Add ArgumentMatchers#assertArg method.

Fixes mockito#2285

src/main/java/org/mockito/ArgumentMatchers.java

@@ -14,6 +14,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Consumer;
 import java.util.regex.Pattern;
 
 import org.mockito.internal.matchers.Any;
@@ -912,6 +913,21 @@ public static <T> T argThat(ArgumentMatcher<T> matcher) {
         return null;
     }
 
+    /**
+     * Allows creating custom argument matchers where matching is considered successful when the consumer given by parameter does not throw an exception.
+     * <p>
+     * Typically used with {@link Mockito#verify(Object)} to execute assertions on parameters passed to the verified method invocation.
+     *
+     * @param consumer executes assertions on the verified argument
+     * @return <code>null</code>.
+     */
+    public static <T> T assertArg(Consumer<T> consumer) {
+        return argThat(argument -> {
+            consumer.accept(argument);
+            return true;
+        });
+    }
+
     /**
      * Allows creating custom <code>char</code> argument matchers.
      * <p>

src/test/java/org/mockitousage/matchers/MatchersTest.java

@@ -52,6 +52,7 @@
 import java.util.RandomAccess;
 import java.util.regex.Pattern;
 
+import org.junit.ComparisonFailure;
 import org.junit.Test;
 import org.mockito.ArgumentMatchers;
 import org.mockito.Mockito;
@@ -624,4 +625,27 @@ public void nullable_matcher() throws Exception {
 
         verify(mock, times(2)).oneArg(nullable(Character.class));
     }
+
+    @Test
+    public void assertArg_matcher() throws Exception {
+        mock.oneArg("hello");
+
+        verify(mock).oneArg(Mockito.assertArg((String it) -> {
+            assertEquals("hello", it);
+        }));
+    }
+
+    @Test
+    public void assertArg_matcher_fails_when_assertion_fails() throws Exception {
+        mock.oneArg("hello");
+
+        try {
+            verify(mock).oneArg(Mockito.assertArg((String it) -> {
+                assertEquals("not-hello", it);
+            }));
+            fail("Should throw an exception");
+        } catch (ComparisonFailure e) {
+            // do nothing
+        }
+    }
 }