Rollup merge of #146596 - bjorn3:dummy_backend, r=SparrowLii

Add a dummy codegen backend

This allows building a rustc capable of running the frontend without any backend present. While this may not seem all that useful, it allows running the frontend of rustc to report errors or running miri to interpret a program without any backend present. This is useful when you are trying to say run miri in the browser as upstream LLVM can't be compiled for wasm yet. Or to run rustc itself in miri like I did a while ago and caught some UB.

1 files changed, 70 insertions, 4 deletions

diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index 58ec72b5b45..be2fd0787b9 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -1,3 +1,4 @@
+use std::any::Any;
 use std::env::consts::{DLL_PREFIX, DLL_SUFFIX};
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -6,13 +7,20 @@ use std::{env, thread};
 
 use rustc_ast as ast;
 use rustc_attr_parsing::{ShouldEmit, validate_attr};
+use rustc_codegen_ssa::back::archive::ArArchiveBuilderBuilder;
+use rustc_codegen_ssa::back::link::link_binary;
 use rustc_codegen_ssa::traits::CodegenBackend;
+use rustc_codegen_ssa::{CodegenResults, CrateInfo};
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::jobserver::Proxy;
 use rustc_data_structures::sync;
 use rustc_errors::LintBuffer;
-use rustc_metadata::{DylibError, load_symbol_from_dylib};
-use rustc_middle::ty::CurrentGcx;
-use rustc_session::config::{Cfg, OutFileName, OutputFilenames, OutputTypes, Sysroot, host_tuple};
+use rustc_metadata::{DylibError, EncodedMetadata, load_symbol_from_dylib};
+use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
+use rustc_middle::ty::{CurrentGcx, TyCtxt};
+use rustc_session::config::{
+    Cfg, CrateType, OutFileName, OutputFilenames, OutputTypes, Sysroot, host_tuple,
+};
 use rustc_session::output::{CRATE_TYPES, categorize_crate_type};
 use rustc_session::{EarlyDiagCtxt, Session, filesearch, lint};
 use rustc_span::edit_distance::find_best_match_for_name;
@@ -316,12 +324,13 @@ pub fn get_codegen_backend(
         let backend = backend_name
             .or(target.default_codegen_backend.as_deref())
             .or(option_env!("CFG_DEFAULT_CODEGEN_BACKEND"))
-            .unwrap_or("llvm");
+            .unwrap_or("dummy");
 
         match backend {
             filename if filename.contains('.') => {
                 load_backend_from_dylib(early_dcx, filename.as_ref())
             }
+            "dummy" => || Box::new(DummyCodegenBackend),
             #[cfg(feature = "llvm")]
             "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,
             backend_name => get_codegen_sysroot(early_dcx, sysroot, backend_name),
@@ -334,6 +343,63 @@ pub fn get_codegen_backend(
     unsafe { load() }
 }
 
+struct DummyCodegenBackend;
+
+impl CodegenBackend for DummyCodegenBackend {
+    fn locale_resource(&self) -> &'static str {
+        ""
+    }
+
+    fn name(&self) -> &'static str {
+        "dummy"
+    }
+
+    fn codegen_crate<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Box<dyn Any> {
+        Box::new(CodegenResults {
+            modules: vec![],
+            allocator_module: None,
+            crate_info: CrateInfo::new(tcx, String::new()),
+        })
+    }
+
+    fn join_codegen(
+        &self,
+        ongoing_codegen: Box<dyn Any>,
+        _sess: &Session,
+        _outputs: &OutputFilenames,
+    ) -> (CodegenResults, FxIndexMap<WorkProductId, WorkProduct>) {
+        (*ongoing_codegen.downcast().unwrap(), FxIndexMap::default())
+    }
+
+    fn link(
+        &self,
+        sess: &Session,
+        codegen_results: CodegenResults,
+        metadata: EncodedMetadata,
+        outputs: &OutputFilenames,
+    ) {
+        // JUSTIFICATION: TyCtxt no longer available here
+        #[allow(rustc::bad_opt_access)]
+        if sess.opts.crate_types.iter().any(|&crate_type| crate_type != CrateType::Rlib) {
+            #[allow(rustc::untranslatable_diagnostic)]
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            sess.dcx().fatal(format!(
+                "crate type {} not supported by the dummy codegen backend",
+                sess.opts.crate_types[0],
+            ));
+        }
+
+        link_binary(
+            sess,
+            &ArArchiveBuilderBuilder,
+            codegen_results,
+            metadata,
+            outputs,
+            self.name(),
+        );
+    }
+}
+
 // This is used for rustdoc, but it uses similar machinery to codegen backend
 // loading, so we leave the code here. It is potentially useful for other tools
 // that want to invoke the rustc binary while linking to rustc as well.