Added count() and count(predicate) mappings for `Assertion.Builde…

…r<Iterable<*>>`

strikt-core/src/main/kotlin/strikt/assertions/Iterable.kt

@@ -2,6 +2,22 @@ package strikt.assertions
 
 import strikt.api.Assertion.Builder
 
+/**
+ * Maps this assertion to an assertion over the count of elements in the subject.
+ *
+ * @see Iterable.count
+ */
+fun Builder<out Iterable<*>>.count(): Builder<Int> =
+  get(Iterable<*>::count)
+
+/**
+ * Maps this assertion to an assertion over the count of elements matching [predicate].
+ *
+ * @see Iterable.count
+ */
+fun <T : Iterable<E>, E> Builder<T>.count(description: String, predicate: (E) -> Boolean): Builder<Int> =
+  get("count matching $description") { count(predicate) }
+
 /**
  * Applies [Iterable.map] with [function] to the subject and returns an
  * assertion builder wrapping the result.

strikt-core/src/test/kotlin/strikt/assertions/IterableAssertions.kt

@@ -884,5 +884,19 @@ internal object IterableAssertions : JUnit5Minutests {
           }
         }
     }
+
+    context("count mapping") {
+      test("maps to an assertion on the iterable's size") {
+        val subject = listOf("catflap", "rubberplant", "marzipan")
+        expectThat(subject).count().isEqualTo(subject.size)
+      }
+
+      test("maps to an assertion on the count of elements matching the predicate") {
+        val subject = listOf("catflap", "rubberplant", "marzipan")
+        expectThat(subject)
+          .count("elements containing 't'") { it.contains("t") }
+          .isEqualTo(2)
+      }
+    }
   }
 }