libunwind's build.rs expects to run in a directory with no stale object files,
which isn't always the case. This makes it the case.

Submitted for upstream review in https://github.com/rust-lang/rust/pull/87977

--- a/library/unwind/build.rs
+++ b/library/unwind/build.rs
@@ -26,8 +26,9 @@ fn main() {
             // Since ndk r23 beta 3 `libgcc` was replaced with `libunwind` thus
             // check if we have `libunwind` available and if so use it. Otherwise
             // fall back to `libgcc` to support older ndk versions.
-            let has_unwind =
-                build.is_flag_supported("-lunwind").expect("Unable to invoke compiler");
+            let has_unwind = build
+                .is_flag_supported("-lunwind")
+                .expect("Unable to invoke compiler");
 
             if has_unwind {
                 println!("cargo:rustc-link-lib=unwind");
@@ -66,7 +67,19 @@ fn main() {
 
 mod llvm_libunwind {
     use std::env;
-    use std::path::Path;
+    use std::path::{Path, PathBuf};
+
+    fn enumerate_object_files_in(out_dir: &Path) -> impl Iterator<Item = PathBuf> {
+        std::fs::read_dir(out_dir).unwrap().filter_map(|entry| {
+            let path = entry.unwrap().path();
+            if let Some(ext) = path.extension() {
+                if ext == "o" {
+                    return Some(path.canonicalize().unwrap());
+                }
+            }
+            None
+        })
+    }
 
     /// Compile the libunwind C/C++ source code.
     pub fn compile() {
@@ -76,6 +89,16 @@ mod llvm_libunwind {
         let mut cpp_cfg = cc::Build::new();
         let root = Path::new("../../src/llvm-project/libunwind");
 
+        // We depend on starting with a fresh build directory each time.
+        let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+        for object in enumerate_object_files_in(&out_dir) {
+            if let Err(x) = std::fs::remove_file(&object) {
+                if x.kind() != std::io::ErrorKind::NotFound {
+                    panic!("Failed removing {}: {}", object.display(), x);
+                }
+            }
+        }
+
         cpp_cfg.cpp(true);
         cpp_cfg.cpp_set_stdlib(None);
         cpp_cfg.flag("-nostdinc++");
@@ -137,7 +160,7 @@ mod llvm_libunwind {
             "UnwindRegistersSave.S",
         ];
 
-        let cpp_sources = vec!["Unwind-EHABI.cpp", "Unwind-seh.cpp", "libunwind.cpp"];
+        let cpp_sources = &["Unwind-EHABI.cpp", "Unwind-seh.cpp", "libunwind.cpp"];
         let cpp_len = cpp_sources.len();
 
         if target.contains("x86_64-fortanix-unknown-sgx") {
@@ -152,22 +175,19 @@ mod llvm_libunwind {
             cpp_cfg.file(root.join("src").join(src).canonicalize().unwrap());
         }
 
-        let out_dir = env::var("OUT_DIR").unwrap();
-        println!("cargo:rustc-link-search=native={}", &out_dir);
+        println!("cargo:rustc-link-search=native={}", out_dir.display());
 
         cpp_cfg.compile("unwind-cpp");
-
-        let mut count = 0;
-        for entry in std::fs::read_dir(&out_dir).unwrap() {
-            let obj = entry.unwrap().path().canonicalize().unwrap();
-            if let Some(ext) = obj.extension() {
-                if ext == "o" {
-                    cc_cfg.object(&obj);
-                    count += 1;
-                }
-            }
+        let objects: Vec<PathBuf> = enumerate_object_files_in(&out_dir).collect();
+        assert_eq!(
+            cpp_len,
+            objects.len(),
+            "Can't get object files from {}",
+            out_dir.display()
+        );
+        for obj in objects {
+            cc_cfg.object(obj);
         }
-        assert_eq!(cpp_len, count, "Can't get object files from {:?}", &out_dir);
         cc_cfg.compile("unwind");
     }
 }