Add Result::{is_ok_with, is_err_with}.

Author: m-ou-se

Diff for: library/core/src/result.rs

@@ -542,6 +542,27 @@ impl<T, E> Result<T, E> {
         matches!(*self, Ok(_))
     }
 
+    /// Returns `true` if the result is [`Ok`] wrapping a value matching the predicate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x: Result<u32, &str> = Ok(2);
+    /// assert_eq!(x.is_ok_with(|x| x > 1), true);
+    ///
+    /// let x: Result<u32, &str> = Ok(0);
+    /// assert_eq!(x.is_ok_with(|x| x > 1), false);
+    ///
+    /// let x: Result<u32, &str> = Err("hey");
+    /// assert_eq!(x.is_ok_with(|x| x > 1), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "is_some_with", issue = "none")]
+    pub fn is_ok_with(&self, f: impl FnOnce(&T) -> bool) -> bool {
+        matches!(self, Ok(x) if f(x))
+    }
+
     /// Returns `true` if the result is [`Err`].
     ///
     /// # Examples
@@ -563,6 +584,27 @@ impl<T, E> Result<T, E> {
         !self.is_ok()
     }
 
+    /// Returns `true` if the result is [`Err`] wrapping a value matching the predicate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x: Result<u32, &str> = Err("abc");
+    /// assert_eq!(x.is_err_with(|x| x.len() > 1), true);
+    ///
+    /// let x: Result<u32, &str> = Err("");
+    /// assert_eq!(x.is_ok_with(|x| x.len() > 1), false);
+    ///
+    /// let x: Result<u32, &str> = Ok(123);
+    /// assert_eq!(x.is_ok_with(|x| x.len() > 1), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "is_some_with", issue = "none")]
+    pub fn is_err_with(&self, f: impl FnOnce(&E) -> bool) -> bool {
+        matches!(self, Err(x) if f(x))
+    }
+
     /////////////////////////////////////////////////////////////////////////
     // Adapter for each variant
     /////////////////////////////////////////////////////////////////////////