Implement let-else type annotations natively #89841

cormacrelf · 2021-10-13T07:52:59Z

Tracking issue: #87335

Fixes #89688, fixes #89807, edit: fixes #89960 as well

As explained in #89688 (comment), the previous desugaring moved the let-else scrutinee into a dummy variable, which meant if you wanted to refer to it again in the else block, it had moved.

This introduces a new hir type, ~~hir::LetExpr~~ hir::Let, which takes over all the fields of hir::ExprKind::Let(...) and adds an optional type annotation. The hir::Let is then treated like a hir::Local when type checking a function body, specifically:

GatherLocalsVisitor overrides a new Visitor::visit_let_expr and does pretty much exactly what it does for visit_local, assigning a local type to the hir::Let ~~(they could be deduplicated but they are right next to each other, so at least we know they're the same)~~
It reuses the code in check_decl_local to typecheck the hir::Let, simply returning 'bool' for the expression type after doing that.
~~FnCtxt::check_expr_let passes this local type in to demand_scrutinee_type, and then imitates check_decl_local's pattern checking~~
~~demand_scrutinee_type (the blindest change for me, please give this extra scrutiny) uses this local type instead of of creating a new one~~
- ~~Just realised the check_expr_with_needs was passing NoExpectation further down, need to pass the type there too. And apparently this Expectation API already exists.~~

Some other misc notes:

~~Is the clippy code supposed to be autoformatted? I tried not to give huge diffs but maybe some rustfmt changes simply haven't hit it yet.~~
in rustc_ast_lowering/src/block.rs, I noticed some existing self.alias_attrs() calls in LoweringContext::lower_stmts seem to be copying attributes from the lowered locals/etc to the statements. Is that right? I'm new at this, I don't know.

rust-highfive · 2021-10-13T07:53:01Z

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

rust-highfive · 2021-10-13T07:53:03Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

camsteffen · 2021-10-13T12:07:34Z

Clippy has a different rustfmt config so the formatting changes would have to be reverted.

I wonder if the type could just be added to ExprKind::Let?

cormacrelf · 2021-10-13T12:30:37Z

I wonder if the type could just be added to ExprKind::Let?

That's what I did initially. From a diff perspective, this way you save having to write those field types in all the visitors, and the knock-on effects of adding the type boil down to adding LetExpr { , .. } around pattern bindings. You win some you lose some. Subjectively I think it helped.

From interoperability, I would note that:

Nearly all other hir nodes that need to be selectively walked in an override-able visitor method are separate types, e.g. hir::Arm.
A few lines down from struct LetExpr is struct Guard, with the current (your?) FIXME: use ExprKind::Let for this that doesn't make much sense to me, ExprKind being an enum with many other variants after all. I think you'd want a separate struct for this reason alone.

cormacrelf · 2021-10-13T12:53:26Z

Re the clippy rustfmt, I don't know how to make it format according to src/tools/clippy/rustfmt.toml. cargo fmt from inside that directory does nothing. So does x fmt, which I think is because the root one ignores all the submodules/subtrees. Bit lost on this one, and can't find anything about it in the guide, "clippy rustfmt" is un-googleable.

compiler/rustc_hir/src/intravisit.rs

compiler/rustc_hir_pretty/src/lib.rs

src/tools/clippy/clippy_utils/src/hir_utils.rs

camsteffen · 2021-10-13T12:59:21Z

Yeah the formatting situation is unfortunate. It is just not supported in the rust repo. Typically there are some formatting discrepancies that get fixed with every clippy repo sync. The only way I know to do it automatically is to patch your changes in the clippy repo and run cargo dev fmt there, then patch back. I believe there is sentiment for improving this by making rustfmt the same as rustc.

Mark-Simulacrum · 2021-10-13T13:20:03Z

r? rust-lang/compiler-team

cormacrelf · 2021-10-13T13:41:34Z

Ah, I think you can run cargo fmt -p clippy_lints / _utils etc and it will do it. I can't tell the vintage of rust code from one sniff, but it has undone the missing match_block_trailing_comma = true. The line length ones can't be reverted automatically because it's happy to leave shorter lines in place.

cormacrelf · 2021-10-13T16:30:18Z

That's due to some unintended variable shadowing after I renamed scrutinee to expr... :/

cormacrelf · 2021-10-13T17:34:57Z

src/tools/clippy/clippy_lints/src/pattern_type_mismatch.rs

+        if let ExprKind::Let(let_expr) = expr.kind {
+            if let Some(expr_ty) = cx.typeck_results().node_type_opt(let_expr.init.hir_id) {
+                if in_external_macro(cx.sess(), let_expr.pat.span) {
                    return;
                }
-                apply_lint(cx, let_pat, expr_ty, DerefPossible::Possible);
+                apply_lint(cx, let_expr.pat, expr_ty, DerefPossible::Possible);


Should this be using the Let's hir_id (instead of the init expression's) to get info out of typeck, because we used that hir_id for the local type in typeck?

I think Let should not have a HirId since Expr already has one.

Does typeck let you add a local type for a hirid that's actually an expr? Currently Let::hir_id is where we mount the local type.

I'm reasonably sure you need a HirId that is reserved for a local Ty, that can be based on the hir::Ty and be separate from the type of the expression. If you remove Let::hir_id, I don't see how this works:

(the expr containing an ExprKind::Let(_) should have type boolean)

the let's init expression has a type

the let's optional hir::Ty is also resolved to a local Ty

the init and the local Ty are assigned to the the same HirId

unifying that resolved Ty with the init expression Ty is somehow still meaningful, not always a tautology

ExprKind::Match doesn't need its own HirId because it does not need to be given a local type independent from the expression. Local does because it does. Ours is emulating Local.

If I'm right then I've answered my own question, after type checking the let_expr.hir_id and the let_expr.init.hir_id have been unified and it shouldn't matter which one you use to query for it.

There is also Pat hirid and the type really dictates the type of the pattern. Not sure if that makes a difference.

Upshot is that maybe you do maybe you don't need one, but it ultimately simplified treating Let and Local identically that it does have one.

cormacrelf · 2021-10-13T17:43:21Z

src/tools/clippy/clippy_lints/src/utils/inspector.rs

+        hir::ExprKind::Let(hir::Let { pat, init, .. }) => {
            print_pat(cx, pat, indent + 1);
-            print_expr(cx, expr, indent + 1);
+            print_expr(cx, init, indent + 1);
        },


Probably need to print the type, and apparently should use the typeck results to do so

cormacrelf · 2021-10-16T08:47:38Z

Mm, update, this doesn't do Deref coercion properly. I have a failing test with an example that works for regular let,. I'm working on it so marking as draft for now.

cormacrelf · 2021-10-17T06:59:22Z

Ok, we're back in business I think. Let-else is now type-checked exactly as a let statement is checked; they use the same code path via a Declaration abstraction (only exists in typeck). Obviously this raises a fair question, why not just use hir::Local all the way? I thought of this, tried it, and it was painful.

The init being optional on a Local means lots of local.init.unwrap_or_else(|| span_bug!(local.span, "...")). It's nice to have a type that maps exactly to the construct, and this remains true for future use cases e.g. Guard::IfLet.
The visitor implementations got knotty, wondering whether you need to .visit_local(local) or just use its component parts. It's less confusing this way, at least for me, it forces you to think about them differently and I think that is generally justified by the non-optional inititaliser.

I have added a bunch of tests, one adapted from @est31's rustc work (see the tracking issue) and a bunch adapted from rfc2005 match ergonomics to check that that all works, especially with type annotations attached.

cormacrelf · 2021-10-17T07:09:21Z

src/test/ui/let-else/let-else-binding-explicit-mut-borrow.rs

+// Slightly different from explicit-mut-annotated -- this won't show an error until borrowck.
+// Should it show a type error instead?
+
+fn main() {
+    let Some(n): &mut Option<i32> = &mut &Some(5i32) else { //~ ERROR cannot borrow data in a `&` reference as mutable
+        return
+    };


This is the most interesting/surprising case to me, not sure if it's wrong or not but it stood out.

The original goes like this:

rust/src/test/ui/rfc-2005-default-binding-mode/explicit-mut.rs

Lines 13 to 15 in 4e89811

match &mut &Some(5i32) {

Some(n) => {

*n += 1; //~ ERROR cannot assign to `*n`, which is behind a `&` reference

Compare to this PR's explicit-mut-annotated.rs where the other combinations of & and &mut produce type mismatch ... types differ in mutability errors.

This seems like a poor message indeed, but since it behaves the same as

let n: &mut Option<i32> = &mut &Some(5i32);

this actually gives me some confidence that the behaviour here is correct (for now.)

cjgillot

Thanks for the PR @cormacrelf. I wonder if we would simplify the code by using directly a hir::Local in ExprKind::Let instead of introducing hir::Let (which has essentially the same fields).

EDIT: sorry, did not see your earlier comment.

bors · 2021-10-18T10:26:36Z

☔ The latest upstream changes (presumably #90000) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2021-10-22T02:53:59Z

☔ The latest upstream changes (presumably #90126) made this pull request unmergeable. Please resolve the merge conflicts.

unify typeck of hir::Local and hir::Let remove extraneous pub(crate/super)

fix clippy format using `cargo fmt -p clippy_{lints,utils}` manually revert rustfmt line truncations rename to hir::Let in clippy Undo the shadowing of various `expr` variables after renaming `scrutinee` reduce destructuring of hir::Let to avoid `expr` collisions cargo fmt -p clippy_{lints,utils} bless new clippy::author output

expands issue 89960

collect explicit-mut passing tests in one file

cormacrelf · 2021-12-13T03:19:45Z

@nagisa i think that's done, thanks for your help!

nagisa · 2021-12-17T19:46:40Z

@bors r+ rollup=never

Thanks!

bors · 2021-12-17T19:46:42Z

📌 Commit fec8a50 has been approved by nagisa

bors · 2021-12-17T22:12:43Z

⌛ Testing commit fec8a50 with merge dde825d...

bors · 2021-12-18T01:12:06Z

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing dde825d to master...

rust-timer · 2021-12-18T06:32:01Z

Finished benchmarking commit (dde825d): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Implement let-else type annotations natively Tracking issue: rust-lang#87335 Fixes rust-lang#89688, fixes rust-lang#89807, edit: fixes rust-lang#89960 as well As explained in rust-lang#89688 (comment), the previous desugaring moved the let-else scrutinee into a dummy variable, which meant if you wanted to refer to it again in the else block, it had moved. This introduces a new hir type, ~~`hir::LetExpr`~~ `hir::Let`, which takes over all the fields of `hir::ExprKind::Let(...)` and adds an optional type annotation. The `hir::Let` is then treated like a `hir::Local` when type checking a function body, specifically: * `GatherLocalsVisitor` overrides a new `Visitor::visit_let_expr` and does pretty much exactly what it does for `visit_local`, assigning a local type to the `hir::Let` ~~(they could be deduplicated but they are right next to each other, so at least we know they're the same)~~ * It reuses the code in `check_decl_local` to typecheck the `hir::Let`, simply returning 'bool' for the expression type after doing that. * ~~`FnCtxt::check_expr_let` passes this local type in to `demand_scrutinee_type`, and then imitates check_decl_local's pattern checking~~ * ~~`demand_scrutinee_type` (the blindest change for me, please give this extra scrutiny) uses this local type instead of of creating a new one~~ * ~~Just realised the `check_expr_with_needs` was passing NoExpectation further down, need to pass the type there too. And apparently this Expectation API already exists.~~ Some other misc notes: * ~~Is the clippy code supposed to be autoformatted? I tried not to give huge diffs but maybe some rustfmt changes simply haven't hit it yet.~~ * in `rustc_ast_lowering/src/block.rs`, I noticed some existing `self.alias_attrs()` calls in `LoweringContext::lower_stmts` seem to be copying attributes from the lowered locals/etc to the statements. Is that right? I'm new at this, I don't know.

…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.

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Tracking Issue for RFC 3137: let-else statements #87335

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Fix ICE with let...else and ref mut #89965

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cormacrelf force-pushed the let-else-typed branch from 38ba92f to 5f711f2 Compare October 18, 2021 12:47

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cormacrelf added 7 commits December 13, 2021 14:02

let-else: add hir::Let and type check it like a hir::Local

af2f0e6

unify typeck of hir::Local and hir::Let remove extraneous pub(crate/super)

let-else: fix attribute aliasing + add test for issue 89807

9b45713

let-else: add tests for moved expressions, copy out of non-copy

61bcd8d

let-else: build out ref/ref mut tests, with/without explicit annotations

102b912

expands issue 89960

let-else: add match-ergonomics tests adapted from rfc2005

2715c5f

collect explicit-mut passing tests in one file

let-else: add deref-coercion tests

fec8a50

cormacrelf force-pushed the let-else-typed branch from f68510a to fec8a50 Compare December 13, 2021 03:03

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 Dec 17, 2021

matthiaskrgr mentioned this pull request Dec 17, 2021

Rollup of 7 pull requests #92049

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bors added the merged-by-bors This PR was explicitly merged by bors. label Dec 18, 2021

bors merged commit dde825d into rust-lang:master Dec 18, 2021

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This was referenced Dec 18, 2021

Fix false positives in unreachable_code lint #90636

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Tweak output for non-exhaustive match expression #91993

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Rollup of 7 pull requests #92031

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spaarmann mentioned this pull request Dec 20, 2021

Wrong suggestion for slice patterns when using let-else statement #92069

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est31 mentioned this pull request Feb 9, 2022

Stabilize let else #93628

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Implement let-else type annotations natively #89841

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	match &mut &Some(5i32) {
	Some(n) => {
	n += 1; //~ ERROR cannot assign to `n`, which is behind a `&` reference

Implement let-else type annotations natively #89841

Implement let-else type annotations natively #89841

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