bootstrap: Don't panic if codegen-backends is set to empty

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

@@ -1332,9 +1332,7 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         cargo.env("CFG_OMIT_GIT_HASH", "1");
     }
 
-    if let Some(backend) = builder.config.default_codegen_backend(target) {
-        cargo.env("CFG_DEFAULT_CODEGEN_BACKEND", backend.name());
-    }
+    cargo.env("CFG_DEFAULT_CODEGEN_BACKEND", builder.config.default_codegen_backend(target).name());
 
     let libdir_relative = builder.config.libdir_relative().unwrap_or_else(|| Path::new("lib"));
     let target_config = builder.config.target_config.get(&target);

src/bootstrap/src/core/build_steps/test.rs

@@ -1850,7 +1850,7 @@ HELP: You can add it into `bootstrap.toml` in `rust.codegen-backends = [{name:?}
             // Tells compiletest which codegen backend to use.
             // It is used to e.g. ignore tests that don't support that codegen backend.
             cmd.arg("--default-codegen-backend")
-                .arg(builder.config.default_codegen_backend(compiler.host).unwrap().name());
+                .arg(builder.config.default_codegen_backend(compiler.host).name());
         }
 
         if builder.build.config.llvm_enzyme {

src/bootstrap/src/core/builder/cargo.rs

@@ -1326,12 +1326,7 @@ impl Builder<'_> {
 
             if let Some(limit) = limit
                 && (build_compiler_stage == 0
-                    || self
-                        .config
-                        .default_codegen_backend(target)
-                        .cloned()
-                        .unwrap_or_default()
-                        .is_llvm())
+                    || self.config.default_codegen_backend(target).is_llvm())
             {
                 rustflags.arg(&format!("-Cllvm-args=-import-instr-limit={limit}"));
             }

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

@@ -1663,8 +1663,9 @@ impl Config {
 
     /// Returns the codegen backend that should be configured as the *default* codegen backend
     /// for a rustc compiled by bootstrap.
-    pub fn default_codegen_backend(&self, target: TargetSelection) -> Option<&CodegenBackendKind> {
-        self.enabled_codegen_backends(target).first()
+    pub fn default_codegen_backend(&self, target: TargetSelection) -> &CodegenBackendKind {
+        // We're guaranteed to have always at least one codegen backend listed.
+        self.enabled_codegen_backends(target).first().unwrap()
     }
 
     pub fn jemalloc(&self, target: TargetSelection) -> bool {

src/bootstrap/src/core/config/toml/rust.rs

@@ -415,6 +415,10 @@ pub(crate) fn parse_codegen_backends(
         };
         found_backends.push(backend);
     }
+    if found_backends.is_empty() {
+        eprintln!("ERROR: `{section}.codegen-backends` should not be set to `[]`");
+        exit!(1);
+    }
     found_backends
 }