Use protected symbols when building rustc_driver

Author: davidlattimore

src/bootstrap/src/core/build_steps/compile.rs

@@ -1057,6 +1057,10 @@ pub fn rustc_cargo(
         cargo.rustflag("-l").rustflag("Enzyme-19");
     }
 
+    if builder.build.config.protected_symbol_definitions {
+        cargo.rustflag("-Zdefault-visibility=protected");
+    }
+
     // We currently don't support cross-crate LTO in stage0. This also isn't hugely necessary
     // and may just be a time sink.
     if compiler.stage != 0 {

src/bootstrap/src/core/config/config.rs

@@ -295,6 +295,7 @@ pub struct Config {
     pub llvm_profile_generate: bool,
     pub llvm_libunwind_default: Option<LlvmLibunwind>,
     pub enable_bolt_settings: bool,
+    pub protected_symbol_definitions: bool,
 
     pub reproducible_artifacts: Vec<String>,
 
@@ -1308,6 +1309,7 @@ impl Config {
         config.llvm_profile_generate = flags.llvm_profile_generate;
         config.enable_bolt_settings = flags.enable_bolt_settings;
         config.bypass_bootstrap_lock = flags.bypass_bootstrap_lock;
+        config.protected_symbol_definitions = flags.protected_symbol_definitions;
 
         // Infer the rest of the configuration.
 

src/bootstrap/src/core/config/flags.rs

@@ -161,6 +161,9 @@ pub struct Flags {
     /// Enable BOLT link flags
     #[arg(global = true, long)]
     pub enable_bolt_settings: bool,
+    /// Use protected symbol definitions in rustc_driver
+    #[arg(global = true, long, default_value_t = true)]
+    pub protected_symbol_definitions: bool,
     /// Skip stage0 compiler validation
     #[arg(global = true, long)]
     pub skip_stage0_validation: bool,