Resolve placeholders in reference config against all other config

Fixes gh-34643

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cassandra/CassandraAutoConfiguration.java

@@ -136,7 +136,7 @@ private Config cassandraConfiguration(CassandraProperties properties) {
 			config = config.withFallback(loadConfig(properties.getConfig()));
 		}
 		config = config.withFallback(SPRING_BOOT_DEFAULTS);
-		config = config.withFallback(ConfigFactory.defaultReference());
+		config = config.withFallback(ConfigFactory.defaultReferenceUnresolved());
 		return config.resolve();
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/cassandra/CassandraAutoConfigurationTests.java

@@ -300,6 +300,18 @@ void driverConfigLoaderWithConfigOverridesDefaults() {
 		});
 	}
 
+	@Test
+	void placeholdersInReferenceConfAreResolvedAgainstConfigDerivedFromSpringCassandraProperties() {
+		this.contextRunner.withPropertyValues("spring.data.cassandra.request.timeout=60s").run((context) -> {
+			DriverExecutionProfile actual = context.getBean(DriverConfigLoader.class)
+				.getInitialConfig()
+				.getDefaultProfile();
+			assertThat(actual.getDuration(DefaultDriverOption.REQUEST_TIMEOUT)).isEqualTo(Duration.ofSeconds(60));
+			assertThat(actual.getDuration(DefaultDriverOption.METADATA_SCHEMA_REQUEST_TIMEOUT))
+				.isEqualTo(Duration.ofSeconds(60));
+		});
+	}
+
 	@Test
 	void driverConfigLoaderWithConfigCreateProfiles() {
 		String configLocation = "org/springframework/boot/autoconfigure/cassandra/profiles.conf";