rollup merge of #23878: Ryman/stable_extremes

`min`-like functions now return the leftmost element/input for equal elements.
`max`-like return the rightmost.

Closes #23687.

cc @HeroesGrave, @aturon, @alexcrichton

1 files changed, 83 insertions, 0 deletions

diff --git a/src/test/run-pass/minmax-stability-issue-23687.rs b/src/test/run-pass/minmax-stability-issue-23687.rs
new file mode 100644
index 00000000000..86dd1a04532
--- /dev/null
+++ b/src/test/run-pass/minmax-stability-issue-23687.rs
@@ -0,0 +1,83 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(core)]
+use std::fmt::Debug;
+use std::cmp::{self, PartialOrd, Ordering};
+use std::iter::MinMaxResult::MinMax;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+struct Foo {
+    n: u8,
+    name: &'static str
+}
+
+impl PartialOrd for Foo {
+    fn partial_cmp(&self, other: &Foo) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Foo {
+    fn cmp(&self, other: &Foo) -> Ordering {
+        self.n.cmp(&other.n)
+    }
+}
+
+fn main() {
+    let a = Foo { n: 4, name: "a" };
+    let b = Foo { n: 4, name: "b" };
+    let c = Foo { n: 8, name: "c" };
+    let d = Foo { n: 8, name: "d" };
+    let e = Foo { n: 22, name: "e" };
+    let f = Foo { n: 22, name: "f" };
+
+    let data = [a, b, c, d, e, f];
+
+    // `min` should return the left when the values are equal
+    assert_eq!(data.iter().min(), Some(&a));
+    assert_eq!(data.iter().min_by(|a| a.n), Some(&a));
+    assert_eq!(cmp::min(a, b), a);
+    assert_eq!(cmp::min(b, a), b);
+    assert_eq!(cmp::partial_min(a, b), Some(a));
+    assert_eq!(cmp::partial_min(b, a), Some(b));
+
+    // `max` should return the right when the values are equal
+    assert_eq!(data.iter().max(), Some(&f));
+    assert_eq!(data.iter().max_by(|a| a.n), Some(&f));
+    assert_eq!(cmp::max(e, f), f);
+    assert_eq!(cmp::max(f, e), e);
+    assert_eq!(cmp::partial_max(e, f), Some(f));
+    assert_eq!(cmp::partial_max(f, e), Some(e));
+
+    // Similar for `min_max`
+    assert_eq!(data.iter().min_max(), MinMax(&a, &f));
+    assert_eq!(data[1..5].iter().min_max(), MinMax(&b, &e));
+    assert_eq!(data[2..4].iter().min_max(), MinMax(&c, &d));
+
+    let mut presorted = data.to_vec();
+    presorted.sort();
+    assert_stable(&presorted);
+
+    let mut presorted = data.to_vec();
+    presorted.sort_by(|a, b| a.cmp(b));
+    assert_stable(&presorted);
+
+    // Assert that sorted and min/max are the same
+    fn assert_stable<T: Ord + Debug>(presorted: &[T]) {
+        for slice in presorted.windows(2) {
+            let a = &slice[0];
+            let b = &slice[1];
+
+            assert_eq!(a, cmp::min(a, b));
+            assert_eq!(b, cmp::max(a, b));
+        }
+    }
+}