Add musl_v1_2_3 feature

This feature, controlled by the environment variable
RUST_LIBC_UNSTABLE_MUSL_1_2_3 will control whether breaking changes up
to musl v1.2.3 will be reflected

Author: xbjfk

build.rs

@@ -21,6 +21,7 @@ const ALLOWED_CFGS: &[&str] = &[
     "libc_ctest",
     // Corresponds to `__USE_TIME_BITS64` in UAPI
     "linux_time_bits64",
+    "musl_v1_2_3"
 ];
 
 // Extra values to allow for check-cfg.
@@ -85,6 +86,11 @@ fn main() {
         _ => (),
     }
 
+    let musl_v1_2_3 = env::var("RUST_LIBC_UNSTABLE_MUSL_1_2_3").is_ok();
+    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_MUSL_1_2_3");
+    if musl_v1_2_3 {
+        set_cfg("musl_v1_2_3");
+    }
     let linux_time_bits64 = env::var("RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64").is_ok();
     println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64");
     if linux_time_bits64 {

libc-test/build.rs

@@ -3656,7 +3656,12 @@ fn test_linux(target: &str) {
     let wasm32 = target.contains("wasm32");
     let uclibc = target.contains("uclibc");
 
+    let musl_v1_2_3 = env::var("RUST_LIBC_UNSTABLE_MUSL_1_2_3").is_ok();
+
     let mut cfg = ctest_cfg();
+    if musl_v1_2_3 {
+        cfg.cfg("musl_v1_2_3", None);
+    }
     cfg.define("_GNU_SOURCE", None);
     // This macro re-defines fscanf,scanf,sscanf to link to the symbols that are
     // deprecated since glibc >= 2.29. This allows Rust binaries to link against