resolve error when attempting to link a universal library on macOS

Previously attempting to link universal libraries into libraries (but not binaries) would produce an error that "File too small to be an archive". This works around this by using `object` to extract a library for the target platform when passed a univeral library.

Fixes #55235

Cargo.lock

@@ -3297,6 +3297,7 @@ dependencies = [
  "rustc_symbol_mangling",
  "rustc_target",
  "smallvec",
+ "tempfile",
  "tracing",
 ]
 

compiler/rustc_codegen_llvm/Cargo.toml

@@ -34,3 +34,4 @@ rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 rustc_ast = { path = "../rustc_ast" }
 rustc_span = { path = "../rustc_span" }
+tempfile = "3.2.0"

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -2,16 +2,20 @@
 
 use std::env;
 use std::ffi::{CStr, CString, OsString};
-use std::io;
+use std::fs;
+use std::io::{self, Write};
 use std::mem;
 use std::path::{Path, PathBuf};
 use std::ptr;
 use std::str;
 
+use object::read::macho::FatArch;
+
 use crate::common;
 use crate::llvm::archive_ro::{ArchiveRO, Child};
 use crate::llvm::{self, ArchiveKind, LLVMMachineType, LLVMRustCOFFShortExport};
 use rustc_codegen_ssa::back::archive::{ArchiveBuilder, ArchiveBuilderBuilder};
+use rustc_data_structures::memmap::Mmap;
 use rustc_session::cstore::DllImport;
 use rustc_session::Session;
 
@@ -53,13 +57,70 @@ fn llvm_machine_type(cpu: &str) -> LLVMMachineType {
     }
 }
 
+fn try_filter_fat_archs(
+    archs: object::read::Result<&[impl FatArch]>,
+    target_arch: object::Architecture,
+    archive_path: &Path,
+    archive_map_data: &[u8],
+) -> io::Result<Option<PathBuf>> {
+    let archs = archs.map_err(|e| io::Error::new(io::ErrorKind::Other, e))?;
+
+    let desired = match archs.iter().filter(|a| a.architecture() == target_arch).next() {
+        Some(a) => a,
+        None => return Ok(None),
+    };
+
+    let (mut new_f, extracted_path) = tempfile::Builder::new()
+        .suffix(archive_path.file_name().unwrap())
+        .tempfile()?
+        .keep()
+        .unwrap();
+
+    new_f.write_all(
+        desired.data(archive_map_data).map_err(|e| io::Error::new(io::ErrorKind::Other, e))?,
+    )?;
+
+    Ok(Some(extracted_path))
+}
+
+fn try_extract_macho_fat_archive(
+    sess: &Session,
+    archive_path: &Path,
+) -> io::Result<Option<PathBuf>> {
+    let archive_map = unsafe { Mmap::map(fs::File::open(&archive_path)?)? };
+    let target_arch = match sess.target.arch.as_ref() {
+        "aarch64" => object::Architecture::Aarch64,
+        "x86_64" => object::Architecture::X86_64,
+        _ => return Ok(None),
+    };
+
+    match object::macho::FatHeader::parse(&*archive_map) {
+        Ok(h) if h.magic.get(object::endian::BigEndian) == object::macho::FAT_MAGIC => {
+            let archs = object::macho::FatHeader::parse_arch32(&*archive_map);
+            try_filter_fat_archs(archs, target_arch, archive_path, &*archive_map)
+        }
+        Ok(h) if h.magic.get(object::endian::BigEndian) == object::macho::FAT_MAGIC_64 => {
+            let archs = object::macho::FatHeader::parse_arch64(&*archive_map);
+            try_filter_fat_archs(archs, target_arch, archive_path, &*archive_map)
+        }
+        // Not a FatHeader at all, just return None.
+        _ => Ok(None),
+    }
+}
+
 impl<'a> ArchiveBuilder<'a> for LlvmArchiveBuilder<'a> {
     fn add_archive(
         &mut self,
         archive: &Path,
         skip: Box<dyn FnMut(&str) -> bool + 'static>,
     ) -> io::Result<()> {
-        let archive_ro = match ArchiveRO::open(archive) {
+        let mut archive = archive.to_path_buf();
+        if self.sess.target.llvm_target.contains("-apple-macosx") {
+            if let Some(new_archive) = try_extract_macho_fat_archive(&self.sess, &archive)? {
+                archive = new_archive
+            }
+        }
+        let archive_ro = match ArchiveRO::open(&archive) {
             Ok(ar) => ar,
             Err(e) => return Err(io::Error::new(io::ErrorKind::Other, e)),
         };
@@ -67,7 +128,7 @@ impl<'a> ArchiveBuilder<'a> for LlvmArchiveBuilder<'a> {
             return Ok(());
         }
         self.additions.push(Addition::Archive {
-            path: archive.to_path_buf(),
+            path: archive,
             archive: archive_ro,
             skip: Box::new(skip),
         });

src/test/run-make/macos-fat-archive/Makefile

@@ -0,0 +1,10 @@
+# only-macos
+
+-include ../../run-make-fulldeps/tools.mk
+
+"$(TMPDIR)"/libnative-library.a: native-library.c
+	$(CC) -arch arm64 -arch x86_64 native-library.c -c -o "$(TMPDIR)"/native-library.o
+	$(AR) crs "$(TMPDIR)"/libnative-library.a "$(TMPDIR)"/native-library.o
+
+all: "$(TMPDIR)"/libnative-library.a
+	$(RUSTC) lib.rs --crate-type=lib -L "native=$(TMPDIR)" -l static=native-library

src/test/run-make/macos-fat-archive/lib.rs

@@ -0,0 +1,3 @@
+extern "C" {
+    pub fn native_func();
+}

src/test/run-make/macos-fat-archive/native-library.c

@@ -0,0 +1 @@
+void native_func() {}