Add #[must_use] to Option and Result overlap methods

This helps users to avoid dropping errors/`None` without checking,
for example Miserlou/Loop/pull/66.

Author: bobrippling

library/core/src/option.rs

@@ -527,6 +527,7 @@ impl<T> Option<T> {
     /// let x: Option<&str> = None;
     /// assert_eq!(x.ok_or(0), Err(0));
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
@@ -553,6 +554,7 @@ impl<T> Option<T> {
     /// let x: Option<&str> = None;
     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
@@ -1241,6 +1243,7 @@ impl<T, E> Option<Result<T, E>> {
     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
     /// assert_eq!(x, y.transpose());
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "transpose_result", since = "1.33.0")]
     pub fn transpose(self) -> Result<Option<T>, E> {

library/core/src/result.rs

@@ -381,6 +381,7 @@ impl<T, E> Result<T, E> {
     /// let x: Result<u32, &str> = Err("Nothing here");
     /// assert_eq!(x.ok(), None);
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn ok(self) -> Option<T> {
@@ -408,6 +409,7 @@ impl<T, E> Result<T, E> {
     /// let x: Result<u32, &str> = Err("Nothing here");
     /// assert_eq!(x.err(), Some("Nothing here"));
     /// ```
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn err(self) -> Option<E> {