Auto merge of #64047 - timvermeulen:cmp_min_max_by, r=cuviper

Add `cmp::{min_by, min_by_key, max_by, max_by_key}`

This adds the following functions to `core::cmp`:

- `min_by`
- `min_by_key`
- `max_by`
- `max_by_key`

`min_by` and `max_by` are somewhat trivial to implement, but not entirely because `min_by` returns the first value in case the two are equal (and `max_by` the second). `min` and `max` can be implemented in terms of `min_by` and `max_by`, but not as easily the other way around.

To give an example of why I think these functions could be useful: the `Iterator::{min_by, min_by_key, max_by, max_by_key}` methods all currently hard-code the behavior mentioned above which is an ever so small duplication of logic. If we delegate them to `cmp::{min_by, max_by}` methods instead, we get the correct behavior for free. (edit: this is now included in the PR)

I added `min_by_key` / `max_by_key` for consistency's sake but I wouldn't mind removing them. I don't have a particular use case in mind for them, and `min_by` / `max_by` seem to be more useful.

Tracking issue: #64460

2 files changed, 24 insertions, 1 deletions

diff --git a/src/libcore/tests/cmp.rs b/src/libcore/tests/cmp.rs
index 4e624e5eb12..5e6778e222a 100644
--- a/src/libcore/tests/cmp.rs
+++ b/src/libcore/tests/cmp.rs
@@ -1,4 +1,4 @@
-use core::cmp::Ordering::{Less, Greater, Equal};
+use core::cmp::{self, Ordering::*};
 
 #[test]
 fn test_int_totalord() {
@@ -29,6 +29,28 @@ fn test_ord_max_min() {
 }
 
 #[test]
+fn test_ord_min_max_by() {
+    let f = |x: &i32, y: &i32| x.abs().cmp(&y.abs());
+    assert_eq!(cmp::min_by(1, -1, f), 1);
+    assert_eq!(cmp::min_by(1, -2, f), 1);
+    assert_eq!(cmp::min_by(2, -1, f), -1);
+    assert_eq!(cmp::max_by(1, -1, f), -1);
+    assert_eq!(cmp::max_by(1, -2, f), -2);
+    assert_eq!(cmp::max_by(2, -1, f), 2);
+}
+
+#[test]
+fn test_ord_min_max_by_key() {
+    let f = |x: &i32| x.abs();
+    assert_eq!(cmp::min_by_key(1, -1, f), 1);
+    assert_eq!(cmp::min_by_key(1, -2, f), 1);
+    assert_eq!(cmp::min_by_key(2, -1, f), -1);
+    assert_eq!(cmp::max_by_key(1, -1, f), -1);
+    assert_eq!(cmp::max_by_key(1, -2, f), -2);
+    assert_eq!(cmp::max_by_key(2, -1, f), 2);
+}
+
+#[test]
 fn test_ordering_reverse() {
     assert_eq!(Less.reverse(), Greater);
     assert_eq!(Equal.reverse(), Equal);
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 050195cd2ef..35661356028 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -34,6 +34,7 @@
 #![feature(iter_partition_in_place)]
 #![feature(iter_is_partitioned)]
 #![feature(iter_order_by)]
+#![feature(cmp_min_max_by)]
 
 extern crate test;