Emit directives for cargo-check-cfg

build.rs

@@ -2,6 +2,7 @@ use std::{collections::BTreeMap, env, sync::atomic::Ordering};
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
+    configure_check_cfg();
 
     let target = env::var("TARGET").unwrap();
     let cwd = env::current_dir().unwrap();
@@ -145,6 +146,46 @@ fn generate_aarch64_outlined_atomics() {
     std::fs::write(dst, buf).unwrap();
 }
 
+/// Emit directives for features we expect to support that aren't in `Cargo.toml`
+fn configure_check_cfg() {
+    // Functions where we can set the "optimized-c" flag
+    const HAS_OPTIMIZED_C: &[&str] = &[
+        "__ashldi3",
+        "__ashlsi3",
+        "__ashrdi3",
+        "__ashrsi3",
+        "__clzsi2",
+        "__divdi3",
+        "__divmoddi4",
+        "__divmodsi4",
+        "__divmodsi4",
+        "__divmodti4",
+        "__lshrdi3",
+        "__lshrsi3",
+        "__moddi3",
+        "__modsi3",
+        "__muldi3",
+        "__udivdi3",
+        "__udivmoddi4",
+        "__udivmodsi4",
+        "__udivsi3",
+        "__umoddi3",
+        "__umodsi3",
+    ];
+
+    for fn_name in HAS_OPTIMIZED_C {
+        println!(
+            "cargo::rustc-check-cfg=cfg({}, values(\"optimized-c\"))",
+            fn_name
+        );
+    }
+
+    println!("cargo::rustc-check-cfg=cfg(feature, values(\"mem-unaligned\")");
+    println!("cargo::rustc-check-cfg=cfg(feature, values(\"unstable\")");
+    println!("cargo::rustc-check-cfg=cfg(assert_no_panic)");
+    println!("cargo::rustc-check-cfg=cfg(kernel_user_helpers)");
+}
+
 #[cfg(feature = "c")]
 mod c {
     extern crate cc;