Rollup merge of #148253 - bjorn3:dummy_backend_target_features, r=JonathanBrouwer

Handle default features and -Ctarget-features in the dummy backend

This prevents a warning about ABI relevant target features not being set on x86 and arm. In addition it is required for miri to report correct features in is_*_feature_detected!() if miri switches to the dummy backend.

Required for https://github.com/rust-lang/miri/pull/4648

compiler/rustc_interface/src/util.rs

@@ -9,8 +9,9 @@ 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::target_features::{self, cfg_target_feature};
 use rustc_codegen_ssa::traits::CodegenBackend;
-use rustc_codegen_ssa::{CodegenResults, CrateInfo};
+use rustc_codegen_ssa::{CodegenResults, CrateInfo, TargetConfig};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::jobserver::Proxy;
 use rustc_data_structures::sync;
@@ -354,6 +355,33 @@ impl CodegenBackend for DummyCodegenBackend {
         "dummy"
     }
 
+    fn target_config(&self, sess: &Session) -> TargetConfig {
+        let (target_features, unstable_target_features) = cfg_target_feature(sess, |feature| {
+            // This is a standin for the list of features a backend is expected to enable.
+            // It would be better to parse target.features instead and handle implied features,
+            // but target.features is a list of LLVM target features, not Rust target features.
+            // The dummy backend doesn't know the mapping between LLVM and Rust target features.
+            sess.target.abi_required_features().required.contains(&feature)
+        });
+
+        // To report warnings about unknown features
+        target_features::flag_to_backend_features::<0>(
+            sess,
+            true,
+            |_| Default::default(),
+            |_, _| {},
+        );
+
+        TargetConfig {
+            target_features,
+            unstable_target_features,
+            has_reliable_f16: true,
+            has_reliable_f16_math: true,
+            has_reliable_f128: true,
+            has_reliable_f128_math: true,
+        }
+    }
+
     fn supported_crate_types(&self, _sess: &Session) -> Vec<CrateType> {
         // This includes bin despite failing on the link step to ensure that you
         // can still get the frontend handling for binaries. For all library