diff --git a/src/tests/ui.md b/src/tests/ui.md
index 25d3efdb..9e4a1104 100644
--- a/src/tests/ui.md
+++ b/src/tests/ui.md
@@ -113,6 +113,8 @@ Compiletest makes the following replacements on the compiler output:
 - The base directory where the test's output goes is replaced with
   `$TEST_BUILD_DIR`. This only comes up in a few rare circumstances. Example:
   `/path/to/rust/build/x86_64-unknown-linux-gnu/test/ui`
+- The real directory to the standard library source is replaced with `$SRC_DIR_REAL`.
+- The real directory to the compiler source is replaced with `$COMPILER_DIR_REAL`.
 - Tabs are replaced with `\t`.
 - Backslashes (`\`) are converted to forward slashes (`/`) within paths (using a
   heuristic). This helps normalize differences with Windows-style paths.