diff --git a/mk/tests.mk b/mk/tests.mk
index 2599e1a6272..9d91795f28b 100644
--- a/mk/tests.mk
+++ b/mk/tests.mk
@@ -25,7 +25,7 @@ $(eval $(call RUST_CRATE,collectionstest))
 TEST_TARGET_CRATES = $(filter-out core rustc_unicode alloc_system libc \
 		     		  alloc_jemalloc,$(TARGET_CRATES)) \
 			collectionstest coretest
-TEST_DOC_CRATES = $(DOC_CRATES) arena flate
+TEST_DOC_CRATES = $(DOC_CRATES) arena flate fmt_macros
 TEST_HOST_CRATES = $(filter-out rustc_typeck rustc_borrowck rustc_resolve \
 		   		rustc_trans rustc_lint,\
                      $(HOST_CRATES))
diff --git a/src/libfmt_macros/lib.rs b/src/libfmt_macros/lib.rs
index ed767ab1e5c..dc69f38e4e7 100644
--- a/src/libfmt_macros/lib.rs
+++ b/src/libfmt_macros/lib.rs
@@ -24,7 +24,8 @@
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/",
-       html_playground_url = "https://play.rust-lang.org/")]
+       html_playground_url = "https://play.rust-lang.org/",
+       test(attr(deny(warnings))))]
 
 #![feature(staged_api)]
 #![feature(unicode)]