Add must_use to StrExt methods

src/lib.rs

@@ -551,24 +551,28 @@ pub trait StrExt: private::Sealed {
  /// potentially without allocating.
  ///
  /// See [`str::to_lowercase`].
+ #[must_use = "this returns a new SmolStr without modifying the original"]
  fn to_lowercase_smolstr(&self) -> SmolStr;
 
  /// Returns the uppercase equivalent of this string slice as a new [`SmolStr`],
  /// potentially without allocating.
  ///
  /// See [`str::to_uppercase`].
+ #[must_use = "this returns a new SmolStr without modifying the original"]
  fn to_uppercase_smolstr(&self) -> SmolStr;
 
  /// Returns the ASCII lowercase equivalent of this string slice as a new [`SmolStr`],
  /// potentially without allocating.
  ///
  /// See [`str::to_ascii_lowercase`].
+ #[must_use = "this returns a new SmolStr without modifying the original"]
  fn to_ascii_lowercase_smolstr(&self) -> SmolStr;
 
  /// Returns the ASCII uppercase equivalent of this string slice as a new [`SmolStr`],
  /// potentially without allocating.
  ///
  /// See [`str::to_ascii_uppercase`].
+ #[must_use = "this returns a new SmolStr without modifying the original"]
  fn to_ascii_uppercase_smolstr(&self) -> SmolStr;
 }