Auto merge of #56566 - ljedrz:fix_clippy_errors, r=cramertj

codegen_utils, driver: fix clippy errors

- remove a superfluous conversion
- remove an explicit `return`
- rename `MetadataOnlyCodegenBackend::new` to `::boxed`
- single-arm `match` > `if let`

src/librustc_codegen_utils/codegen_backend.rs

@@ -86,8 +86,8 @@ impl MetadataLoader for NoLlvmMetadataLoader {
 
         let mut buf = Vec::new();
         io::copy(&mut file, &mut buf).unwrap();
-        let buf: OwningRef<Vec<u8>, [u8]> = OwningRef::new(buf).into();
-        return Ok(rustc_erase_owner!(buf.map_owner_box()));
+        let buf: OwningRef<Vec<u8>, [u8]> = OwningRef::new(buf);
+        Ok(rustc_erase_owner!(buf.map_owner_box()))
     }
 
     fn get_dylib_metadata(&self, target: &Target, filename: &Path) -> Result<MetadataRef, String> {
@@ -103,7 +103,7 @@ pub struct OngoingCodegen {
 }
 
 impl MetadataOnlyCodegenBackend {
-    pub fn new() -> Box<dyn CodegenBackend> {
+    pub fn boxed() -> Box<dyn CodegenBackend> {
         box MetadataOnlyCodegenBackend(())
     }
 }
@@ -165,15 +165,12 @@ impl CodegenBackend for MetadataOnlyCodegenBackend {
                 tcx,
                 collector::MonoItemCollectionMode::Eager
             ).0 {
-            match mono_item {
-                MonoItem::Fn(inst) => {
-                    let def_id = inst.def_id();
-                    if def_id.is_local()  {
-                        let _ = inst.def.is_inline(tcx);
-                        let _ = tcx.codegen_fn_attrs(def_id);
-                    }
+            if let MonoItem::Fn(inst) = mono_item {
+                let def_id = inst.def_id();
+                if def_id.is_local()  {
+                    let _ = inst.def.is_inline(tcx);
+                    let _ = tcx.codegen_fn_attrs(def_id);
                 }
-                _ => {}
             }
         }
         tcx.sess.abort_if_errors();
@@ -181,7 +178,7 @@ impl CodegenBackend for MetadataOnlyCodegenBackend {
         let metadata = tcx.encode_metadata();
 
         box OngoingCodegen {
-            metadata: metadata,
+            metadata,
             metadata_version: tcx.metadata_encoding_version().to_vec(),
             crate_name: tcx.crate_name(LOCAL_CRATE),
         }

src/librustc_driver/lib.rs

@@ -258,7 +258,7 @@ pub fn get_codegen_backend(sess: &Session) -> Box<dyn CodegenBackend> {
             .unwrap_or(&sess.target.target.options.codegen_backend);
         let backend = match &codegen_name[..] {
             "metadata_only" => {
-                rustc_codegen_utils::codegen_backend::MetadataOnlyCodegenBackend::new
+                rustc_codegen_utils::codegen_backend::MetadataOnlyCodegenBackend::boxed
             }
             filename if filename.contains(".") => {
                 load_backend_from_dylib(filename.as_ref())
@@ -291,7 +291,7 @@ fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend> {
     // let's just return a dummy creation function which won't be used in
     // general anyway.
     if cfg!(test) {
-        return rustc_codegen_utils::codegen_backend::MetadataOnlyCodegenBackend::new
+        return rustc_codegen_utils::codegen_backend::MetadataOnlyCodegenBackend::boxed
     }
 
     let target = session::config::host_triple();

src/test/run-make-fulldeps/hotplug_codegen_backend/the_backend.rs

@@ -78,5 +78,5 @@ impl CodegenBackend for TheBackend {
 /// This is the entrypoint for a hot plugged rustc_codegen_llvm
 #[no_mangle]
 pub fn __rustc_codegen_backend() -> Box<CodegenBackend> {
-    Box::new(TheBackend(MetadataOnlyCodegenBackend::new()))
+    Box::new(TheBackend(MetadataOnlyCodegenBackend::boxed()))
 }