Make the libstd build script smaller

Remove all rustc-link-lib from the std build script. Also remove use of
feature = "restricted-std" where not necessary.

library/std/build.rs

@@ -3,66 +3,23 @@ use std::env;
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
-    if target.contains("linux") {
+    if target.contains("freebsd") {
-        if target.contains("android") {
-            println!("cargo:rustc-link-lib=dl");
-            println!("cargo:rustc-link-lib=log");
-            println!("cargo:rustc-link-lib=gcc");
-        }
-    } else if target.contains("freebsd") {
-        println!("cargo:rustc-link-lib=execinfo");
-        println!("cargo:rustc-link-lib=pthread");
         if env::var("RUST_STD_FREEBSD_12_ABI").is_ok() {
             println!("cargo:rustc-cfg=freebsd12");
         }
-    } else if target.contains("netbsd") {
+    } else if target.contains("linux")
-        println!("cargo:rustc-link-lib=pthread");
+        || target.contains("netbsd")
-        println!("cargo:rustc-link-lib=rt");
+        || target.contains("dragonfly")
-    } else if target.contains("dragonfly") || target.contains("openbsd") {
+        || target.contains("openbsd")
-        println!("cargo:rustc-link-lib=pthread");
+        || target.contains("solaris")
-    } else if target.contains("solaris") {
+        || target.contains("illumos")
-        println!("cargo:rustc-link-lib=socket");
+        || target.contains("apple-darwin")
-        println!("cargo:rustc-link-lib=posix4");
+        || target.contains("apple-ios")
-        println!("cargo:rustc-link-lib=pthread");
+        || target.contains("uwp")
-        println!("cargo:rustc-link-lib=resolv");
+        || target.contains("windows")
-    } else if target.contains("illumos") {
+        || target.contains("fuchsia")
-        println!("cargo:rustc-link-lib=socket");
+        || target.contains("cloudabi")
-        println!("cargo:rustc-link-lib=posix4");
+        || (target.contains("sgx") && target.contains("fortanix"))
-        println!("cargo:rustc-link-lib=pthread");
-        println!("cargo:rustc-link-lib=resolv");
-        println!("cargo:rustc-link-lib=nsl");
-        // Use libumem for the (malloc-compatible) allocator
-        println!("cargo:rustc-link-lib=umem");
-    } else if target.contains("apple-darwin") {
-        println!("cargo:rustc-link-lib=System");
-
-        // res_init and friends require -lresolv on macOS/iOS.
-        // See #41582 and http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html
-        println!("cargo:rustc-link-lib=resolv");
-    } else if target.contains("apple-ios") {
-        println!("cargo:rustc-link-lib=System");
-        println!("cargo:rustc-link-lib=objc");
-        println!("cargo:rustc-link-lib=framework=Security");
-        println!("cargo:rustc-link-lib=framework=Foundation");
-        println!("cargo:rustc-link-lib=resolv");
-    } else if target.contains("uwp") {
-        println!("cargo:rustc-link-lib=ws2_32");
-        // For BCryptGenRandom
-        println!("cargo:rustc-link-lib=bcrypt");
-    } else if target.contains("windows") {
-        println!("cargo:rustc-link-lib=advapi32");
-        println!("cargo:rustc-link-lib=ws2_32");
-        println!("cargo:rustc-link-lib=userenv");
-    } else if target.contains("fuchsia") {
-        println!("cargo:rustc-link-lib=zircon");
-        println!("cargo:rustc-link-lib=fdio");
-    } else if target.contains("cloudabi") {
-        if cfg!(feature = "backtrace") {
-            println!("cargo:rustc-link-lib=unwind");
-        }
-        println!("cargo:rustc-link-lib=c");
-        println!("cargo:rustc-link-lib=compiler_rt");
-    } else if (target.contains("sgx") && target.contains("fortanix"))
         || target.contains("hermit")
         || target.contains("l4re")
         || target.contains("redox")

library/std/src/lib.rs

@@ -563,5 +563,5 @@ include!("keyword_docs.rs");
 // This is required to avoid an unstable error when `restricted-std` is not
 // enabled. The use of #![feature(restricted_std)] in rustc-std-workspace-std
 // is unconditional, so the unstable feature needs to be defined somewhere.
-#[cfg_attr(not(feature = "restricted-std"), unstable(feature = "restricted_std", issue = "none"))]
+#[unstable(feature = "restricted_std", issue = "none")]
 mod __restricted_std_workaround {}

library/std/src/sys/cloudabi/mod.rs

@@ -66,3 +66,8 @@ pub fn hashmap_random_keys() -> (u64, u64) {
         v.assume_init()
     }
 }
+
+#[cfg_attr(feature = "backtrace", link(name = "unwind"))]
+#[link(name = "c")]
+#[link(name = "compiler_rt")]
+extern "C" {}

library/std/src/sys/unix/mod.rs

@@ -234,3 +234,55 @@ pub fn cvt_nz(error: libc::c_int) -> crate::io::Result<()> {
 pub fn abort_internal() -> ! {
     unsafe { libc::abort() }
 }
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "android")] {
+        #[link(name = "dl")]
+        #[link(name = "log")]
+        #[link(name = "gcc")]
+        extern "C" {}
+    } else if #[cfg(target_os = "freebsd")] {
+        #[link(name = "execinfo")]
+        #[link(name = "pthread")]
+        extern "C" {}
+    } else if #[cfg(target_os = "netbsd")] {
+        #[link(name = "pthread")]
+        #[link(name = "rt")]
+        extern "C" {}
+    } else if #[cfg(any(target_os = "dragonfly", target_os = "openbsd"))] {
+        #[link(name = "pthread")]
+        extern "C" {}
+    } else if #[cfg(target_os = "solaris")] {
+        #[link(name = "socket")]
+        #[link(name = "posix4")]
+        #[link(name = "pthread")]
+        #[link(name = "resolv")]
+        extern "C" {}
+    } else if #[cfg(target_os = "illumos")] {
+        #[link(name = "socket")]
+        #[link(name = "posix4")]
+        #[link(name = "pthread")]
+        #[link(name = "resolv")]
+        #[link(name = "nsl")]
+        // Use libumem for the (malloc-compatible) allocator
+        #[link(name = "umem")]
+        extern "C" {}
+    } else if #[cfg(target_os = "macos")] {
+        #[link(name = "System")]
+        // res_init and friends require -lresolv on macOS/iOS.
+        // See #41582 and http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html
+        #[link(name = "resolv")]
+        extern "C" {}
+    } else if #[cfg(target_os = "ios")] {
+        #[link(name = "System")]
+        #[link(name = "objc")]
+        #[link(name = "Security", kind = "framework")]
+        #[link(name = "Foundation", kind = "framework")]
+        #[link(name = "resolv")]
+        extern "C" {}
+    } else if #[cfg(target_os = "fuchsia")] {
+        #[link(name = "zircon")]
+        #[link(name = "fdio")]
+        extern "C" {}
+    }
+}

library/std/src/sys/windows/mod.rs

@@ -270,3 +270,17 @@ pub fn abort_internal() -> ! {
     }
     crate::intrinsics::abort();
 }
+
+cfg_if::cfg_if! {
+    if #[cfg(target_vendor = "uwp")] {
+        #[link(name = "ws2_32")]
+        // For BCryptGenRandom
+        #[link(name = "bcrypt")]
+        extern "C" {}
+    } else {
+        #[link(name = "advapi32")]
+        #[link(name = "ws2_32")]
+        #[link(name = "userenv")]
+        extern "C" {}
+    }
+}