Introduce macro to create static UncasedStrs

Add a static_uncased_str macro, which can create `&'static UncasedStr`s
from `&'static str`s. This won't be necessary once rust-lang/rust#53605
lands, but that's at least a few months away.

src/borrowed.rs

@@ -242,3 +242,20 @@ impl Hash for UncasedStr {
         self.0.bytes().for_each(|b| hasher.write_u8(b.to_ascii_lowercase()));
     }
 }
+
+/// Cost-free conversion from a `&'static str` to a `&'static UncasedStr`.
+///
+/// This macro can be used in the definition of a static or const variable,
+/// while [`UncasedStr::new`] cannot.
+///
+/// When it is possible to mark `UncasedStr::new` as a `const fn`
+/// (see [rust-lang/rust#53605]), this macro will be removed.
+///
+/// [rust-lang/rust#53605]: https://github.com/rust-lang/rust/issues/53605
+#[macro_export]
+macro_rules! static_uncased_str {
+    ($string:expr) => {{
+        // This is safe for the same reason that `UncasedStr::new` is safe.
+        unsafe { ::core::mem::transmute::<&'static str, &'static UncasedStr>($string) }
+    }}
+}

src/tests.rs

@@ -1,6 +1,6 @@
 #![allow(deprecated)]
 
-use crate::UncasedStr;
+use crate::{static_uncased_str, UncasedStr};
 
 use core::hash::{Hash, Hasher, SipHasher};
 
@@ -58,3 +58,9 @@ fn test_case_cmp() {
     assert!(UncasedStr::new("aA") > UncasedStr::new("a"));
     assert!(UncasedStr::new("aA") > UncasedStr::new("A"));
 }
+
+#[test]
+fn test_static() {
+    const FOO: &UncasedStr = static_uncased_str!("FOO");
+    assert_eq!(FOO, UncasedStr::new("foo"));
+}