Don't require dlltool with the dummy backend on MinGW

The dummy backend should be able to cross-compile to any target without
requiring any external tool or library other than the rust standard
library.

Author: bjorn3

compiler/rustc_interface/src/util.rs

@@ -7,7 +7,7 @@ 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::archive::{ArArchiveBuilderBuilder, ArchiveBuilderBuilder};
 use rustc_codegen_ssa::back::link::link_binary;
 use rustc_codegen_ssa::target_features::{self, cfg_target_feature};
 use rustc_codegen_ssa::traits::CodegenBackend;
@@ -440,7 +440,7 @@ impl CodegenBackend for DummyCodegenBackend {
 
         link_binary(
             sess,
-            &ArArchiveBuilderBuilder,
+            &DummyArchiveBuilderBuilder,
             codegen_results,
             metadata,
             outputs,
@@ -449,6 +449,28 @@ impl CodegenBackend for DummyCodegenBackend {
     }
 }
 
+struct DummyArchiveBuilderBuilder;
+
+impl ArchiveBuilderBuilder for DummyArchiveBuilderBuilder {
+    fn new_archive_builder<'a>(
+        &self,
+        sess: &'a Session,
+    ) -> Box<dyn rustc_codegen_ssa::back::archive::ArchiveBuilder + 'a> {
+        ArArchiveBuilderBuilder.new_archive_builder(sess)
+    }
+
+    fn create_dll_import_lib(
+        &self,
+        sess: &Session,
+        _lib_name: &str,
+        _items: Vec<rustc_codegen_ssa::back::archive::ImportLibraryItem>,
+        output_path: &Path,
+    ) {
+        // Build an empty static library to avoid calling an external dlltool on mingw
+        ArArchiveBuilderBuilder.new_archive_builder(sess).build(output_path);
+    }
+}
+
 // 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.