Make new symbol mangling scheme default for compiler itself.

config.toml.example

@@ -603,7 +603,11 @@ changelog-seen = 2
 
 # Enable symbol-mangling-version v0. This can be helpful when profiling rustc,
 # as generics will be preserved in symbols (rather than erased into opaque T).
-#new-symbol-mangling = false
+# When no setting is given, the new scheme will be used when compiling the
+# compiler and its tools and the legacy scheme will be used when compiling the
+# standard library.
+# If an explicit setting is given, it will be used for all parts of the codebase.
+#new-symbol-mangling = true|false (see comment)
 
 # =============================================================================
 # Options for specific targets

src/bootstrap/builder.rs

@@ -972,8 +972,26 @@ impl<'a> Builder<'a> {
             }
         }
 
-        if self.config.rust_new_symbol_mangling {
+        let use_new_symbol_mangling = match self.config.rust_new_symbol_mangling {
+            Some(setting) => {
+                // If an explicit setting is given, use that
+                setting
+            }
+            None => {
+                if mode == Mode::Std {
+                    // The standard library defaults to the legacy scheme
+                    false
+                } else {
+                    // The compiler and tools default to the new scheme
+                    true
+                }
+            }
+        };
+
+        if use_new_symbol_mangling {
             rustflags.arg("-Zsymbol-mangling-version=v0");
+        } else {
+            rustflags.arg("-Zsymbol-mangling-version=legacy");
         }
 
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,

src/bootstrap/config.rs

@@ -140,7 +140,7 @@ pub struct Config {
     pub rust_verify_llvm_ir: bool,
     pub rust_thin_lto_import_instr_limit: Option<u32>,
     pub rust_remap_debuginfo: bool,
-    pub rust_new_symbol_mangling: bool,
+    pub rust_new_symbol_mangling: Option<bool>,
     pub rust_profile_use: Option<String>,
     pub rust_profile_generate: Option<String>,
     pub llvm_profile_use: Option<String>,
@@ -870,7 +870,7 @@ impl Config {
             config.rust_run_dsymutil = rust.run_dsymutil.unwrap_or(false);
             optimize = rust.optimize;
             ignore_git = rust.ignore_git;
-            set(&mut config.rust_new_symbol_mangling, rust.new_symbol_mangling);
+            config.rust_new_symbol_mangling = rust.new_symbol_mangling;
             set(&mut config.rust_optimize_tests, rust.optimize_tests);
             set(&mut config.codegen_tests, rust.codegen_tests);
             set(&mut config.rust_rpath, rust.rpath);