Support reading thin archives in ArArchiveBuilder

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -106,9 +106,11 @@ pub struct LlvmArchiveBuilderBuilder;
 
 impl ArchiveBuilderBuilder for LlvmArchiveBuilderBuilder {
     fn new_archive_builder<'a>(&self, sess: &'a Session) -> Box<dyn ArchiveBuilder + 'a> {
-        // FIXME use ArArchiveBuilder on most targets again once reading thin archives is
-        // implemented
-        if true {
+        // Keeping LlvmArchiveBuilder around in case of a regression caused by using
+        // ArArchiveBuilder.
+        // FIXME(#128955) remove a couple of months after #128936 gets merged in case
+        // no regression is found.
+        if false {
             Box::new(LlvmArchiveBuilder { sess, additions: Vec::new() })
         } else {
             Box::new(ArArchiveBuilder::new(sess, &LLVM_OBJECT_READER))

compiler/rustc_codegen_ssa/src/back/archive.rs

@@ -307,10 +307,15 @@ impl<'a> ArchiveBuilder for ArArchiveBuilder<'a> {
             let file_name = String::from_utf8(entry.name().to_vec())
                 .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))?;
             if !skip(&file_name) {
-                self.entries.push((
-                    file_name.into_bytes(),
-                    ArchiveEntry::FromArchive { archive_index, file_range: entry.file_range() },
-                ));
+                if entry.is_thin() {
+                    let member_path = archive_path.parent().unwrap().join(Path::new(&file_name));
+                    self.entries.push((file_name.into_bytes(), ArchiveEntry::File(member_path)));
+                } else {
+                    self.entries.push((
+                        file_name.into_bytes(),
+                        ArchiveEntry::FromArchive { archive_index, file_range: entry.file_range() },
+                    ));
+                }
             }
         }
 

src/tools/run-make-support/src/external_deps/llvm.rs

@@ -271,6 +271,12 @@ impl LlvmAr {
         self
     }
 
+    /// Like `obj_to_ar` except creating a thin archive.
+    pub fn obj_to_thin_ar(&mut self) -> &mut Self {
+        self.cmd.arg("rcus").arg("--thin");
+        self
+    }
+
     /// Extract archive members back to files.
     pub fn extract(&mut self) -> &mut Self {
         self.cmd.arg("x");

tests/run-make/staticlib-thin-archive/bin.rs

@@ -0,0 +1,5 @@
+fn main() {
+    unsafe {
+        rust_lib::simple_fn();
+    }
+}

tests/run-make/staticlib-thin-archive/rmake.rs

@@ -0,0 +1,28 @@
+// Regression test for https://github.com/rust-lang/rust/issues/107407 which
+// checks that rustc can read thin archive. Before the object crate added thin
+// archive support rustc would add emit object files to the staticlib and after
+// the object crate added thin archive support it would previously crash the
+// compiler due to a missing special case for thin archive members.
+use std::path::Path;
+
+use run_make_support::{llvm_ar, rust_lib_name, rustc, static_lib_name};
+
+fn main() {
+    std::fs::create_dir("archive").unwrap();
+
+    // Build a thin archive
+    rustc().input("simple_obj.rs").emit("obj").output("archive/simple_obj.o").run();
+    llvm_ar()
+        .obj_to_thin_ar()
+        .output_input(
+            Path::new("archive").join(static_lib_name("thin_archive")),
+            "archive/simple_obj.o",
+        )
+        .run();
+
+    // Build an rlib which includes the members of this thin archive
+    rustc().input("rust_lib.rs").library_search_path("archive").run();
+
+    // Build a binary which requires a symbol from the thin archive
+    rustc().input("bin.rs").extern_("rust_lib", rust_lib_name("rust_lib")).run();
+}

tests/run-make/staticlib-thin-archive/rust_lib.rs

@@ -0,0 +1,6 @@
+#![crate_type = "rlib"]
+
+#[link(name = "thin_archive", kind = "static")]
+extern "C" {
+    pub fn simple_fn();
+}

tests/run-make/staticlib-thin-archive/simple_obj.rs

@@ -0,0 +1,4 @@
+#![crate_type = "staticlib"]
+
+#[no_mangle]
+extern "C" fn simple_fn() {}