Use protected symbols when building rustc_driver

Author: davidlattimore

config.example.toml

@@ -780,6 +780,11 @@
 #  - If building for a zkvm target, "compiler-builtins-mem" will be added.
 #std-features = ["panic_unwind"]
 
+# Whether to use protected symbols when compiling rustc. Doing so produces a binary that starts
+# faster. Setting this to true when linking with GNU ld < 2.40 will likely result in link errors.
+# Defaults to true when using lld to link rustc.
+#use-protected-symbols = true
+
 # =============================================================================
 # Options for specific targets
 #

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.rust_use_protected_symbols {
+        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

@@ -291,6 +291,7 @@ pub struct Config {
     pub rust_lto: RustcLto,
     pub rust_validate_mir_opts: Option<u32>,
     pub rust_std_features: BTreeSet<String>,
+    pub rust_use_protected_symbols: bool,
     pub llvm_profile_use: Option<String>,
     pub llvm_profile_generate: bool,
     pub llvm_libunwind_default: Option<LlvmLibunwind>,
@@ -1166,6 +1167,7 @@ define_config! {
         lto: Option<String> = "lto",
         validate_mir_opts: Option<u32> = "validate-mir-opts",
         std_features: Option<BTreeSet<String>> = "std-features",
+        use_protected_symbols: Option<bool> = "use-protected-symbols",
     }
 }
 
@@ -1223,6 +1225,7 @@ impl Config {
             dist_include_mingw_linker: true,
             dist_compression_profile: "fast".into(),
             rustc_parallel: true,
+            rust_use_protected_symbols: false,
 
             stdout_is_tty: std::io::stdout().is_terminal(),
             stderr_is_tty: std::io::stderr().is_terminal(),
@@ -1725,6 +1728,7 @@ impl Config {
                 strip,
                 lld_mode,
                 std_features: std_features_toml,
+                use_protected_symbols,
             } = rust;
 
             is_user_configured_rust_channel = channel.is_some();
@@ -1769,6 +1773,12 @@ impl Config {
             set(&mut config.lld_mode, lld_mode);
             set(&mut config.llvm_bitcode_linker_enabled, llvm_bitcode_linker);
 
+            // Default to using protected symbols only when linking with LLD.
+            if config.lld_mode != LldMode::Unused {
+                config.rust_use_protected_symbols = true;
+            }
+            set(&mut config.rust_use_protected_symbols, use_protected_symbols);
+
             config.rust_randomize_layout = randomize_layout.unwrap_or_default();
             config.llvm_tools_enabled = llvm_tools.unwrap_or(true);
             config.rustc_parallel =
@@ -3102,6 +3112,7 @@ fn check_incompatible_options_for_ci_rustc(
         download_rustc: _,
         validate_mir_opts: _,
         frame_pointers: _,
+        use_protected_symbols: _,
     } = ci_rust_config;
 
     // There are two kinds of checks for CI rustc incompatible options: