Always generated object code for #![no_builtins]

src/librustc_codegen_ssa/back/write.rs

@@ -142,8 +142,22 @@ impl ModuleConfig {
         let emit_obj = if !should_emit_obj {
             EmitObj::None
         } else if sess.target.target.options.obj_is_bitcode
-            || sess.opts.cg.linker_plugin_lto.enabled()
+            || (sess.opts.cg.linker_plugin_lto.enabled() && !no_builtins)
         {
+            // This case is selected if the target uses objects as bitcode, or
+            // if linker plugin LTO is enabled. In the linker plugin LTO case
+            // the assumption is that the final link-step will read the bitcode
+            // and convert it to object code. This may be done by either the
+            // native linker or rustc itself.
+            //
+            // Note, however, that the linker-plugin-lto requested here is
+            // explicitly ignored for `#![no_builtins]` crates. These crates are
+            // specifically ignored by rustc's LTO passes and wouldn't work if
+            // loaded into the linker. These crates define symbols that LLVM
+            // lowers intrinsics to, and these symbol dependencies aren't known
+            // until after codegen. As a result any crate marked
+            // `#![no_builtins]` is assumed to not participate in LTO and
+            // instead goes on to generate object code.
             EmitObj::Bitcode
         } else if need_bitcode_in_object(sess) {
             EmitObj::ObjectCode(BitcodeSection::Full)