Rollup merge of rust-lang#102574 - aDotInTheVoid:const_collections_wi…

…th_hasher, r=oli-obk,fee1-dead

Make Hash{Set,Map}::with_hasher unstably const

Makes  [`HashMap::with_hasher`](https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html#method.with_hasher) and [`HashSet::with_hasher`](https://doc.rust-lang.org/stable/std/collections/hash_set/struct.HashSet.html#method.with_hasher) `const`.

This allows

```rust
static GlobalState: Mutex<HashMap<i32, i32, SomeHasher>> = Mutex::new(HashMap::with_hasher(SomeHasher::new()))
```

Tracking issue: rust-lang#102575

library/std/src/collections/hash/map.rs

@@ -280,7 +280,8 @@ impl<K, V, S> HashMap<K, V, S> {
     /// ```
     #[inline]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
-    pub fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
+    #[rustc_const_unstable(feature = "const_collections_with_hasher", issue = "102575")]
+    pub const fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
         HashMap { base: base::HashMap::with_hasher(hash_builder) }
     }
 

library/std/src/collections/hash/map/tests.rs

@@ -1115,3 +1115,9 @@ fn from_array() {
     // that's a problem!
     let _must_not_require_type_annotation = HashMap::from([(1, 2)]);
 }
+
+#[test]
+fn const_with_hasher() {
+    const X: HashMap<(), (), ()> = HashMap::with_hasher(());
+    assert_eq!(X.len(), 0);
+}

library/std/src/collections/hash/set.rs

@@ -376,7 +376,8 @@ impl<T, S> HashSet<T, S> {
     /// ```
     #[inline]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
-    pub fn with_hasher(hasher: S) -> HashSet<T, S> {
+    #[rustc_const_unstable(feature = "const_collections_with_hasher", issue = "102575")]
+    pub const fn with_hasher(hasher: S) -> HashSet<T, S> {
         HashSet { base: base::HashSet::with_hasher(hasher) }
     }
 

library/std/src/collections/hash/set/tests.rs

@@ -496,3 +496,9 @@ fn from_array() {
     // that's a problem!
     let _must_not_require_type_annotation = HashSet::from([1, 2]);
 }
+
+#[test]
+fn const_with_hasher() {
+    const X: HashSet<(), ()> = HashSet::with_hasher(());
+    assert_eq!(X.len(), 0);
+}

library/std/src/lib.rs

@@ -351,6 +351,7 @@
 // Only used in tests/benchmarks:
 //
 // Only for const-ness:
+#![feature(const_collections_with_hasher)]
 #![feature(const_io_structs)]
 #![feature(const_ip)]
 #![feature(const_ipv4)]