Replace fake "restricted-std" Cargo feature by custom cfg

library/std/build.rs

@@ -10,6 +10,7 @@ fn main() {
     if target_os == "netbsd" && env::var("RUSTC_STD_NETBSD10").is_ok() {
         println!("cargo:rustc-cfg=netbsd10");
     }
+    println!("cargo:rustc-check-cfg=cfg(restricted_std)");
     if target_os == "linux"
         || target_os == "android"
         || target_os == "netbsd"
@@ -59,7 +60,7 @@ fn main() {
         // - arch=avr
         // - JSON targets
         // - Any new targets that have not been explicitly added above.
-        println!("cargo:rustc-cfg=feature=\"restricted-std\"");
+        println!("cargo:rustc-cfg=restricted_std");
     }
     println!("cargo:rustc-env=STD_ENV_ARCH={}", env::var("CARGO_CFG_TARGET_ARCH").unwrap());
     println!("cargo:rustc-cfg=backtrace_in_libstd");

library/std/src/lib.rs

@@ -213,9 +213,9 @@
 //! [array]: prim@array
 //! [slice]: prim@slice
 
-#![cfg_attr(not(feature = "restricted-std"), stable(feature = "rust1", since = "1.0.0"))]
+#![cfg_attr(not(restricted_std), stable(feature = "rust1", since = "1.0.0"))]
 #![cfg_attr(
-    feature = "restricted-std",
+    restricted_std,
     unstable(
         feature = "restricted_std",
         issue = "none",

src/bootstrap/src/lib.rs

@@ -94,6 +94,7 @@
     (Some(Mode::Std), "no_sync", None),
     (Some(Mode::Std), "netbsd10", None),
     (Some(Mode::Std), "backtrace_in_libstd", None),
+    (Some(Mode::Std), "restricted_std", None),
     /* Extra values not defined in the built-in targets yet, but used in std */
     (Some(Mode::Std), "target_env", Some(&["libnx", "p2"])),
     (Some(Mode::Std), "target_os", Some(&["visionos"])),