Rollup merge of rust-lang#129550 - kornelski:boxasstr, r=joshtriplett…

…,dtolnay

Add str.as_str() for easy Deref to string slices

Working with `Box<str>` is cumbersome, because in places like `iter.filter()` it can end up being `&Box<str>` or even `&&Box<str>`, and such type doesn't always get auto-dereferenced as expected.

Dereferencing such box to `&str` requires ugly syntax like `&**boxed_str` or `&***boxed_str`, with the exact amount of `*`s.

`Box<str>` is [not easily comparable with other string types](rust-lang#129852) via `PartialEq`. `Box<str>` won't work for lookups in types like `HashSet<String>`, because `Borrow<String>` won't take types like `&Box<str>`. OTOH `set.contains(s.as_str())` works nicely regardless of levels of indirection.

`String` has a simple solution for this: the `as_str()` method, and `Box<str>` should too.

alloc/src/lib.rs

@@ -93,6 +93,7 @@
 // tidy-alphabetical-start
 #![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_len))]
+#![cfg_attr(test, feature(str_as_str))]
 #![feature(alloc_layout_extra)]
 #![feature(allocator_api)]
 #![feature(array_chunks)]

alloc/src/rc/tests.rs

@@ -448,7 +448,11 @@ fn test_from_box_str() {
     use std::string::String;
 
     let s = String::from("foo").into_boxed_str();
+    assert_eq!((&&&s).as_str(), "foo");
+
     let r: Rc<str> = Rc::from(s);
+    assert_eq!((&r).as_str(), "foo");
+    assert_eq!(r.as_str(), "foo");
 
     assert_eq!(&r[..], "foo");
 }

core/src/str/mod.rs

@@ -2740,6 +2740,17 @@ impl str {
     pub fn substr_range(&self, substr: &str) -> Option<Range<usize>> {
         self.as_bytes().subslice_range(substr.as_bytes())
     }
+
+    /// Returns the same string as a string slice `&str`.
+    ///
+    /// This method is redundant when used directly on `&str`, but
+    /// it helps dereferencing other string-like types to string slices,
+    /// for example references to `Box<str>` or `Arc<str>`.
+    #[inline]
+    #[unstable(feature = "str_as_str", issue = "130366")]
+    pub fn as_str(&self) -> &str {
+        self
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]