Temporarily make the test cfg a well known bootstrap cfg

src/bootstrap/src/core/builder/cargo.rs

@@ -624,6 +624,8 @@ impl Builder<'_> {
         // get warnings about it being unexpected.
         hostflags.arg("-Zunstable-options");
         hostflags.arg("--check-cfg=cfg(bootstrap)");
+        // #[cfg(bootstrap)] as we are transition `test` to userspace cfg
+        hostflags.arg("--check-cfg=cfg(test)");
 
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,
         // but this breaks CI. At the very least, stage0 `rustdoc` needs `--cfg bootstrap`. See

src/bootstrap/src/lib.rs

@@ -77,6 +77,8 @@ const LLD_FILE_NAMES: &[&str] = &["ld.lld", "ld64.lld", "lld-link", "wasm-ld"];
 #[allow(clippy::type_complexity)] // It's fine for hard-coded list and type is explained above.
 const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     (None, "bootstrap", None),
+    // #[cfg(bootstrap)] to be removed when Cargo is updated
+    (None, "test", None),
     (Some(Mode::Rustc), "llvm_enzyme", None),
     (Some(Mode::Codegen), "llvm_enzyme", None),
     (Some(Mode::ToolRustc), "llvm_enzyme", None),