addresss reviews

library/alloc/Cargo.toml

@@ -43,9 +43,12 @@ optimize_for_size = ["core/optimize_for_size"]
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
+# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
+# for rust-lang/rust. But for users of `-Zbuild-std` it does.
+# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
-    'cfg(no_global_oom_handling)',
     'cfg(bootstrap)',
+    'cfg(no_global_oom_handling)',
     'cfg(no_rc)',
     'cfg(no_sync)',
 ]

library/core/Cargo.toml

@@ -39,9 +39,13 @@ debug_refcell = []
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
+# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
+# for rust-lang/rust. But for users of `-Zbuild-std` it does.
+# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
     'cfg(no_fp_fmt_parse)',
     'cfg(bootstrap)',
     'cfg(stdarch_intel_sde)',
-    'cfg(feature, values("all_lane_counts"))',
+    # This matches `EXTRA_CHECK_CFGS` in `src/bootstrap/src/lib.rs`.
+    'cfg(feature, values(any()))',
 ]

src/bootstrap/src/lib.rs

@@ -84,6 +84,7 @@ const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     (Some(Mode::ToolRustc), "rust_analyzer", None),
     (Some(Mode::ToolStd), "rust_analyzer", None),
     (Some(Mode::Codegen), "parallel_compiler", None),
+    // NOTE: consider updating `check-cfg` entries in `std/Cargo.toml` too.
     (Some(Mode::Std), "stdarch_intel_sde", None),
     (Some(Mode::Std), "no_fp_fmt_parse", None),
     (Some(Mode::Std), "no_global_oom_handling", None),