Only allow bootstrap cfg in rustc & related

library/core/src/iter/sources/generator.rs

@@ -9,8 +9,6 @@
 ///
 /// ```
 /// #![feature(iter_macro, coroutines)]
-/// # #[cfg(not(bootstrap))]
-/// # {
 ///
 /// let it = std::iter::iter!{|| {
 ///     yield 1;
@@ -19,11 +17,10 @@
 /// } }();
 /// let v: Vec<_> = it.collect();
 /// assert_eq!(v, [1, 2, 3]);
-/// # }
 /// ```
 #[unstable(feature = "iter_macro", issue = "none", reason = "generators are unstable")]
 #[allow_internal_unstable(coroutines, iter_from_coroutine)]
-#[cfg_attr(not(bootstrap), rustc_builtin_macro)]
+#[rustc_builtin_macro]
 pub macro iter($($t:tt)*) {
     /* compiler-builtin */
 }

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

@@ -636,6 +636,15 @@ impl Builder<'_> {
         for (restricted_mode, name, values) in EXTRA_CHECK_CFGS {
             if restricted_mode.is_none() || *restricted_mode == Some(mode) {
                 rustflags.arg(&check_cfg_arg(name, *values));
+
+                if *name == "bootstrap" {
+                    // Cargo doesn't pass RUSTFLAGS to proc_macros:
+                    // https://github.com/rust-lang/cargo/issues/4423
+                    // Thus, if we are on stage 0, we explicitly set `--cfg=bootstrap`.
+                    // We also declare that the flag is expected, which we need to do to not
+                    // get warnings about it being unexpected.
+                    hostflags.arg(check_cfg_arg(name, *values));
+                }
             }
         }
 
@@ -645,13 +654,6 @@ impl Builder<'_> {
         if stage == 0 {
             hostflags.arg("--cfg=bootstrap");
         }
-        // Cargo doesn't pass RUSTFLAGS to proc_macros:
-        // https://github.com/rust-lang/cargo/issues/4423
-        // Thus, if we are on stage 0, we explicitly set `--cfg=bootstrap`.
-        // We also declare that the flag is expected, which we need to do to not
-        // get warnings about it being unexpected.
-        hostflags.arg("-Zunstable-options");
-        hostflags.arg("--check-cfg=cfg(bootstrap)");
 
         // 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

@@ -81,7 +81,10 @@ const LLD_FILE_NAMES: &[&str] = &["ld.lld", "ld64.lld", "lld-link", "wasm-ld"];
 /// (Mode restriction, config name, config values (if any))
 #[expect(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),
+    (Some(Mode::Rustc), "bootstrap", None),
+    (Some(Mode::Codegen), "bootstrap", None),
+    (Some(Mode::ToolRustc), "bootstrap", None),
+    (Some(Mode::ToolStd), "bootstrap", None),
     (Some(Mode::Rustc), "llvm_enzyme", None),
     (Some(Mode::Codegen), "llvm_enzyme", None),
     (Some(Mode::ToolRustc), "llvm_enzyme", None),