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update patterns.md for const pattern RFC #1456

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merged 3 commits into from
May 27, 2024
Merged

update patterns.md for const pattern RFC #1456

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3fb728d
update patterns.md for const pattern RFC
RalfJung Jan 26, 2024
c6719b5
mention associated consts
RalfJung May 3, 2024
2f63507
clarifications
RalfJung May 7, 2024
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35 changes: 23 additions & 12 deletions src/patterns.md
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Expand Up @@ -144,12 +144,6 @@ Since negative numbers are not [literals], literal patterns also accept an optio

<div class="warning">

Floating-point literals are currently accepted, but due to the complexity of comparing them, they are going to be forbidden on literal patterns in a future version of Rust (see [issue #41620](https://github.com/rust-lang/rust/issues/41620)).

</div>

<div class="warning">

C string and raw C string literals are accepted in literal patterns, but `&CStr`
doesn't implement structural equality (`#[derive(Eq, PartialEq)]`) and therefore
any such `match` on a `&CStr` will be rejected with a type error.
Expand Down Expand Up @@ -451,6 +445,7 @@ If the bounds is written as a path, after macro resolution, the path must resolv

The type and value of the bounds is dependent upon how it is written out.
If the bounds is a [path], the pattern has the type and value of the [constant] the path resolves to.
For float range patterns, the constant may not be a `NaN`.
If it is a literal, it has the type and value of the corresponding [literal expression].
If is a literal preceded by a `-`, it has the same type as the corresponding [literal expression] and the value of [negating] the value of the corresponding literal expression.

Expand Down Expand Up @@ -534,9 +529,6 @@ For example, `0u8..=255u8` is irrefutable.
The range of values for an integer type is the closed range from its minimum to maximum value.
The range of values for a `char` type are precisely those ranges containing all Unicode Scalar Values: `'\u{0000}'..='\u{D7FF}'` and `'\u{E000}'..='\u{10FFFF}'`.

Floating point range patterns are deprecated and may be removed in a future Rust release.
See [issue #41620](https://github.com/rust-lang/rust/issues/41620) for more information.

> **Edition Differences**: Before the 2021 edition, range patterns with both a lower and upper bound may also be written using `...` in place of `..=`, with the same meaning.

> **Note**: Although range patterns use the same syntax as [range expressions], there are no exclusive range patterns.
Expand Down Expand Up @@ -776,12 +768,31 @@ Unqualified path patterns can refer to:

Qualified path patterns can only refer to associated constants.

Constants cannot be a union type.
Struct and enum constants must have `#[derive(PartialEq, Eq)]` (not merely implemented).

Path patterns are irrefutable when they refer to structs or an enum variant when the enum has only one variant or a constant whose type is irrefutable.
They are refutable when they refer to refutable constants or enum variants for enums with multiple variants.

### Constant patterns

When a constant `C` of type `T` is used as a pattern, we first check that `T: PartialEq`.
Furthermore we require that the value of `C` *has (recursive) structural equality*, which is defined recursively as follows:

- Integers as well as `bool` and `char` values always have structural equality.
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And strings?

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Ah right, they should also be added here, good catch.

- Tuples, arrays, and slices have structural equality if all their fields/elements have structural equality.
(In particular, `()` and `[]` always have structural equality.)
- References have structural equality if the value they point to has structural equality.
- A value of `struct` or `enum` type has structural equality if its `PartialEq` instance is derived via `#[derive(PartialEq)]`,
and all fields (for enums: of the active variant) have structural equality.
- A raw pointer has structural equality if it was defined as a constant integer (and then cast/transmuted).
- A float value has structural equality if it is not a `NaN`.
- Nothing else has structural equality.

In particular, the value of `C` must be known at pattern-building time (which is pre-monomorphization).
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This means that associated consts cannot be used as patterns.
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I don't understand this part. Path patterns can be associated consts. Can you clarify what it is referring to?

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@RalfJung RalfJung May 6, 2024 •
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This is intended to explain why this code does not compile:

trait Trait {
    const C: i32;
}

fn f<T: Trait>(x: i32) -> bool {
    matches!(x, T::C) //~ERROR: associated consts cannot be referenced in patterns
}


After ensuring all conditions are met, the constant value is translated into a pattern, and now behaves exactly as-if that pattern had been written directly.
In particular, it fully participates in exhaustiveness checking.
(For raw pointers, constants are the only way to write such patterns. Only `_` is ever considered exhaustive for these types.)

## Or-patterns

_Or-patterns_ are patterns that match on one of two or more sub-patterns (for example `A | B | C`).
Expand Down