Fix building std_detect as a dependency of std (#1089)

Author: Amanieu

Diff for: crates/std_detect/Cargo.toml

@@ -25,12 +25,22 @@ maintenance = { status = "experimental" }
 libc = { version = "0.2", optional = true, default-features = false }
 cfg-if = "0.1.10"
 
+# When built as part of libstd
+core = { version = "1.0.0", optional = true, package = "rustc-std-workspace-core" }
+compiler_builtins = { version = "0.1.2", optional = true }
+alloc = { version = "1.0.0", optional = true, package = "rustc-std-workspace-alloc" }
+
 [dev-dependencies]
 auxv = "0.3.3"
 cupid = "0.6.0"
 
 [features]
 default = [ "std_detect_dlsym_getauxval", "std_detect_file_io" ]
-std_detect_file_io = []
+std_detect_file_io = [ "libc" ]
 std_detect_dlsym_getauxval = [ "libc" ]
-std_detect_env_override = []
+std_detect_env_override = [ "libc" ]
+rustc-dep-of-std = [
+    "core",
+    "compiler_builtins",
+    "alloc",
+]

Diff for: crates/std_detect/src/detect/os/linux/auxvec.rs

@@ -89,7 +89,7 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
 
     #[cfg(not(feature = "std_detect_dlsym_getauxval"))]
     {
-        let hwcap = unsafe { libc::getauxval(AT_HWCAP) };
+        let hwcap = unsafe { libc::getauxval(AT_HWCAP as libc::c_ulong) as usize };
 
         // Targets with only AT_HWCAP:
         #[cfg(any(target_arch = "aarch64", target_arch = "mips", target_arch = "mips64"))]
@@ -106,7 +106,7 @@ pub(crate) fn auxv() -> Result<AuxVec, ()> {
             target_arch = "powerpc64"
         ))]
         {
-            let hwcap2 = unsafe { libc::getauxval(AT_HWCAP2) };
+            let hwcap2 = unsafe { libc::getauxval(AT_HWCAP2 as libc::c_ulong) as usize };
             if hwcap != 0 && hwcap2 != 0 {
                 return Ok(AuxVec { hwcap, hwcap2 });
             }

Diff for: crates/std_detect/src/lib.rs

@@ -20,6 +20,7 @@
 #![cfg_attr(feature = "std_detect_file_io", feature(vec_spare_capacity))]
 #![no_std]
 
+#[cfg_attr(feature = "rustc-dep-of-std", allow(unused_extern_crates))]
 #[cfg(feature = "std_detect_file_io")]
 extern crate alloc;