged.

…ing, r=petrochenkov Implement destructuring assignment for structs and slices This is the second step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the second part of rust-lang#71156, which was split up to allow for easier review. Note that the first PR (rust-lang#78748) is not merged yet, so it is included as the first commit in this one. I thought this would allow the review to start earlier because I have some time this weekend to respond to reviews. If ``@petrochenkov`` prefers to wait until the first PR is merged, I totally understand, of course. This PR implements destructuring assignment for (tuple) structs and slices. In order to do this, the following *parser change* was necessary: struct expressions are not required to have a base expression, i.e. `Struct { a: 1, .. }` becomes legal (in order to act like a struct pattern). Unfortunately, this PR slightly regresses the diagnostics implemented in rust-lang#77283. However, it is only a missing help message in `src/test/ui/issues/issue-77218.rs`. Other instances of this diagnostic are not affected. Since I don't exactly understand how this help message works and how to fix it yet, I was hoping it's OK to regress this temporarily and fix it in a follow-up PR. Thanks to ``@varkor`` who helped with the implementation, particularly around the struct rest changes. r? ``@petrochenkov``

…etrochenkov Make `_` an expression, to discard values in destructuring assignments This is the third and final step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the third and final part of rust-lang#71156, which was split up to allow for easier review. With this PR, an underscore `_` is parsed as an expression but is allowed *only* on the left-hand side of a destructuring assignment. There it simply discards a value, similarly to the wildcard `_` in patterns. For instance, ```rust (a, _) = (1, 2) ``` will simply assign 1 to `a` and discard the 2. Note that for consistency, ``` _ = foo ``` is also allowed and equivalent to just `foo`. Thanks to ````@varkor```` who helped with the implementation, particularly around pre-expansion gating. r? ````@petrochenkov````

rust-highfive assigned estebank Apr 14, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 14, 2020

varkor marked this pull request as draft April 14, 2020 23:14

varkor mentioned this pull request Apr 14, 2020

Tracking Issue for destructuring_assignment #71126

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This was referenced Apr 17, 2020

Destructuring assignment rust-lang/rfcs#2909

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Destructuring assignment rust-lang/rfcs#372

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fanzier force-pushed the destructuring-assignment branch from 0c87939 to bcd08a3 Compare May 3, 2020 10:47

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 Jun 13, 2020

varkor and others added 8 commits November 4, 2020 14:23

Add additional tests for trailing ..

ac37e3c

Add pre-expansion gating

f015a2e

Make pre-expansion gate slightly less obnoxious

9a5d725

Fix ui-fulldeps test

fbced7d

Fix rebase issues

60552c7

Add support for StructRest in clippy

8614f44

Prevent default match bindings in destructuring assignment

a0d4c55

Fix rebase issue and test

c25779a

fanzier force-pushed the destructuring-assignment branch from 5c8c229 to c25779a Compare November 4, 2020 16:04

fanzier marked this pull request as draft November 4, 2020 16:07

fanzier mentioned this pull request Nov 4, 2020

Implement destructuring assignment for tuples #78748

Merged

fanzier mentioned this pull request Nov 7, 2020

Implement destructuring assignment for structs and slices #78836

Merged

slanterns mentioned this pull request Nov 11, 2020

test: add ()=()=()=... to weird-exprs.rs #78948

Merged

jyn514 mentioned this pull request Nov 12, 2020

frontend: Show feature flags in topbar and separate page rust-lang/docs.rs#1144

Merged

fanzier mentioned this pull request Nov 13, 2020

Make _ an expression, to discard values in destructuring assignments #79016

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fanzier closed this Nov 14, 2020

varkor added the F-destructuring_assignment `#![feature(destructuring_assignment)]` label Nov 19, 2020

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