Let-else: break out scopes when a let-else pattern fails to match #99518

dingxiangfei2009 · 2022-07-20T16:39:57Z

This PR will commit to a new behavior so that values from initializer expressions are dropped earlier when a let-else pattern fails to match.

Fix #98672.
Close #93951.
cc @camsteffen @est31

rust-highfive · 2022-07-20T16:40:00Z

r? @lcnr

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

est31 · 2022-07-20T18:13:29Z

Nice, this was fast!

Btw, if you are adding a regression test for #93951, you should add "closes #93951" to the PR description.

lcnr · 2022-07-21T12:27:07Z

i expect this to also cause some programs to stop compiling because temporaries are now dropped earlier? 🤔 would be good to also add a test where this is the case

reassigning as I don't have a lot of time during the next few weeks

r? rust-lang/compiler

est31 · 2022-07-21T13:57:56Z

@lcnr I think this change is doing the opposite: the set of programs that the compiler accepts after this change A is a strict superset of the programs before B. (Let's just not look at const eval for now).

You see B \ A = ∅ as any program that now fails at compile time would have needed to access temporaries from the else block. But there is no way to do that.

You see A \ B ≠ ∅ as now programs compile that access resources in the else block that were used by the temporaries. E.g. references to common variables. Example program that now compiles thanks to the PR (@dingxiangfei2009 feel free to integrate it as test):

#![feature(let_else)]

struct Foo<'a>(&'a mut u32);

impl<'a> Drop for Foo<'a> {
    fn drop(&mut self) {
        *self.0 = 0;
    }
}

fn main() {
    let mut foo = 0;
    let Foo(0) = Foo(&mut foo) else {
        *&mut foo = 1;
        todo!()
    };
}

This change still might break programs at runtime, but the PR already has a runtime test.

compiler-errors · 2022-07-28T21:13:07Z

I'm not the best person to review this.

r? compiler

oli-obk · 2022-07-29T13:10:20Z

@bors r+

bors · 2022-07-29T13:10:22Z

📌 Commit 60be2de has been approved by oli-obk

It is now in the queue for this repository.

Rollup of 8 pull requests Successful merges: - rust-lang#99227 (Fix thumbv4t-none-eabi frame pointer setting) - rust-lang#99518 (Let-else: break out scopes when a let-else pattern fails to match) - rust-lang#99671 (Suggest dereferencing index when trying to use a reference of usize as index) - rust-lang#99831 (Add Fuchsia platform support documentation) - rust-lang#99881 (fix ICE when computing codegen_fn_attrs on closure with non-fn parent) - rust-lang#99888 (Streamline lint checking) - rust-lang#99891 (Adjust an expr span to account for macros) - rust-lang#99904 (Cleanup html whitespace) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

…-higher-up, r=oli-obk let-else: break out to one scope higher for let-else ``@est31`` This PR follows up with rust-lang#99518 which is to break out to the last remainder scope. It breaks to the out-most `region_scope` of the block if the first statement is a `let-else`.

…-higher-up, r=oli-obk let-else: break out to one scope higher for let-else ```@est31``` This PR follows up with rust-lang#99518 which is to break out to the last remainder scope. It breaks to the out-most `region_scope` of the block if the first statement is a `let-else`.

…plett Stabilize `let else` :tada: **Stabilizes the `let else` feature, added by [RFC 3137](rust-lang/rfcs#3137 🎉 Reference PR: rust-lang/reference#1156 closes rust-lang#87335 (`let else` tracking issue) FCP: rust-lang#93628 (comment) ---------- ## Stabilization report ### Summary The feature allows refutable patterns in `let` statements if the expression is followed by a diverging `else`: ```Rust fn get_count_item(s: &str) -> (u64, &str) { let mut it = s.split(' '); let (Some(count_str), Some(item)) = (it.next(), it.next()) else { panic!("Can't segment count item pair: '{s}'"); }; let Ok(count) = u64::from_str(count_str) else { panic!("Can't parse integer: '{count_str}'"); }; (count, item) } assert_eq!(get_count_item("3 chairs"), (3, "chairs")); ``` ### Differences from the RFC / Desugaring Outside of desugaring I'm not aware of any differences between the implementation and the RFC. The chosen desugaring has been changed from the RFC's [original](https://rust-lang.github.io/rfcs/3137-let-else.html#reference-level-explanations). You can read a detailed discussion of the implementation history of it in `@cormacrelf` 's [summary](rust-lang#93628 (comment)) in this thread, as well as the [followup](rust-lang#93628 (comment)). Since that followup, further changes have happened to the desugaring, in rust-lang#98574, rust-lang#99518, rust-lang#99954. The later changes were mostly about the drop order: On match, temporaries drop in the same order as they would for a `let` declaration. On mismatch, temporaries drop before the `else` block. ### Test cases In chronological order as they were merged. Added by df9a2e0 (rust-lang#87688): * [`ui/pattern/usefulness/top-level-alternation.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/pattern/usefulness/top-level-alternation.rs) to ensure the unreachable pattern lint visits patterns inside `let else`. Added by 5b95df4 (rust-lang#87688): * [`ui/let-else/let-else-bool-binop-init.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-bool-binop-init.rs) to ensure that no lazy boolean expressions (using `&&` or `||`) are allowed in the expression, as the RFC mandates. * [`ui/let-else/let-else-brace-before-else.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-brace-before-else.rs) to ensure that no `}` directly preceding the `else` is allowed in the expression, as the RFC mandates. * [`ui/let-else/let-else-check.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-check.rs) to ensure that `#[allow(...)]` attributes added to the entire `let` statement apply for the `else` block. * [`ui/let-else/let-else-irrefutable.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-irrefutable.rs) to ensure that the `irrefutable_let_patterns` lint fires. * [`ui/let-else/let-else-missing-semicolon.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-missing-semicolon.rs) to ensure the presence of semicolons at the end of the `let` statement. * [`ui/let-else/let-else-non-diverging.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-non-diverging.rs) to ensure the `else` block diverges. * [`ui/let-else/let-else-run-pass.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-run-pass.rs) to ensure the feature works in some simple test case settings. * [`ui/let-else/let-else-scope.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-scope.rs) to ensure the bindings created by the outer `let` expression are not available in the `else` block of it. Added by bf7c32a (rust-lang#89965): * [`ui/let-else/issue-89960.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/issue-89960.rs) as a regression test for the ICE-on-error bug rust-lang#89960 . Later in 102b912 this got removed in favour of more comprehensive tests. Added by 8565419 (rust-lang#89974): * [`ui/let-else/let-else-if.rs`](https://github.com/rust-lang/rust/blob/1.58.1/src/test/ui/let-else/let-else-if.rs) to test for the improved error message that points out that `let else if` is not possible. Added by 9b45713: * [`ui/let-else/let-else-allow-unused.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-allow-unused.rs) as a regression test for rust-lang#89807, to ensure that `#[allow(...)]` attributes added to the entire `let` statement apply for bindings created by the `let else` pattern. Added by 61bcd8d (rust-lang#89841): * [`ui/let-else/let-else-non-copy.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-non-copy.rs) to ensure that a copy is performed out of non-copy wrapper types. This mirrors `if let` behaviour. The test case bases on rustc internal changes originally meant for rust-lang#89933 but then removed from the PR due to the error prior to the improvements of rust-lang#89841. * [`ui/let-else/let-else-source-expr-nomove-pass.rs `](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-source-expr-nomove-pass.rs) to ensure that while there is a move of the binding in the successful case, the `else` case can still access the non-matching value. This mirrors `if let` behaviour. Added by 102b912 (rust-lang#89841): * [`ui/let-else/let-else-ref-bindings.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-ref-bindings.rs) and [`ui/let-else/let-else-ref-bindings-pass.rs `](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-ref-bindings-pass.rs) to check `ref` and `ref mut` keywords in the pattern work correctly and error when needed. Added by 2715c5f (rust-lang#89841): * Match ergonomic tests adapted from the `rfc2005` test suite. Added by fec8a50 (rust-lang#89841): * [`ui/let-else/let-else-deref-coercion-annotated.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-deref-coercion-annotated.rs) and [`ui/let-else/let-else-deref-coercion.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-deref-coercion.rs) to check deref coercions. #### Added since this stabilization report was originally written (2022-02-09) Added by 76ea566 (rust-lang#94211): * [`ui/let-else/let-else-destructuring.rs`](https://github.com/rust-lang/rust/blob/1.63.0/src/test/ui/let-else/let-else-destructuring.rs) to give a nice error message if an user tries to do an assignment with a (possibly refutable) pattern and an `else` block, like asked for in rust-lang#93995. Added by e7730dc (rust-lang#94208): * [`ui/let-else/let-else-allow-in-expr.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-allow-in-expr.rs) to test whether `#[allow(unused_variables)]` works in the expr, as well as its non presence, as well as putting it on the entire `let else` *affects* the expr, too. This was adding a missing test as pointed out by the stabilization report. * Expansion of `ui/let-else/let-else-allow-unused.rs` and `ui/let-else/let-else-check.rs` to ensure that non-presence of `#[allow(unused)]` does issue the unused lint. This was adding a missing test case as pointed out by the stabilization report. Added by 5bd7106 (rust-lang#94208): * [`ui/let-else/let-else-slicing-error.rs`](https://github.com/rust-lang/rust/blob/1.61.0/src/test/ui/let-else/let-else-slicing-error.rs), a regression test for rust-lang#92069, which got fixed without addition of a regression test. This resolves a missing test as pointed out by the stabilization report. Added by 5374688 (rust-lang#98574): * [`src/test/ui/async-await/async-await-let-else.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/async-await/async-await-let-else.rs) to test the interaction of async/await with `let else` Added by 6c529de (rust-lang#98574): * [`src/test/ui/let-else/let-else-temporary-lifetime.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/let-else-temporary-lifetime.rs) as a (partial) regression test for rust-lang#98672 Added by 9b56640 (rust-lang#99518): * [`src/test/ui/let-else/let-else-temp-borrowck.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/let-else-temporary-lifetime.rs) as a regression test for rust-lang#93951 * Extension of `src/test/ui/let-else/let-else-temporary-lifetime.rs` to include a partial regression test for rust-lang#98672 (especially regarding `else` drop order) Added by baf9a7c (rust-lang#99518): * Extension of `src/test/ui/let-else/let-else-temporary-lifetime.rs` to include a partial regression test for rust-lang#93951, similar to `let-else-temp-borrowck.rs` Added by 60be2de (rust-lang#99518): * Extension of `src/test/ui/let-else/let-else-temporary-lifetime.rs` to include a program that can now be compiled thanks to borrow checker implications of rust-lang#99518 Added by 47a7a91 (rust-lang#100132): * [`src/test/ui/let-else/issue-100103.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/issue-100103.rs), as a regression test for rust-lang#100103, to ensure that there is no ICE when doing `Err(...)?` inside else blocks. Added by e3c5bd6 (rust-lang#100443): * [`src/test/ui/let-else/let-else-then-diverge.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/let-else-then-diverge.rs), to verify that there is no unreachable code error with the current desugaring. Added by 9818526 (rust-lang#100443): * [`src/test/ui/let-else/issue-94176.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/issue-94176.rs), to make sure that a correct span is emitted for a missing trailing expression error. Regression test for rust-lang#94176. Added by e182d12 (rust-lang#100434): * [src/test/ui/unpretty/pretty-let-else.rs](https://github.com/rust-lang/rust/blob/master/src/test/ui/unpretty/pretty-let-else.rs), as a regression test to ensure pretty printing works for `let else` (this bug surfaced in many different ways) Added by e262856 (rust-lang#99954): * [`src/test/ui/let-else/let-else-temporary-lifetime.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/let-else-temporary-lifetime.rs) extended to contain & borrows as well, as this was identified as an earlier issue with the desugaring: rust-lang#98672 (comment) Added by 2d8460e (rust-lang#99291): * [`src/test/ui/let-else/let-else-drop-order.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/let-else-drop-order.rs) a matrix based test for various drop order behaviour of `let else`. Especially, it verifies equality of `let` and `let else` drop orders, [resolving](rust-lang#93628 (comment)) a [stabilization blocker](rust-lang#93628 (comment)). Added by 1b87ce0 (rust-lang#101410): * Edit to `src/test/ui/let-else/let-else-temporary-lifetime.rs` to add the `-Zvalidate-mir` flag, as a regression test for rust-lang#99228 Added by af591eb (rust-lang#101410): * [`src/test/ui/let-else/issue-99975.rs`](https://github.com/rust-lang/rust/blob/master/src/test/ui/let-else/issue-99975.rs) as a regression test for the ICE rust-lang#99975. Added by this PR: * `ui/let-else/let-else.rs`, a simple run-pass check, similar to `ui/let-else/let-else-run-pass.rs`. ### Things not currently tested * ~~The `#[allow(...)]` tests check whether allow works, but they don't check whether the non-presence of allow causes a lint to fire.~~ → *test added by e7730dc* * ~~There is no `#[allow(...)]` test for the expression, as there are tests for the pattern and the else block.~~ → *test added by e7730dc* * ~~`let-else-brace-before-else.rs` forbids the `let ... = {} else {}` pattern and there is a rustfix to obtain `let ... = ({}) else {}`. I'm not sure whether the `.fixed` files are checked by the tooling that they compile. But if there is no such check, it would be neat to make sure that `let ... = ({}) else {}` compiles.~~ → *test added by e7730dc* * ~~rust-lang#92069 got closed as fixed, but no regression test was added. Not sure it's worth to add one.~~ → *test added by 5bd7106* * ~~consistency between `let else` and `if let` regarding lifetimes and drop order: rust-lang#93628 (comment) → *test added by 2d8460e* Edit: they are all tested now. ### Possible future work / Refutable destructuring assignments [RFC 2909](https://rust-lang.github.io/rfcs/2909-destructuring-assignment.html) specifies destructuring assignment, allowing statements like `FooBar { a, b, c } = foo();`. As it was stabilized, destructuring assignment only allows *irrefutable* patterns, which before the advent of `let else` were the only patterns that `let` supported. So the combination of `let else` and destructuring assignments gives reason to think about extensions of the destructuring assignments feature that allow refutable patterns, discussed in rust-lang#93995. A naive mapping of `let else` to destructuring assignments in the form of `Some(v) = foo() else { ... };` might not be the ideal way. `let else` needs a diverging `else` clause as it introduces new bindings, while assignments have a default behaviour to fall back to if the pattern does not match, in the form of not performing the assignment. Thus, there is no good case to require divergence, or even an `else` clause at all, beyond the need for having *some* introducer syntax so that it is clear to readers that the assignment is not a given (enums and structs look similar). There are better candidates for introducer syntax however than an empty `else {}` clause, like `maybe` which could be added as a keyword on an edition boundary: ```Rust let mut v = 0; maybe Some(v) = foo(&v); maybe Some(v) = foo(&v) else { bar() }; ``` Further design discussion is left to an RFC, or the linked issue.

break out scopes when let-else fails to match

9b56640

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jul 20, 2022

rust-highfive assigned lcnr Jul 20, 2022

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 20, 2022

est31 mentioned this pull request Jul 21, 2022

Stabilize let else #93628

Merged

rust-highfive assigned compiler-errors and unassigned lcnr Jul 21, 2022

provide a test for rust-lang#93951

baf9a7c

dingxiangfei2009 force-pushed the let-else-additional-tests branch from 94e1bdf to baf9a7c Compare July 21, 2022 15:40

include a demo that more programs can be compiled

60be2de

est31 mentioned this pull request Jul 24, 2022

Add let else drop order tests #99291

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rust-highfive assigned oli-obk and unassigned compiler-errors Jul 28, 2022

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 Jul 29, 2022

JohnTitor mentioned this pull request Jul 29, 2022

Rollup of 8 pull requests #99925

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bors merged commit 955091b into rust-lang:master Jul 30, 2022

rustbot added this to the 1.64.0 milestone Jul 30, 2022

dingxiangfei2009 deleted the let-else-additional-tests branch July 30, 2022 14:40

est31 mentioned this pull request Jul 30, 2022

Let-else does not drop temporaries at the end of the statement #98672

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dingxiangfei2009 mentioned this pull request Jul 30, 2022

let-else: break out to one scope higher for let-else #99954

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est31 mentioned this pull request Aug 10, 2022

"temporary value dropped while borrowed" occurs with let-chains #100276

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Let-else: break out scopes when a let-else pattern fails to match #99518

Let-else: break out scopes when a let-else pattern fails to match #99518

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Let-else: break out scopes when a let-else pattern fails to match #99518

Let-else: break out scopes when a let-else pattern fails to match #99518

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