Add option to pass a custom codegen backend from a driver

compiler/rustc_driver/src/lib.rs

@@ -141,6 +141,9 @@ pub fn run_compiler(
     callbacks: &mut (dyn Callbacks + Send),
     file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
     emitter: Option<Box<dyn Write + Send>>,
+    make_codegen_backend: Option<
+        Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>,
+    >,
 ) -> interface::Result<()> {
     let mut args = Vec::new();
     for arg in at_args {
@@ -162,6 +165,11 @@ pub fn run_compiler(
     let sopts = config::build_session_options(&matches);
     let cfg = interface::parse_cfgspecs(matches.opt_strs("cfg"));
 
+    // We wrap `make_codegen_backend` in another `Option` such that `dummy_config` can take
+    // ownership of it when necessary, while also allowing the non-dummy config to take ownership
+    // when `dummy_config` is not used.
+    let mut make_codegen_backend = Some(make_codegen_backend);
+
     let mut dummy_config = |sopts, cfg, diagnostic_output| {
         let mut config = interface::Config {
             opts: sopts,
@@ -177,6 +185,7 @@ pub fn run_compiler(
             lint_caps: Default::default(),
             register_lints: None,
             override_queries: None,
+            make_codegen_backend: make_codegen_backend.take().unwrap(),
             registry: diagnostics_registry(),
         };
         callbacks.config(&mut config);
@@ -253,6 +262,7 @@ pub fn run_compiler(
         lint_caps: Default::default(),
         register_lints: None,
         override_queries: None,
+        make_codegen_backend: make_codegen_backend.unwrap(),
         registry: diagnostics_registry(),
     };
 
@@ -1265,7 +1275,7 @@ pub fn main() -> ! {
                 })
             })
             .collect::<Vec<_>>();
-        run_compiler(&args, &mut callbacks, None, None)
+        run_compiler(&args, &mut callbacks, None, None, None)
     });
     // The extra `\t` is necessary to align this label with the others.
     print_time_passes_entry(callbacks.time_passes, "\ttotal", start.elapsed());

compiler/rustc_interface/src/interface.rs

@@ -154,6 +154,10 @@ pub struct Config {
     pub override_queries:
         Option<fn(&Session, &mut ty::query::Providers, &mut ty::query::Providers)>,
 
+    /// This is a callback from the driver that is called to create a codegen backend.
+    pub make_codegen_backend:
+        Option<Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>>,
+
     /// Registry of diagnostics codes.
     pub registry: Registry,
 }
@@ -167,6 +171,7 @@ pub fn create_compiler_and_run<R>(config: Config, f: impl FnOnce(&Compiler) -> R
         config.file_loader,
         config.input_path.clone(),
         config.lint_caps,
+        config.make_codegen_backend,
         registry.clone(),
     );
 

compiler/rustc_interface/src/util.rs

@@ -63,9 +63,17 @@ pub fn create_session(
     file_loader: Option<Box<dyn FileLoader + Send + Sync + 'static>>,
     input_path: Option<PathBuf>,
     lint_caps: FxHashMap<lint::LintId, lint::Level>,
+    make_codegen_backend: Option<
+        Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>,
+    >,
     descriptions: Registry,
 ) -> (Lrc<Session>, Lrc<Box<dyn CodegenBackend>>) {
-    let codegen_backend = get_codegen_backend(&sopts);
+    let codegen_backend = if let Some(make_codegen_backend) = make_codegen_backend {
+        make_codegen_backend(&sopts)
+    } else {
+        get_codegen_backend(&sopts)
+    };
+
     // target_override is documented to be called before init(), so this is okay
     let target_override = codegen_backend.target_override(&sopts);