Mark map_or as #[must_use]

Author: WaffleLapkin

core/src/option.rs

@@ -1125,6 +1125,7 @@ impl<T> Option<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "if you don't need the returned value, use `if let` instead"]
     pub fn map_or<U, F>(self, default: U, f: F) -> U
     where
         F: FnOnce(T) -> U,

core/src/result.rs

@@ -768,6 +768,7 @@ impl<T, E> Result<T, E> {
     /// ```
     #[inline]
     #[stable(feature = "result_map_or", since = "1.41.0")]
+    #[must_use = "if you don't need the returned value, use `if let` instead"]
     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
         match self {
             Ok(t) => f(t),