Add 'spaceship_operator' RFC;

Author: bjoernager

text/0000-spaceship-operator.md

@@ -0,0 +1,134 @@
+- Feature Name: `spaceship_operator`
+- Start Date: 2025-02-21
+- RFC PR: TBA
+- Rust Issue: TBA
+
+# Summary
+[summary]: #summary
+
+Add the spaceship operator `<=>` to Rust, equivalent to calling `PartialOrd::partial_cmp` with the same operands.
+
+# Motivation
+[motivation]: #motivation
+
+Suppose a case where a three-way comparison was explicitly needed to be tested in all possible outcomes (for example, in a binary searcher).
+
+A usual approach would be to use `if`-statements for all of these paths:
+
+```rust
+let lhs = todo!();
+let rhs = todo!();
+
+if lhs < rhs {
+	todo!();
+} else if lhs == rhs {
+	todo!();
+} else if lhs > rhs {
+	todo!();
+} else {
+	todo!();
+}
+```
+
+This could also be simplified to:
+
+```rust
+use core::cmp::Ordering;
+
+let lhs = todo!();
+let rhs = todo!();
+
+match lhs.partial_cmp(&rhs) {
+	Some(Ordering::Less)    => todo!(),
+	Some(Ordering::Equal)   => todo!(),
+	Some(Ordering::Greater) => todo!(),
+	None                    => todo!(),
+}
+```
+
+This RFC simply proposes that the following would be possible as well:
+
+```rust
+use core::cmp::Ordering;
+
+let lhs = todo!();
+let rhs = todo!();
+
+match lhs <=> rhs {
+	Some(Ordering::Less)    => todo!(),
+	Some(Ordering::Equal)   => todo!(),
+	Some(Ordering::Greater) => todo!(),
+	None                    => todo!(),
+}
+```
+
+# Guide- and reference-level explanation
+[guide-and-reference-level-explanation]: #guide-and-reference-level-explanation
+
+Usage of this operator will map identically to calling `PartialOrd::partial_cmp` (or `Ord::cmp` if this is desired instead):
+
+```rust
+use core::cmp::Ordering;
+
+// `T` and `U` are unspecified here, but both im-
+// plement `PartialOrd` for demonstration's sake.
+
+let lhs: T = todo!();
+let rhs: U = todo!();
+
+// `cmp0` and `cmp1` are defined completely equiva-
+// lently:
+
+let cmp0: Option<Ordering> = <T as PartialOrd<U>>::partial_cmp(&lhs, &rhs);
+let cmp1: Option<Ordering> = lhs <=> rhs;
+```
+
+The operator would be implemented similarly to the existing ones (see [Appendix B](https://doc.rust-lang.org/nightly/book/appendix-02-operators.html) of the book).
+In cases where `PartialOrd` is not implement between the compared objects, a compilation error would be yielded.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The greatest drawback, as far as I can tell, would be that this operator may seem confusing to programmers that are unfamiliar with this construct in other languages (see [prior art](prior-art) for a list hereof).
+But the same can also be said for a lot of other features in Rust.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The main (and perhaps only) use for this operator would be to reduce boilerplate in cases where three-way comparisons are needed.
+
+There is the possibility that this operation is not considered fundamental enough for an operator to be considered worthwhile.
+Currently, Rust has the `PartialOrd::partial_cmp` and `Ord::cmp` methods that achieve the same *operation* as proposed here.
+Not merging this RFC would (obviously) not change status quo, and thus one would simply resort to using either of these functions instead.
+
+I personally believe that the same reasonings for the preexisting operators also apply (to some extent) in this case.
+I will, however, concede that the demand for this operator will not be as great as that for e.g. `==`, but in the end this will also be a matter of taste.
+
+# Prior art
+[prior-art]: #prior-art
+
+The *spaceship* operator itself currently exists in other languages, albeit with slightly different syntaxes.
+
+## C++
+
+In C++, this feature has existed since ISO/IEC 14882:2020 (C++20), stable since 15 December 2020 (see [`cppreference.com`](https://en.cppreference.com/w/cpp/language/operator_comparison)).
+C++, however, does not strictly define the precise semantics of this operator, merely requiring that the returned type must itself be comparable against `0`.
+
+For integer types, such as `int` or `long long int`, the returned type is `::std::strong_ordering`, which is equivalent to Rust's `core::cmp::Ordering`.
+Floating-point types, like `long double` or `::std::bfloat_16`, on the other hand, return `::std::weak_ordering`, which is equivalent to Rust's `Option<core::cmp::Ordering>`.
+
+## PHP
+
+PHP has had the spaceship operator since PHP 7, stable since 1 December 2015 (see the [PHP Manual](https://www.php.net/manual/en/language.operators.comparison.php)).
+Here, it is defined as returning `-1` as an equivalent to our `Some(Ordering::Less)`, `0` as an equivalent to our `Some(Ordering::Equal)`, and `1` as an equivalent to our `Some(Ordering::Greater)`.
+
+The operator is only defined for integral types.
+
+## Perl
+
+Perl defines the spaceship operator the same way as PHP (see the [Perldoc Browser](https://perldoc.perl.org/perlop#Equality-Operators)), with the added support for floating-point types (where `NaN` yields `undef` as an equivalent to our `None`).
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- Should this operator map to `PartialOrd::partial_cmp` or `Ord::cmp`? <sub>(Maybe obvious, but still worth discussing).</sub>