diff --git a/src/test/incremental/hygiene/load_cached_hygiene.rs b/src/test/incremental/hygiene/load_cached_hygiene.rs
index 8124141418b..d6a5cb993a4 100644
--- a/src/test/incremental/hygiene/load_cached_hygiene.rs
+++ b/src/test/incremental/hygiene/load_cached_hygiene.rs
@@ -1,5 +1,5 @@
 // revisions:rpass1 rpass2
-// compile-flags: -Z query-dep-graph
+// compile-flags: -Z query-dep-graph -O
 // aux-build:cached_hygiene.rs
 
 // This tests the folllowing scenario
@@ -19,7 +19,12 @@
 // the metadata. Specifically, we were not resetting `orig_id`
 // for an `EpxnData` generate in the current crate, which would cause
 // us to serialize the `ExpnId` pointing to a garbage location in
-// the metadata.
+// the metadata.o
+
+// NOTE: We're explicitly passing the `-O` optimization flag because if optimizations are not
+// enabled, then rustc will ignore the `#[inline(always)]` attribute which means we do not load
+// the optimized mir for the unmodified function to be loaded and so the CGU containing that
+// function will be reused.
 
 #![feature(rustc_attrs)]
 
diff --git a/src/test/incremental/remapped_paths_cc/main.rs b/src/test/incremental/remapped_paths_cc/main.rs
index b01f02444ea..735635029da 100644
--- a/src/test/incremental/remapped_paths_cc/main.rs
+++ b/src/test/incremental/remapped_paths_cc/main.rs
@@ -1,11 +1,18 @@
 // revisions:rpass1 rpass2 rpass3
-// compile-flags: -Z query-dep-graph -g
+// compile-flags: -Z query-dep-graph -g -O
 // aux-build:extern_crate.rs
 
 // ignore-asmjs wasm2js does not support source maps yet
+
 // This test case makes sure that we detect if paths emitted into debuginfo
 // are changed, even when the change happens in an external crate.
 
+// NOTE: We're explicitly passing the `-O` optimization flag because if no optimizations are
+// requested, rustc will ignore the `#[inline]` attribute. This is a performance optimization for
+// non-optimized builds which causes us to generate fewer copies of inlined functions when
+// runtime performance doesn't matter. Without this flag, the function will go into a different
+// CGU which can be reused by this crate.
+
 #![feature(rustc_attrs)]
 
 #![rustc_partition_reused(module="main", cfg="rpass2")]