Use the first codegen backend in the config.toml as default

compiler/rustc_interface/src/util.rs

@@ -236,13 +236,9 @@ pub fn get_codegen_backend(
     static LOAD: SyncOnceCell<unsafe fn() -> Box<dyn CodegenBackend>> = SyncOnceCell::new();
 
     let load = LOAD.get_or_init(|| {
-        #[cfg(feature = "llvm")]
-        const DEFAULT_CODEGEN_BACKEND: &str = "llvm";
+        let default_codegen_backend = option_env!("CFG_DEFAULT_CODEGEN_BACKEND").unwrap_or("llvm");
 
-        #[cfg(not(feature = "llvm"))]
-        const DEFAULT_CODEGEN_BACKEND: &str = "cranelift";
-
-        match backend_name.unwrap_or(DEFAULT_CODEGEN_BACKEND) {
+        match backend_name.unwrap_or(default_codegen_backend) {
             filename if filename.contains('.') => load_backend_from_dylib(filename.as_ref()),
             #[cfg(feature = "llvm")]
             "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,

config.toml.example

@@ -551,7 +551,9 @@ changelog-seen = 2
 
 # This is an array of the codegen backends that will be compiled for the rustc
 # that's being compiled. The default is to only build the LLVM codegen backend,
-# and currently the only standard options supported are `"llvm"` and `"cranelift"`.
+# and currently the only standard options supported are `"llvm"`, `"cranelift"`
+# and `"gcc"`. The first backend in this list will be used as default by rustc
+# when no explicit backend is specified.
 #codegen-backends = ["llvm"]
 
 # Indicates whether LLD will be compiled and made available in the sysroot for

src/bootstrap/compile.rs

@@ -662,6 +662,10 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         .env("CFG_RELEASE_CHANNEL", &builder.config.channel)
         .env("CFG_VERSION", builder.rust_version());
 
+    if let Some(backend) = builder.config.rust_codegen_backends.get(0) {
+        cargo.env("CFG_DEFAULT_CODEGEN_BACKEND", backend);
+    }
+
     let libdir_relative = builder.config.libdir_relative().unwrap_or_else(|| Path::new("lib"));
     let target_config = builder.config.target_config.get(&target);