Rollup merge of #62431 - czipperz:add-messages-to-must-use-is_-methods, r=scottmcm

Add messages to `Option`'s and `Result`'s `must_use` annotation for `is_*`

r? @RalfJung

src/libcore/option.rs

@@ -178,7 +178,7 @@ impl<T> Option<T> {
     /// ```
     ///
     /// [`Some`]: #variant.Some
-    #[must_use]
+    #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_some(&self) -> bool {
@@ -201,7 +201,8 @@ impl<T> Option<T> {
     /// ```
     ///
     /// [`None`]: #variant.None
-    #[must_use]
+    #[must_use = "if you intended to assert that this doesn't have a value, consider \
+                  `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_none(&self) -> bool {

src/libcore/result.rs

@@ -277,7 +277,7 @@ impl<T, E> Result<T, E> {
     /// let x: Result<i32, &str> = Err("Some error message");
     /// assert_eq!(x.is_ok(), false);
     /// ```
-    #[must_use]
+    #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_ok(&self) -> bool {
@@ -302,7 +302,7 @@ impl<T, E> Result<T, E> {
     /// let x: Result<i32, &str> = Err("Some error message");
     /// assert_eq!(x.is_err(), true);
     /// ```
-    #[must_use]
+    #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_err(&self) -> bool {