Add RUST_LIBC_UNSTABLE_EMCC_VERSION for wasm32-unknown-emscripten

Author: guusw

.github/workflows/ci.yaml

@@ -179,7 +179,6 @@ jobs:
           - powerpc64le-unknown-linux-gnu
           - riscv64gc-unknown-linux-gnu
           - s390x-unknown-linux-gnu
-          - wasm32-unknown-emscripten
           - wasm32-wasip1
           - wasm32-wasip2
           - x86_64-linux-android
@@ -195,6 +194,9 @@ jobs:
             env:
               RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS: 64
             artifact-tag: offset-bits64
+          - target: wasm32-unknown-emscripten
+            env:
+              RUST_LIBC_UNSTABLE_EMCC_VERSION: 30142
           # FIXME(ppc): SIGILL running tests, see
           # https://github.com/rust-lang/libc/pull/4254#issuecomment-2636288713
           # - target: powerpc-unknown-linux-gnu

build.rs

@@ -82,7 +82,15 @@ fn main() {
         _ => set_cfg("freebsd15"),
     }
 
-    match emcc_version_code() {
+    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_EMCC_VERSION");
+    let emcc_version = if let Ok(version) = env::var("RUST_LIBC_UNSTABLE_EMCC_VERSION") {
+        let vers: i32 = version.parse().unwrap();
+        println!("cargo:warning=setting emcc version to {vers}");
+        Some(vers as u64)
+    } else {
+        emcc_version_code()
+    };
+    match emcc_version {
         Some(v) if (v < 30142) => set_cfg("emscripten_old_stat_abi"),
         // Non-Emscripten or version >= 3.1.42.
         _ => (),