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src/bootstrap/check.rs

@@ -140,6 +140,9 @@ impl Step for Std {
             cargo_subcommand(builder.kind),
         );
 
+        // If we're not in stage 0, tests and examples will fail to compile
+        // from `core` definitions being loaded from two different `libcore`
+        // .rmeta and .rlib files.
         if compiler.stage == 0 {
             cargo.arg("--all-targets");
         }