cstyle-export-rules: export global symbols from upstream static libraries

[cezarbbb]

This is a demo I wrote based on an idea from this Zulip thread, and it's currently in a very early stage (it doesn't yet limit unsupported platforms or consider LTO).

I wrote a test to check the effect of enabling this option. First, I wrote an upstream C static library and compiled it into an object file.

int c_add(int a, int b) {
    return a + b;
}

int c_sub(int a, int b) {
    return a - b;
}

Then I created a cdylib project downstream.

Cargo.toml

[package]
name = "downstream-cdylib"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

src/lib.rs

extern "C" {
    fn c_add(a: i32, b: i32) -> i32;
}

#[no_mangle]
pub extern "C" fn downstream_add(a: i32, b: i32) -> i32 {
    unsafe { c_add(a, b) }
}

Then I tested by enabling and disabling cstyle-export-rules in the following two toolchains, and checked their symbol tables and the size of the generated .so files.
default:

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

+c-e-r

rustc 1.91.0-nightly (383b9c447 2025-08-03)
binary: rustc
commit-hash: 383b9c447b61641e1f1a3850253944a897a60827
commit-date: 2025-08-03
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 20.1.8

Here are the results:

1. RUSTFLAGS="-Z cstyle-export-rules" cargo +c-e-r rustc --release -- -C link-arg=c_add.o
size: 451KB

                 U abort@GLIBC_2.2.5
                 U bcmp@GLIBC_2.2.5
00000000000520ad T c_add
                 U calloc@GLIBC_2.2.5
                 U close@GLIBC_2.2.5
00000000000520c5 T c_sub
                 w __cxa_finalize@GLIBC_2.2.5
                 w __cxa_thread_atexit_impl@GLIBC_2.18
                 U dl_iterate_phdr@GLIBC_2.2.5
0000000000015160 T downstream_add
                 U __errno_location@GLIBC_2.2.5
                 U free@GLIBC_2.2.5
                 U fstat64@GLIBC_2.33
                 U getcwd@GLIBC_2.2.5
                 U getenv@GLIBC_2.2.5
                 w __gmon_start__
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
                 U lseek64@GLIBC_2.2.5
                 U malloc@GLIBC_2.2.5
                 U memcpy@GLIBC_2.14
                 U memmove@GLIBC_2.2.5
                 U memset@GLIBC_2.2.5
                 U mmap64@GLIBC_2.2.5
                 U munmap@GLIBC_2.2.5
                 U open64@GLIBC_2.2.5
                 U posix_memalign@GLIBC_2.2.5
                 U pthread_key_create@GLIBC_2.34
                 U pthread_key_delete@GLIBC_2.34
                 U pthread_setspecific@GLIBC_2.34
                 U read@GLIBC_2.2.5
                 U readlink@GLIBC_2.2.5
                 U realloc@GLIBC_2.2.5
                 U realpath@GLIBC_2.3
0000000000052074 T _RNvCs6OckbHhR1MK_7___rustc17___rust_probestack
                 U stat64@GLIBC_2.33
                 w statx@GLIBC_2.28
                 U strlen@GLIBC_2.2.5
                 U syscall@GLIBC_2.2.5
                 U __tls_get_addr@GLIBC_2.3
                 U _Unwind_Backtrace@GCC_3.3
                 U _Unwind_DeleteException@GCC_3.0
                 U _Unwind_GetDataRelBase@GCC_3.0
                 U _Unwind_GetIP@GCC_3.0
                 U _Unwind_GetIPInfo@GCC_4.2.0
                 U _Unwind_GetLanguageSpecificData@GCC_3.0
                 U _Unwind_GetRegionStart@GCC_3.0
                 U _Unwind_GetTextRelBase@GCC_3.0
                 U _Unwind_RaiseException@GCC_3.0
                 U _Unwind_Resume@GCC_3.0
                 U _Unwind_SetGR@GCC_3.0
                 U _Unwind_SetIP@GCC_3.0
                 U write@GLIBC_2.2.5
                 U writev@GLIBC_2.2.5
0000000000044bcc T _ZN119_$LT$core..iter..adapters..copied..Copied$LT$I$GT$$u20$as$u20$core..iter..traits..double_ended..DoubleEndedIterator$GT$9try_rfold17h00588b6766da7b19E
000000000001a800 T _ZN3std3sys9backtrace26__rust_end_short_backtrace17h9a45a5fa40018bf0E
00000000000414a0 T _ZN4core5slice4sort8unstable7ipnsort17hd24504d702ce9a58E
0000000000041590 T _ZN4core5slice4sort8unstable8heapsort8heapsort17h5983f0d9c6b77eb4E

2. cargo +c-e-r rustc --release -- -C link-arg=c_add.o
size: 450KB

                 U abort@GLIBC_2.2.5
                 U bcmp@GLIBC_2.2.5
                 U calloc@GLIBC_2.2.5
                 U close@GLIBC_2.2.5
                 w __cxa_finalize@GLIBC_2.2.5
                 w __cxa_thread_atexit_impl@GLIBC_2.18
                 U dl_iterate_phdr@GLIBC_2.2.5
0000000000014ea0 T downstream_add
                 U __errno_location@GLIBC_2.2.5
                 U free@GLIBC_2.2.5
                 U fstat64@GLIBC_2.33
                 U getcwd@GLIBC_2.2.5
                 U getenv@GLIBC_2.2.5
                 w __gmon_start__
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
                 U lseek64@GLIBC_2.2.5
                 U malloc@GLIBC_2.2.5
                 U memcpy@GLIBC_2.14
                 U memmove@GLIBC_2.2.5
                 U memset@GLIBC_2.2.5
                 U mmap64@GLIBC_2.2.5
                 U munmap@GLIBC_2.2.5
                 U open64@GLIBC_2.2.5
                 U posix_memalign@GLIBC_2.2.5
                 U pthread_key_create@GLIBC_2.34
                 U pthread_key_delete@GLIBC_2.34
                 U pthread_setspecific@GLIBC_2.34
                 U read@GLIBC_2.2.5
                 U readlink@GLIBC_2.2.5
                 U realloc@GLIBC_2.2.5
                 U realpath@GLIBC_2.3
                 U stat64@GLIBC_2.33
                 w statx@GLIBC_2.28
                 U strlen@GLIBC_2.2.5
                 U syscall@GLIBC_2.2.5
                 U __tls_get_addr@GLIBC_2.3
                 U _Unwind_Backtrace@GCC_3.3
                 U _Unwind_DeleteException@GCC_3.0
                 U _Unwind_GetDataRelBase@GCC_3.0
                 U _Unwind_GetIP@GCC_3.0
                 U _Unwind_GetIPInfo@GCC_4.2.0
                 U _Unwind_GetLanguageSpecificData@GCC_3.0
                 U _Unwind_GetRegionStart@GCC_3.0
                 U _Unwind_GetTextRelBase@GCC_3.0
                 U _Unwind_RaiseException@GCC_3.0
                 U _Unwind_Resume@GCC_3.0
                 U _Unwind_SetGR@GCC_3.0
                 U _Unwind_SetIP@GCC_3.0
                 U write@GLIBC_2.2.5
                 U writev@GLIBC_2.2.5

3. cargo rustc --release -- -C link-arg=c_add.o
size: 16KB

                 w __cxa_finalize@GLIBC_2.2.5
0000000000001100 T downstream_add
                 w __gmon_start__
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable

It appears that the upstream c_add and c_sub were successfully exported, but many more symbols were exported, and the file size increased significantly. I'm not sure why this is happening.
r? @bjorn3

[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is older than 28 days.

  It's recommended to update your branch according to the rustc-dev-guide.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_codegen_ssa/src/back/linker.rs:886:
                     }
                     writeln!(f, "\n  local:\n    *;\n}};")?;
                 }
-                
             };
             if let Err(error) = res {
                 self.sess.dcx().emit_fatal(errors::VersionScriptWriteFailure { error });
Diff in /checkout/compiler/rustc_codegen_ssa/src/base.rs:890:
         let crate_types = tcx.crate_types().to_vec();
         let exported_symbols = crate_types
             .iter()
-            .map(|&c| (c, if tcx.sess.opts.unstable_opts.cstyle_export_rules {
-                crate::back::linker::exported_symbols_cstyle(tcx, c)
-            } else {
-                crate::back::linker::exported_symbols(tcx, c)
-            }))
+            .map(|&c| {
+                (
+                    c,
+                    if tcx.sess.opts.unstable_opts.cstyle_export_rules {
+                        crate::back::linker::exported_symbols_cstyle(tcx, c)
+                    } else {
+                        crate::back::linker::exported_symbols(tcx, c)
+                    },
+                )
+            })
             .collect();
         let linked_symbols =
             crate_types.iter().map(|&c| (c, crate::back::linker::linked_symbols(tcx, c))).collect();
fmt: checked 6611 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:57
  local time: Wed Dec 24 08:17:44 UTC 2025
  network time: Wed, 24 Dec 2025 08:17:45 GMT

[bjorn3]

This lists all exported symbols, while you need all non-exported symbols too.

[cezarbbb]

So this is the correct position to iterate through all exported symbols and invert the selection rules of the original exported_symbols?

[bjorn3]

|| is_compiler_builtins

[cezarbbb]

I know that the original criterion for selecting symbols into exported_symbols was info.level.is_below_threshold(export_threshold) && !tcx.is_compiler_builtins(cnum), but here we cannot perform a negation operation, i.e., info.level.is_below_threshold(export_threshold) || tcx.is_compiler_builtins(cnum), as this will result in the following error:

exit status: 1
  |
  = note:  "cc" "-Wl,--version-script=/tmp/rustcenvqxB/list" "-Wl,--no-undefined-version" "-m64" "/tmp/rustcenvqxB/symbols.o" "<2 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/root/cezarbbb/include-libs-test3/downstream-cdylib/target/debug/deps/libupstream-8102da413a3d22f9.rlib" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/tmp/rustcenvqxB/raw-dylibs" "-B<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld" "-fuse-ld=lld" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "/root/cezarbbb/include-libs-test3/downstream-cdylib/target/debug/deps/libdownstream_cdylib.so" "-Wl,--gc-sections" "-shared" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: rust-lld: error: version script assignment of 'local' to symbol '__rustc_proc_macro_decls_908feb11bfb2de80__' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol 'rust_metadata_downstream_cdylib_908feb11bfb2de80' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__addtf3' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__ashlti3' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__ashrti3' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__bswapdi2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__bswapsi2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__clzdi2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__clzti2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__ctzdi2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__ctzti2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__divmodti4' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__divtf3' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__divti3' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__eqtf2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__extenddftf2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__extendhfdf2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__extendhftf2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__extendsfdf2' failed: symbol not defined
          rust-lld: error: version script assignment of 'local' to symbol '__extendsftf2' failed: symbol not defined
          rust-lld: error: too many errors emitted, stopping now (use --error-limit=0 to see all errors) collect2: error: ld returned 1 exit status

It seems linker is not selecting some of certain compilerbuiltin functions, resulting in undefined symbols?

[bjorn3]

Right. All compiler-builtins symbols have hidden visibility, so I guess they shouldn't be listed in the version script at all.

[bjorn3]

For proc-macros we should keep using an allowlist containing just two symbols.

[cezarbbb]

The same situation occurs when adding proc_macro, as can be seen in the error message above(__rustc_proc_macro_decls_908feb11bfb2de80__ and rust_metadata_downstream_cdylib_908feb11bfb2de80).

[bjorn3]

These symbols must be exported from proc-macros. If they aren't, rustc will fail to load the proc-macro. In any case it looks like you put those symbols on the denylist rather than the allowlist.