Add the Test.assertError() helper function

This matcher function asserts that the result of an executed operation,
such as scripts and transactions, resulted in an error that contains
a given sub-string.

runtime/stdlib/contracts/test.cdc

@@ -171,6 +171,10 @@ pub contract Test {
         /// The resulted status of an executed operation.
         ///
         pub let status: ResultStatus
+
+        /// The optionally resulted error of an executed operation.
+        ///
+        pub let error: Error?
     }
 
     /// The result of a transaction execution.
@@ -346,4 +350,28 @@ pub contract Test {
         })
     }
 
+    /// Asserts that the result of an executed operation, such as
+    /// scripts & transactions, has errored and the error message
+    /// contains the given sub-string.
+    ///
+    pub fun assertError(_ result: {Result}, errorMessage: String) {
+        pre {
+            result.status == ResultStatus.failed: "no error was found"
+        }
+
+        var found = false
+        let msg = result.error!.message
+        let msgLength = msg.length - errorMessage.length + 1
+        var i = 0
+        while i < msgLength {
+            if msg.slice(from: i, upTo: i + errorMessage.length) == errorMessage {
+                found = true
+                break
+            }
+            i = i + 1
+        }
+
+        assert(found, message: "the error message did not contain the given sub-string")
+    }
+
 }

runtime/stdlib/test_test.go

@@ -1154,6 +1154,112 @@ func TestTestBeFailedMatcher(t *testing.T) {
 	})
 }
 
+func TestTestAssertErrorMatcher(t *testing.T) {
+
+	t.Parallel()
+
+	t.Run("with ScriptResult", func(t *testing.T) {
+		t.Parallel()
+
+		const script = `
+            import Test
+
+            pub fun testMatch() {
+                let result = Test.ScriptResult(
+                    status: Test.ResultStatus.failed,
+                    returnValue: nil,
+                    error: Test.Error("computation exceeding limit")
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+
+            pub fun testNoMatch() {
+                let result = Test.ScriptResult(
+                    status: Test.ResultStatus.failed,
+                    returnValue: nil,
+                    error: Test.Error("computation exceeding memory")
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+
+            pub fun testNoError() {
+                let result = Test.ScriptResult(
+                    status: Test.ResultStatus.succeeded,
+                    returnValue: 42,
+                    error: nil
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+		`
+
+		inter, err := newTestContractInterpreter(t, script)
+		require.NoError(t, err)
+
+		_, err = inter.Invoke("testMatch")
+		require.NoError(t, err)
+
+		_, err = inter.Invoke("testNoMatch")
+		require.Error(t, err)
+		assert.ErrorContains(t, err, "the error message did not contain the given sub-string")
+
+		_, err = inter.Invoke("testNoError")
+		require.Error(t, err)
+		assert.ErrorContains(t, err, "no error was found")
+	})
+
+	t.Run("with TransactionResult", func(t *testing.T) {
+		t.Parallel()
+
+		const script = `
+            import Test
+
+            pub fun testMatch() {
+                let result = Test.TransactionResult(
+                    status: Test.ResultStatus.failed,
+                    error: Test.Error("computation exceeding limit")
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+
+            pub fun testNoMatch() {
+                let result = Test.TransactionResult(
+                    status: Test.ResultStatus.failed,
+                    error: Test.Error("computation exceeding memory")
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+
+            pub fun testNoError() {
+                let result = Test.TransactionResult(
+                    status: Test.ResultStatus.succeeded,
+                    error: nil
+                )
+
+                Test.assertError(result, errorMessage: "exceeding limit")
+            }
+		`
+
+		inter, err := newTestContractInterpreter(t, script)
+		require.NoError(t, err)
+
+		_, err = inter.Invoke("testMatch")
+		require.NoError(t, err)
+
+		_, err = inter.Invoke("testNoMatch")
+		require.Error(t, err)
+		assert.ErrorContains(t, err, "the error message did not contain the given sub-string")
+
+		_, err = inter.Invoke("testNoError")
+		require.Error(t, err)
+		assert.ErrorContains(t, err, "no error was found")
+	})
+}
+
 func TestTestBeNilMatcher(t *testing.T) {
 
 	t.Parallel()