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Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 
Stabilize `let else`

🎉  **Stabilizes the `let else` feature, added by [RFC 3137](https://github.com/rust-lang/rfcs/pull/3137).** 🎉

Reference PR: https://github.com/rust-lang/reference/pull/1156

closes #87335 (`let else` tracking issue)

FCP: https://github.com/rust-lang/rust/pull/93628#issuecomment-1029383585

----------

## 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](https://github.com/rust-lang/rust/pull/93628#issuecomment-1041143670) in this thread, as well as the [followup](https://github.com/rust-lang/rust/pull/93628#issuecomment-1046598419). Since that followup, further changes have happened to the desugaring, in #98574, #99518, #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 df9a2e0687 (#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 5b95df4bdc (#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 bf7c32a447 (#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 #89960 . Later in 102b9125e1 this got removed in favour of more comprehensive tests.

Added by 856541963c (#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 9b45713b6c:

* [`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 #89807, to ensure that `#[allow(...)]` attributes added to the entire `let` statement apply for bindings created by the `let else` pattern.

Added by 61bcd8d307 (#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 #89933 but then removed from the PR due to the error prior to the improvements of #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 102b9125e1 (#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 2715c5f984 (#89841):

* Match ergonomic tests adapted from the `rfc2005` test suite.

Added by fec8a507a2 (#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 76ea566677 (#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 #93995.

Added by e7730dcb7e (#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 5bd71063b3 (#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 #92069, which got fixed without addition of a regression test. This resolves a missing test as pointed out by the stabilization report.

Added by 5374688e1d (#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 6c529ded86 (#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 #98672

Added by 9b56640106 (#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 #93951
* Extension of `src/test/ui/let-else/let-else-temporary-lifetime.rs` to include a partial regression test for #98672 (especially regarding `else` drop order)

Added by baf9a7cb57 (#99518):

* Extension of `src/test/ui/let-else/let-else-temporary-lifetime.rs` to include a partial regression test for #93951, similar to `let-else-temp-borrowck.rs`

Added by 60be2de8b7 (#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 #99518

Added by 47a7a91c96 (#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 #100103, to ensure that there is no ICE when doing `Err(...)?` inside else blocks.

Added by e3c5bd617d (#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 981852677c (#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 #94176.

Added by e182d12a84 (#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 e26285603c (#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: https://github.com/rust-lang/rust/issues/98672#issuecomment-1200196921

Added by 2d8460ef43 (#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](https://github.com/rust-lang/rust/pull/93628#issuecomment-1238498468) a [stabilization blocker](https://github.com/rust-lang/rust/pull/93628#issuecomment-1055738523).

Added by 1b87ce0d40 (#101410):

* Edit to `src/test/ui/let-else/let-else-temporary-lifetime.rs` to add the `-Zvalidate-mir` flag, as a regression test for #99228

Added by af591ebe4d (#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 #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 e7730dcb7eb29a10ee73f269f4dc6e9d606db0da*
* ~~There is no `#[allow(...)]` test for the expression, as there are tests for the pattern and the else block.~~ → *test added by e7730dcb7eb29a10ee73f269f4dc6e9d606db0da*
* ~~`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 e7730dcb7eb29a10ee73f269f4dc6e9d606db0da*
* ~~#92069 got closed as fixed, but no regression test was added. Not sure it's worth to add one.~~ → *test added by 5bd71063b3810d977aa376d1e6dd7cec359330cc*
* ~~consistency between `let else` and `if let` regarding lifetimes and drop order: https://github.com/rust-lang/rust/pull/93628#issuecomment-1055738523~~ → *test added by 2d8460ef43d902f34ba2133fe38f66ee8d2fdafc*

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 #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.
2022-09-17 15:31:06 +05:30
..
rustc
rustc_apfloat
rustc_arena Make use of [wrapping_]byte_{add,sub} 2022-08-23 19:32:37 +04:00
rustc_ast Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_ast_lowering Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_ast_passes Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_ast_pretty make mk_attr_id part of ParseSess 2022-09-14 08:49:10 +08:00
rustc_attr Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_borrowck Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_builtin_macros Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_codegen_cranelift Auto merge of #98588 - b-naber:valtrees-cleanup, r=lcnr 2022-09-17 03:04:22 +00:00
rustc_codegen_gcc Remove dead broken code from const zst handling in backends 2022-09-06 14:09:49 +00:00
rustc_codegen_llvm Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_codegen_ssa Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_const_eval Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_data_structures Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_driver Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_error_codes Stabilize #![feature(label_break_value)] 2022-08-23 21:14:12 -05:00
rustc_error_messages Rollup merge of #101782 - JhonnyBillM:refactor-symbol-mangling-diags-migration, r=davidtwco 2022-09-16 20:37:15 -07:00
rustc_errors Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_expand Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_feature Stabilize the let_else feature 2022-09-15 21:06:45 +02:00
rustc_fs_util
rustc_graphviz
rustc_hir Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_hir_pretty Handle generic parameters. 2022-09-09 01:31:46 +00:00
rustc_incremental Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_index Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_infer Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_interface Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_lexer remove unnecessary PartialOrd and Ord 2022-09-08 06:15:33 +09:00
rustc_lint Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_lint_defs Compute lint_levels by definition 2022-09-14 19:02:44 +02:00
rustc_llvm Introduce a fallible variant of LLVMConstIntGetZExtValue 2022-09-09 15:54:14 +02:00
rustc_log
rustc_macros Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_metadata Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_middle Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_mir_build Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_mir_dataflow Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_mir_transform Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_monomorphize Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_parse Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_parse_format Separate CountIsStar from CountIsParam in rustc_parse_format. 2022-08-25 14:49:09 +02:00
rustc_passes Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_plugin_impl
rustc_privacy Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_query_impl Rollup merge of #101787 - compiler-errors:cache-rpitit, r=petrochenkov 2022-09-16 11:17:01 +05:30
rustc_query_system Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_resolve Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_save_analysis Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_serialize Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_session Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_smir Only keep one version of ImplicitSelfKind. 2022-09-13 19:18:23 +02:00
rustc_span Only enable the let_else feature on bootstrap 2022-09-15 21:06:45 +02:00
rustc_symbol_mangling Rollup merge of #101782 - JhonnyBillM:refactor-symbol-mangling-diags-migration, r=davidtwco 2022-09-16 20:37:15 -07:00
rustc_target Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_trait_selection Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_traits Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_transmute Auto merge of #100726 - jswrenn:transmute, r=oli-obk 2022-09-04 07:55:44 +00:00
rustc_ty_utils Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30
rustc_type_ir Address code review comments 2022-09-13 14:50:12 -07:00
rustc_typeck Rollup merge of #93628 - est31:stabilize_let_else, r=joshtriplett 2022-09-17 15:31:06 +05:30