Tracking issue for RFC 1872: `exhaustive_patterns` feature #51085

kennytm · 2018-05-26T14:59:46Z

This tracks the exhaustive_patterns feature which allows uninhabited variant to be omitted
(bug report: #12609; relevant RFC: rust-lang/rfcs#1872).

fn safe_unwrap<T>(x: Result<T, !>) -> T {
    match x {
        Ok(y) => y,
    }
}

Implementation (separated out from never_type in Stabilise feature(never_type). Introduce feature(exhaustive_patterns) #47630)
blocking issue: Incorrect "unreachable match arm" warning #117119
Stabilization

The text was updated successfully, but these errors were encountered:

…lexcrichton Ensure every unstable language feature has a tracking issue. Filled in the missing numbers: * `abi_ptx` → #38788 * `generators` → #43122 * `global_allocator` → #27389 Reused existing tracking issues because they were decomposed from a larger feature * `*_target_feature` → #44839 (reusing the old `target_feature` number) * `proc_macros_*` → #38356 (reusing the to-be-stabilized `proc_macros` number) Filed new issues * `exhaustive_patterns` → #51085 * `pattern_parentheses` → #51087 * `wasm_custom_section` and `wasm_import_module` → #51088

varkor · 2018-10-16T19:45:23Z

The lack of this feature has unintuitive consequences (e.g. #55123). I haven't seen any problems arising from it either.

cc @rust-lang/lang: could this be put forward for stabilisation?

nikomatsakis · 2018-10-16T20:04:00Z

@varkor I am mildly reluctant until we have agreed upon the story around "never patterns" -- but I suppose given a sufficiently conservative definition of "uninhabited pattern" I'd be all right with it. Do you think you could write-up a "stabilization report" that includes examples of the tests we have and also documents the limits of what sorts of things would be considered uninhabited in stable code?

(That said, I think "never patterns" gives us a path forward that addresses my concerns around aggressive uninhabited definitions, by just turning the "ambiguous" cases into lints in the presence of unsafe code. I'd be happier though if we had an RFC on the topic.)

nikic · 2018-10-26T17:01:28Z

The exhaustive_patterns feature currently seems to have a pretty big performance issue. Having perf top running while compiling librustc, I noticed that it spends an unreasonable amount of time inside ty::inhabitedness. Disabling exhaustive_patterns (it's not actually used in librustc/librustc_mir) shaved off ~30s (of 12min total) from stage1 compiler artifact compile time (in a totally unscientific benchmark, so take with a grain of salt).

I would expect that this issue will resolve itself if there is a consensus to go with references never being uninhabited (ref #54125 (comment)), as the reference handling is what I believe makes this expensive (not looking through references both reduces the amount of types we need to look at, and allows us to drop the expensive hashmap and nested hashsets used to avoid unbounded recursion). If the consensus goes in the other direction, then this code should probably be optimized a bit prior to stabilization of the feature.

jonas-schievink · 2019-05-13T15:34:36Z

cc #51221, using this feature in let-bindings seems to ICE unless the never_type feature is also enabled

Nadrieril · 2020-11-17T17:48:14Z

The exhaustive_patterns feature currently seems to have a pretty big performance issue. Having perf top running while compiling librustc, I noticed that it spends an unreasonable amount of time inside ty::inhabitedness.

It is now the case that "references never are uninhabited", at least as far as is_ty_uninhabited_from is concerned. Is the performance issue still there?
Otherwise, I expect that making is_ty_uninhabited_from into a query or something would significantly improve perf, because match checking repeatedly calls it on the same type and its subcomponents.

varkor · 2020-11-19T18:14:49Z

Is the performance issue still there?

I suspect this requires some investigation. (Perhaps it would be sufficient to open a PR enabling the feature and do a perf run.)

…mpiler-errors Exhaustiveness: Reveal empty opaques in depth Follow-up to rust-lang#116821. As noted [there](rust-lang#116821 (comment)), the current implementation doesn't detect emptiness of opaques when the opaque is nested inside a type. This doesn't matter for stable behavior (which ignores nested empty types anyway) but does matter for the [`exhaustive_patterns`](https://github.com/rust-lang/rust/issues/51085)/[`min_exhaustive_patterns`](https://github.com/rust-lang/rust/pull/118803) features. This PR fixes this behavior by adding `InhabitedPredicate::apply_reveal_opaque` that considers opaque types when determining inhabitedness. r? `@compiler-errors`

Nadrieril · 2024-01-05T10:35:11Z

I'm proposing a (hopefully uncontroversial) subset of this feature under the min_exhaustive_patterns feature gate (#118803). I have hopes this can be stabilized quicker without needing to figure out never patterns or unsafe subtleties.

… r=compiler-errors Add the `min_exhaustive_patterns` feature gate ## Motivation Pattern-matching on empty types is tricky around unsafe code. For that reason, current stable rust conservatively requires arms for empty types in all but the simplest case. It has long been the intention to allow omitting empty arms when it's safe to do so. The [`exhaustive_patterns`](rust-lang#51085) feature allows the omission of all empty arms, but hasn't been stabilized because that was deemed dangerous around unsafe code. ## Proposal This feature aims to stabilize an uncontroversial subset of exhaustive_patterns. Namely: when `min_exhaustive_patterns` is enabled and the data we're matching on is guaranteed to be valid by rust's operational semantics, then we allow empty arms to be omitted. E.g.: ```rust let x: Result<T, !> = foo(); match x { // ok Ok(y) => ..., } let Ok(y) = x; // ok ``` If the place is not guaranteed to hold valid data (namely ptr dereferences, ref dereferences (conservatively) and union field accesses), then we keep stable behavior i.e. we (usually) require arms for the empty cases. ```rust unsafe { let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // still required } } let foo: Result<u32, &!> = ...; match foo { Ok(x) => { ... } Err(&_) => { ... } // still required because of the dereference } unsafe { let ptr: *const ! = ...; match *ptr {} // already allowed on stable } ``` Note that we conservatively consider that a valid reference can point to invalid data, hence we don't allow arms of type `&!` and similar cases to be omitted. This could eventually change depending on [opsem decisions](rust-lang/unsafe-code-guidelines#413). Whenever opsem is undecided on a case, we conservatively keep today's stable behavior. I proposed this behavior in the [`never_patterns`](rust-lang#118155) feature gate but it makes sense on its own and could be stabilized more quickly. The two proposals nicely complement each other. ## Unresolved Questions Part of the question is whether this requires an RFC. I'd argue this doesn't need one since there is no design question beyond the intent to omit unreachable patterns, but I'm aware the problem can be framed in ways that require design (I'm thinking of the [original never patterns proposal](https://smallcultfollowing.com/babysteps/blog/2018/08/13/never-patterns-exhaustive-matching-and-uninhabited-types-oh-my/), which would frame this behavior as "auto-nevering" happening). EDIT: I initially proposed a future-compatibility lint as part of this feature, I don't anymore.

Rollup merge of rust-lang#118803 - Nadrieril:min-exhaustive-patterns, r=compiler-errors Add the `min_exhaustive_patterns` feature gate ## Motivation Pattern-matching on empty types is tricky around unsafe code. For that reason, current stable rust conservatively requires arms for empty types in all but the simplest case. It has long been the intention to allow omitting empty arms when it's safe to do so. The [`exhaustive_patterns`](rust-lang#51085) feature allows the omission of all empty arms, but hasn't been stabilized because that was deemed dangerous around unsafe code. ## Proposal This feature aims to stabilize an uncontroversial subset of exhaustive_patterns. Namely: when `min_exhaustive_patterns` is enabled and the data we're matching on is guaranteed to be valid by rust's operational semantics, then we allow empty arms to be omitted. E.g.: ```rust let x: Result<T, !> = foo(); match x { // ok Ok(y) => ..., } let Ok(y) = x; // ok ``` If the place is not guaranteed to hold valid data (namely ptr dereferences, ref dereferences (conservatively) and union field accesses), then we keep stable behavior i.e. we (usually) require arms for the empty cases. ```rust unsafe { let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // still required } } let foo: Result<u32, &!> = ...; match foo { Ok(x) => { ... } Err(&_) => { ... } // still required because of the dereference } unsafe { let ptr: *const ! = ...; match *ptr {} // already allowed on stable } ``` Note that we conservatively consider that a valid reference can point to invalid data, hence we don't allow arms of type `&!` and similar cases to be omitted. This could eventually change depending on [opsem decisions](rust-lang/unsafe-code-guidelines#413). Whenever opsem is undecided on a case, we conservatively keep today's stable behavior. I proposed this behavior in the [`never_patterns`](rust-lang#118155) feature gate but it makes sense on its own and could be stabilized more quickly. The two proposals nicely complement each other. ## Unresolved Questions Part of the question is whether this requires an RFC. I'd argue this doesn't need one since there is no design question beyond the intent to omit unreachable patterns, but I'm aware the problem can be framed in ways that require design (I'm thinking of the [original never patterns proposal](https://smallcultfollowing.com/babysteps/blog/2018/08/13/never-patterns-exhaustive-matching-and-uninhabited-types-oh-my/), which would frame this behavior as "auto-nevering" happening). EDIT: I initially proposed a future-compatibility lint as part of this feature, I don't anymore.

Nadrieril · 2024-02-27T17:00:22Z

Call for Testing

Hi all! The min_exhaustive_patterns feature implements a subset of exhaustive_patterns that can be stabilized soon. The feature is implemented and I am now calling for testing.

min_exhaustive_patterns covers an important part of exhaustive_patterns: it allows omitting an empty pattern when the empty value is matched by-value.

Testing the feature

To test the feature, enable #![feature(min_exhaustive_patterns)]. Compared to stable rust, this feature makes pattern matching take into account empty types in many situations (for details about which, see the tracking issue). In practice, this means you can get extra "unreachable pattern" warnings, and code such as the following is now allowed:

enum Void {}

fn foo() -> Result<String, Void> { ... }

fn bar() {
   let Ok(s) = foo();
   // or
   match foo() {
      Ok(s) => ...,
   }
}

If you already use #![feature(exhaustive_patterns)], try to use min_exhaustive_patterns instead. This will be enough unless you use empty types behind references, pointers, or unions. If min_exhaustive_patterns doesn't cover your usecase (and this isn't a bug), you'll have to keep using exhaustive_patterns for now.

Feedback

If you find a bug or the compiler crashes, please file an issue and tag me.

If you the feature is not doing what you think it should be doing, have a look at the feature description first, and file an issue if you found a case that doesn't match this description.

…ttmcm Use `min_exhaustive_patterns` in core & std [`min_exhaustive_patterns`](rust-lang#119612) provides a subset of the functionality of [`exhaustive_patterns`](rust-lang#51085) which is likely to be stabilized much earlier than the full feature. The subset covers all the compiler and std use cases. `compiler/` [already uses it](rust-lang@9dd6eda); this PR switches `std` over.

Rollup merge of rust-lang#122255 - Nadrieril:min_exh_pats-libs, r=scottmcm Use `min_exhaustive_patterns` in core & std [`min_exhaustive_patterns`](rust-lang#119612) provides a subset of the functionality of [`exhaustive_patterns`](rust-lang#51085) which is likely to be stabilized much earlier than the full feature. The subset covers all the compiler and std use cases. `compiler/` [already uses it](rust-lang@9dd6eda); this PR switches `std` over.

Nadrieril · 2024-03-21T01:06:56Z

I'm proposing to stabilize the min_exhaustive_patterns subset: #122792

… r=fee1-dead Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of.

…=<try> Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of.

…=fee1-dead Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of.

…=<try> Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of. try-job: dist-aarch64-apple

…=fee1-dead Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of. try-job: dist-aarch64-apple

Nadrieril · 2024-08-10T15:43:26Z

min_exhaustive_patterns is now stabilized! This allows omitting empty arms when matching by-value. The other cases (behind a reference, a pointer, or inside an union) still require exhaustive_patterns.

Stabilize `min_exhaustive_patterns` ## Stabilisation report I propose we stabilize the [`min_exhaustive_patterns`](rust-lang/rust#119612) language feature. With this feature, patterns of empty types are considered unreachable when matched by-value. This allows: ```rust enum Void {} fn foo() -> Result<u32, Void>; fn main() { let Ok(x) = foo(); // also match foo() { Ok(x) => ..., } } ``` This is a subset of the long-unstable [`exhaustive_patterns`](rust-lang/rust#51085) feature. That feature is blocked because omitting empty patterns is tricky when *not* matched by-value. This PR stabilizes the by-value case, which is not tricky. The not-by-value cases (behind references, pointers, and unions) stay as they are today, e.g. ```rust enum Void {} fn foo() -> Result<u32, &Void>; fn main() { let Ok(x) = foo(); // ERROR: missing `Err(_)` } ``` The consequence on existing code is some extra "unreachable pattern" warnings. This is fully backwards-compatible. ### Comparison with today's rust This proposal only affects match checking of empty types (i.e. types with no valid values). Non-empty types behave the same with or without this feature. Note that everything below is phrased in terms of `match` but applies equallly to `if let` and other pattern-matching expressions. To be precise, a visibly empty type is: - an enum with no variants; - the never type `!`; - a struct with a *visible* field of a visibly empty type (and no #[non_exhaustive] annotation); - a tuple where one of the types is visibly empty; - en enum with all variants visibly empty (and no `#[non_exhaustive]` annotation); - a `[T; N]` with `N != 0` and `T` visibly empty; - all other types are nonempty. (An extra change was proposed below: that we ignore #[non_exhaustive] for structs since adding fields cannot turn an empty struct into a non-empty one) For normal types, exhaustiveness checking requires that we list all variants (or use a wildcard). For empty types it's more subtle: in some cases we require a `_` pattern even though there are no valid values that can match it. This is where the difference lies regarding this feature. #### Today's rust Under today's rust, a `_` is required for all empty types, except specifically: if the matched expression is of type `!` (the never type) or `EmptyEnum` (where `EmptyEnum` is an enum with no variants), then the `_` is not required. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required } let foo: &! = ...; match foo { _ => ..., // required } fn blah(foo: (u32, !)) { match foo { _ => ..., // required } } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required } } ``` #### After this PR After this PR, a pattern of an empty type can be omitted if (and only if): - the match scrutinee expression has type `!` or `EmptyEnum` (like before); - *or* the empty type is matched by value (that's the new behavior). In all other cases, a `_` is required to match on an empty type. ```rust let foo: Result<u32, !> = ...; match foo { Ok(x) => ..., // `Err` not required } let foo: Result<u32, &!> = ...; match foo { Ok(x) => ..., Err(_) => ..., // required because `!` is under a dereference } let foo: &! = ...; match foo { _ => ..., // required because `!` is under a dereference } fn blah(foo: (u32, !)) { match foo {} // allowed } unsafe { let ptr: *const ! = ...; match *ptr {} // allowed let ptr: *const (u32, !) = ...; match *ptr { (x, _) => { ... } // required because the matched place is under a (pointer) dereference } let ptr: *const Result<u32, !> = ...; match *ptr { Ok(x) => { ... } Err(_) => { ... } // required because the matched place is under a (pointer) dereference } } ``` ### Documentation The reference does not say anything specific about exhaustiveness checking, hence there is nothing to update there. The nomicon does, I opened rust-lang/nomicon#445 to reflect the changes. ### Tests The relevant tests are in `tests/ui/pattern/usefulness/empty-types.rs`. ### Unresolved Questions None that I know of. try-job: dist-aarch64-apple

safinaskar · 2024-10-16T02:54:42Z

Is this code supposed to work in the future?

struct A(Box<A>);
fn f(x: A) -> ! {
  match x {}

// Or this:
  match x {
    !,
  }
}

A is uninhibited in safe code, so theoretically the code above should compile. Is this planned? (Note: I don't need this feature, this is not feature request, it is just pure curiosity.)

RalfJung · 2024-10-16T05:30:02Z

No, I am not aware of any plans trying to reason about cyclic types.

kennytm added T-lang Relevant to the language team, which will review and decide on the PR/issue. B-unstable Blocker: Implemented in the nightly compiler and unstable. C-tracking-issue Category: An issue tracking the progress of sth. like the implementation of an RFC labels May 26, 2018

kennytm mentioned this issue May 26, 2018

Ensure every unstable language feature has a tracking issue. #51090

Merged

Nemo157 mentioned this issue Jul 16, 2018

Tracking issue for promoting ! to a type (RFC 1216) #35121

Open

2 tasks

nikic mentioned this issue Dec 21, 2018

A perf regression between 2018-12-20 and 2018-12-21 #57028

Open

est31 mentioned this issue Jul 18, 2019

Compiling libra on stable Rust diem/diem#270

Closed

Centril mentioned this issue Mar 4, 2020

Exhaustiveness checker errors on unconstructible enum variant #69696

Closed

lily-commure mentioned this issue May 7, 2020

Ok(T) is not an irrefutable pattern even when its type is Result<T, !> #71984

Closed

jonas-schievink added A-exhaustiveness-checking Relating to exhaustiveness / usefulness checking of patterns A-patterns Relating to patterns and pattern matching labels May 22, 2020

Mark-Simulacrum mentioned this issue Nov 17, 2020

Handle empty matches cleanly in exhaustiveness checking #78995

Merged

camelid mentioned this issue Nov 25, 2020

[DO NOT MERGE] Enable exhaustive_patterns by default #79394

Closed

camelid self-assigned this Nov 25, 2020

camelid removed their assignment Nov 25, 2020

Nadrieril mentioned this issue Nov 26, 2020

Add a benchmark for exhaustive_patterns rust-lang/rustc-perf#801

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Nadrieril mentioned this issue Nov 22, 2023

Tracking Issue for Never Patterns (never_patterns) #118155

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5 tasks

Nadrieril mentioned this issue Dec 10, 2023

Add the min_exhaustive_patterns feature gate #118803

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Nadrieril mentioned this issue Dec 22, 2023

Exhaustiveness: Reveal empty opaques in depth #119218

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Nadrieril mentioned this issue Jan 5, 2024

Tracking Issue for min_exhaustive_patterns #119612

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Nadrieril mentioned this issue Mar 9, 2024

Use min_exhaustive_patterns in core & std #122255

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Nadrieril mentioned this issue Mar 20, 2024

Stabilize min_exhaustive_patterns #122792

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Nadrieril mentioned this issue Jul 27, 2024

Patterns of empty types rust-lang/rust-project-goals#115

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MajoraSans mentioned this issue Oct 21, 2024

Tracking issue for experimental features in 1.49 implemented in gccrs Rust-GCC/gccrs#3210

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Compilation error when matching reference to empty enum #131452

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Tracking issue for RFC 1872: `exhaustive_patterns` feature #51085

Tracking issue for RFC 1872: `exhaustive_patterns` feature #51085

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Tracking issue for RFC 1872: exhaustive_patterns feature #51085

Tracking issue for RFC 1872: exhaustive_patterns feature #51085

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Tracking issue for RFC 1872: `exhaustive_patterns` feature #51085

Tracking issue for RFC 1872: `exhaustive_patterns` feature #51085

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