Rollup merge of rust-lang#98039 - tnballo:master, r=thomcc

Fix `panic` message for `BTreeSet`'s `range` API and document `panic` cases

Currently, the `panic` cases for [`BTreeSet`'s `range` API](https://doc.rust-lang.org/std/collections/struct.BTreeSet.html#method.range) are undocumented and produce a slightly wrong `panic` message (says `BTreeMap` instead of `BTreeSet`).

Panic case 1 code:

```rust
use std::collections::BTreeSet;
use std::ops::Bound::Excluded;

fn main() {
    let mut set = BTreeSet::new();
    set.insert(3);
    set.insert(5);
    set.insert(8);

    for &elem in set.range((Excluded(&3), Excluded(&3))) {
        println!("{elem}");
    }
}
```

Panic case 1 message:

```
thread 'main' panicked at 'range start and end are equal and excluded in BTreeMap', /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/collections/btree/search.rs:105:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Panic case 2 code:

```rust
use std::collections::BTreeSet;
use std::ops::Bound::Included;

fn main() {
    let mut set = BTreeSet::new();
    set.insert(3);
    set.insert(5);
    set.insert(8);

    for &elem in set.range((Included(&8), Included(&3))) {
        println!("{elem}");
    }
}
```

Panic case 2:

```
thread 'main' panicked at 'range start is greater than range end in BTreeMap', /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/collections/btree/search.rs:110:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

This PR fixes the output messages to say `BTreeSet`, adds the relevant unit tests, and updates the documentation for the API.

library/alloc/src/collections/btree/map.rs

@@ -16,6 +16,7 @@ use super::dedup_sorted_iter::DedupSortedIter;
 use super::navigate::{LazyLeafRange, LeafRange};
 use super::node::{self, marker, ForceResult::*, Handle, NodeRef, Root};
 use super::search::SearchResult::*;
+use super::set_val::SetValZST;
 
 mod entry;
 
@@ -271,7 +272,7 @@ impl<K: Clone, V: Clone, A: Allocator + Clone> Clone for BTreeMap<K, V, A> {
     }
 }
 
-impl<K, Q: ?Sized, A: Allocator + Clone> super::Recover<Q> for BTreeMap<K, (), A>
+impl<K, Q: ?Sized, A: Allocator + Clone> super::Recover<Q> for BTreeMap<K, SetValZST, A>
 where
     K: Borrow<Q> + Ord,
     Q: Ord,
@@ -318,7 +319,7 @@ where
                     alloc: (*map.alloc).clone(),
                     _marker: PhantomData,
                 }
-                .insert(());
+                .insert(SetValZST::default());
                 None
             }
         }

library/alloc/src/collections/btree/map/tests.rs

@@ -897,6 +897,39 @@ fn test_range_mut() {
     map.check();
 }
 
+#[should_panic(expected = "range start is greater than range end in BTreeMap")]
+#[test]
+fn test_range_panic_1() {
+    let mut map = BTreeMap::new();
+    map.insert(3, "a");
+    map.insert(5, "b");
+    map.insert(8, "c");
+
+    let _invalid_range = map.range((Included(&8), Included(&3)));
+}
+
+#[should_panic(expected = "range start and end are equal and excluded in BTreeMap")]
+#[test]
+fn test_range_panic_2() {
+    let mut map = BTreeMap::new();
+    map.insert(3, "a");
+    map.insert(5, "b");
+    map.insert(8, "c");
+
+    let _invalid_range = map.range((Excluded(&5), Excluded(&5)));
+}
+
+#[should_panic(expected = "range start and end are equal and excluded in BTreeMap")]
+#[test]
+fn test_range_panic_3() {
+    let mut map: BTreeMap<i32, ()> = BTreeMap::new();
+    map.insert(3, ());
+    map.insert(5, ());
+    map.insert(8, ());
+
+    let _invalid_range = map.range((Excluded(&5), Excluded(&5)));
+}
+
 #[test]
 fn test_retain() {
     let mut map = BTreeMap::from_iter((0..100).map(|x| (x, x * 10)));

library/alloc/src/collections/btree/mod.rs

@@ -10,6 +10,7 @@ mod node;
 mod remove;
 mod search;
 pub mod set;
+mod set_val;
 mod split;
 
 #[doc(hidden)]

library/alloc/src/collections/btree/search.rs

@@ -97,17 +97,28 @@ impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::Lea
         K: Borrow<Q>,
         R: RangeBounds<Q>,
     {
+        // Determine if map or set is being searched
+        let is_set = <V as super::set_val::IsSetVal>::is_set_val();
+
         // Inlining these variables should be avoided. We assume the bounds reported by `range`
         // remain the same, but an adversarial implementation could change between calls (#81138).
         let (start, end) = (range.start_bound(), range.end_bound());
         match (start, end) {
             (Bound::Excluded(s), Bound::Excluded(e)) if s == e => {
-                panic!("range start and end are equal and excluded in BTreeMap")
+                if is_set {
+                    panic!("range start and end are equal and excluded in BTreeSet")
+                } else {
+                    panic!("range start and end are equal and excluded in BTreeMap")
+                }
             }
             (Bound::Included(s) | Bound::Excluded(s), Bound::Included(e) | Bound::Excluded(e))
                 if s > e =>
             {
-                panic!("range start is greater than range end in BTreeMap")
+                if is_set {
+                    panic!("range start is greater than range end in BTreeSet")
+                } else {
+                    panic!("range start is greater than range end in BTreeMap")
+                }
             }
             _ => {}
         }

library/alloc/src/collections/btree/set.rs

@@ -13,6 +13,7 @@ use core::ops::{BitAnd, BitOr, BitXor, RangeBounds, Sub};
 
 use super::map::{BTreeMap, Keys};
 use super::merge_iter::MergeIterInner;
+use super::set_val::SetValZST;
 use super::Recover;
 
 use crate::alloc::{Allocator, Global};
@@ -81,7 +82,7 @@ pub struct BTreeSet<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
 > {
-    map: BTreeMap<T, (), A>,
+    map: BTreeMap<T, SetValZST, A>,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -135,7 +136,7 @@ impl<T: Clone, A: Allocator + Clone> Clone for BTreeSet<T, A> {
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a, T: 'a> {
-    iter: Keys<'a, T, ()>,
+    iter: Keys<'a, T, SetValZST>,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
@@ -158,7 +159,7 @@ pub struct IntoIter<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
 > {
-    iter: super::map::IntoIter<T, (), A>,
+    iter: super::map::IntoIter<T, SetValZST, A>,
 }
 
 /// An iterator over a sub-range of items in a `BTreeSet`.
@@ -171,7 +172,7 @@ pub struct IntoIter<
 #[derive(Debug)]
 #[stable(feature = "btree_range", since = "1.17.0")]
 pub struct Range<'a, T: 'a> {
-    iter: super::map::Range<'a, T, ()>,
+    iter: super::map::Range<'a, T, SetValZST>,
 }
 
 /// A lazy iterator producing elements in the difference of `BTreeSet`s.
@@ -375,6 +376,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
     /// range from 4 to 10.
     ///
+    /// # Panics
+    ///
+    /// Panics if range `start > end`.
+    /// Panics if range `start == end` and both bounds are `Excluded`.
+    ///
     /// # Examples
     ///
     /// ```
@@ -905,7 +911,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     where
         T: Ord,
     {
-        self.map.insert(value, ()).is_none()
+        self.map.insert(value, SetValZST::default()).is_none()
     }
 
     /// Adds a value to the set, replacing the existing element, if any, that is
@@ -1210,7 +1216,7 @@ impl<T: Ord> FromIterator<T> for BTreeSet<T> {
 
 impl<T: Ord, A: Allocator + Clone> BTreeSet<T, A> {
     fn from_sorted_iter<I: Iterator<Item = T>>(iter: I, alloc: A) -> BTreeSet<T, A> {
-        let iter = iter.map(|k| (k, ()));
+        let iter = iter.map(|k| (k, SetValZST::default()));
         let map = BTreeMap::bulk_build_from_sorted_iter(iter, alloc);
         BTreeSet { map }
     }
@@ -1234,7 +1240,7 @@ impl<T: Ord, const N: usize> From<[T; N]> for BTreeSet<T> {
 
         // use stable sort to preserve the insertion order.
         arr.sort();
-        let iter = IntoIterator::into_iter(arr).map(|k| (k, ()));
+        let iter = IntoIterator::into_iter(arr).map(|k| (k, SetValZST::default()));
         let map = BTreeMap::bulk_build_from_sorted_iter(iter, Global);
         BTreeSet { map }
     }
@@ -1284,7 +1290,7 @@ pub struct DrainFilter<
     F: 'a + FnMut(&T) -> bool,
 {
     pred: F,
-    inner: super::map::DrainFilterInner<'a, T, ()>,
+    inner: super::map::DrainFilterInner<'a, T, SetValZST>,
     /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     alloc: A,
 }
@@ -1319,7 +1325,7 @@ where
 
     fn next(&mut self) -> Option<T> {
         let pred = &mut self.pred;
-        let mut mapped_pred = |k: &T, _v: &mut ()| pred(k);
+        let mut mapped_pred = |k: &T, _v: &mut SetValZST| pred(k);
         self.inner.next(&mut mapped_pred, self.alloc.clone()).map(|(k, _)| k)
     }
 

library/alloc/src/collections/btree/set/tests.rs

@@ -5,6 +5,7 @@ use crate::vec::Vec;
 use std::cmp::Ordering;
 use std::hash::{Hash, Hasher};
 use std::iter::FromIterator;
+use std::ops::Bound::{Excluded, Included};
 use std::panic::{catch_unwind, AssertUnwindSafe};
 
 #[test]
@@ -831,3 +832,25 @@ fn from_array() {
     let unordered_duplicates = BTreeSet::from([4, 1, 4, 3, 2]);
     assert_eq!(set, unordered_duplicates);
 }
+
+#[should_panic(expected = "range start is greater than range end in BTreeSet")]
+#[test]
+fn test_range_panic_1() {
+    let mut set = BTreeSet::new();
+    set.insert(3);
+    set.insert(5);
+    set.insert(8);
+
+    let _invalid_range = set.range((Included(&8), Included(&3)));
+}
+
+#[should_panic(expected = "range start and end are equal and excluded in BTreeSet")]
+#[test]
+fn test_range_panic_2() {
+    let mut set = BTreeSet::new();
+    set.insert(3);
+    set.insert(5);
+    set.insert(8);
+
+    let _invalid_range = set.range((Excluded(&5), Excluded(&5)));
+}

library/alloc/src/collections/btree/set_val.rs

@@ -0,0 +1,29 @@
+/// Zero-Sized Type (ZST) for internal `BTreeSet` values.
+/// Used instead of `()` to differentiate between:
+/// * `BTreeMap<T, ()>` (possible user-defined map)
+/// * `BTreeMap<T, SetValZST>` (internal set representation)
+#[derive(Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Clone, Default)]
+pub struct SetValZST;
+
+/// A trait to differentiate between `BTreeMap` and `BTreeSet` values.
+/// Returns `true` only for type `SetValZST`, `false` for all other types (blanket implementation).
+/// `TypeId` requires a `'static` lifetime, use of this trait avoids that restriction.
+///
+/// [`TypeId`]: std::any::TypeId
+pub trait IsSetVal {
+    fn is_set_val() -> bool;
+}
+
+// Blanket implementation
+impl<V> IsSetVal for V {
+    default fn is_set_val() -> bool {
+        false
+    }
+}
+
+// Specialization
+impl IsSetVal for SetValZST {
+    fn is_set_val() -> bool {
+        true
+    }
+}