explain why we use in-tree std for compiletest

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/test.rs

@@ -686,6 +686,8 @@ impl Step for CompiletestTest {
         let mut cargo = tool::prepare_tool_cargo(
             builder,
             compiler,
+            // compiletest uses libtest internals; make it use the in-tree std to make sure it never breaks
+            // when std sources change.
             Mode::ToolStd,
             host,
             "test",