Don't attempt to export compiler-builtins symbols from rust dylibs

They are marked with hidden visibility to prevent them from getting
exported, so we shouldn't ask the linker to export them anyway. The only
thing that does it cause a warning on macOS.

Author: bjorn3

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1782,7 +1782,10 @@ fn exported_symbols_for_non_proc_macro(tcx: TyCtxt<'_>, crate_type: CrateType) -
     let mut symbols = Vec::new();
     let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
     for_each_exported_symbols_include_dep(tcx, crate_type, |symbol, info, cnum| {
-        if info.level.is_below_threshold(export_threshold) {
+        // Do not export mangled symbols from cdylibs and don't attempt to export compiler-builtins
+        // from any cdylib. The latter doesn't work anyway as we use hidden visibility for
+        // compiler-builtins. Most linkers silently ignore it, but ld64 gives a warning.
+        if info.level.is_below_threshold(export_threshold) && !tcx.is_compiler_builtins(cnum) {
             symbols.push(symbol_export::exporting_symbol_name_for_instance_in_crate(
                 tcx, symbol, cnum,
             ));