compiletest: compile rmake.rs with -Dunused_must_use

Author: jieyouxu

src/tools/compiletest/src/runtest.rs

@@ -3507,6 +3507,10 @@ impl<'test> TestCx<'test> {
             .env_remove("MFLAGS")
             .env_remove("CARGO_MAKEFLAGS");
 
+        // In test code we want to be very pedantic about values being silently discarded that are
+        // annotated with `#[must_use]`.
+        cmd.arg("-Dunused_must_use");
+
         if std::env::var_os("COMPILETEST_FORCE_STAGE0").is_some() {
             let mut stage0_sysroot = build_root.clone();
             stage0_sysroot.push("stage0-sysroot");