Auto merge of #83640 - bjorn3:shared_metadata_reader, r=nagisa

Use the object crate for metadata reading

This allows sharing the metadata reader between cg_llvm, cg_clif and other codegen backends.

This is not currently useful for rlib reading with cg_spirv ([rust-gpu](https://github.com/EmbarkStudios/rust-gpu/)) as it uses tar rather than ar as .rlib format, but it is useful for dylib reading required for loading proc macros. (cc `@eddyb)`

The object crate is already trusted as dependency of libstd through backtrace. As far as I know it supports reading all object file formats used by targets for which we support rust dylibs with crate metadata, but I am not certain. If this happens to not be the case, I could keep using LLVM for reading dylib metadata.

Marked as WIP for a perf run and as it is based on #83637.

Author: bors

src/lib.rs

@@ -165,7 +165,7 @@ impl CodegenBackend for CraneliftCodegenBackend {
     }
 
     fn metadata_loader(&self) -> Box<dyn MetadataLoader + Sync> {
-        Box::new(crate::metadata::CraneliftMetadataLoader)
+        Box::new(rustc_codegen_ssa::back::metadata::DefaultMetadataLoader)
     }
 
     fn provide(&self, _providers: &mut Providers) {}

src/metadata.rs

@@ -1,73 +1,9 @@
-//! Reading and writing of the rustc metadata for rlibs and dylibs
+//! Writing of the rustc metadata for dylibs
 
-use std::fs::File;
-use std::path::Path;
-
-use rustc_codegen_ssa::METADATA_FILENAME;
-use rustc_data_structures::memmap::Mmap;
-use rustc_data_structures::owning_ref::OwningRef;
-use rustc_data_structures::rustc_erase_owner;
-use rustc_data_structures::sync::MetadataRef;
-use rustc_middle::middle::cstore::MetadataLoader;
 use rustc_middle::ty::TyCtxt;
-use rustc_target::spec::Target;
 
 use crate::backend::WriteMetadata;
 
-/// The metadata loader used by cg_clif.
-///
-/// The metadata is stored in the same format as cg_llvm.
-///
-/// # Metadata location
-///
-/// <dl>
-/// <dt>rlib</dt>
-/// <dd>The metadata can be found in the `lib.rmeta` file inside of the ar archive.</dd>
-/// <dt>dylib</dt>
-/// <dd>The metadata can be found in the `.rustc` section of the shared library.</dd>
-/// </dl>
-pub(crate) struct CraneliftMetadataLoader;
-
-fn load_metadata_with(
-    path: &Path,
-    f: impl for<'a> FnOnce(&'a [u8]) -> Result<&'a [u8], String>,
-) -> Result<MetadataRef, String> {
-    let file = File::open(path).map_err(|e| format!("{:?}", e))?;
-    let data = unsafe { Mmap::map(file) }.map_err(|e| format!("{:?}", e))?;
-    let metadata = OwningRef::new(data).try_map(f)?;
-    return Ok(rustc_erase_owner!(metadata.map_owner_box()));
-}
-
-impl MetadataLoader for CraneliftMetadataLoader {
-    fn get_rlib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
-        load_metadata_with(path, |data| {
-            let archive = object::read::archive::ArchiveFile::parse(&*data)
-                .map_err(|e| format!("{:?}", e))?;
-
-            for entry_result in archive.members() {
-                let entry = entry_result.map_err(|e| format!("{:?}", e))?;
-                if entry.name() == METADATA_FILENAME.as_bytes() {
-                    return Ok(entry.data());
-                }
-            }
-
-            Err("couldn't find metadata entry".to_string())
-        })
-    }
-
-    fn get_dylib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
-        use object::{Object, ObjectSection};
-
-        load_metadata_with(path, |data| {
-            let file = object::File::parse(&data).map_err(|e| format!("parse: {:?}", e))?;
-            file.section_by_name(".rustc")
-                .ok_or("no .rustc section")?
-                .data()
-                .map_err(|e| format!("failed to read .rustc section: {:?}", e))
-        })
-    }
-}
-
 // Adapted from https://github.com/rust-lang/rust/blob/da573206f87b5510de4b0ee1a9c044127e409bd3/src/librustc_codegen_llvm/base.rs#L47-L112
 pub(crate) fn write_metadata<O: WriteMetadata>(tcx: TyCtxt<'_>, object: &mut O) {
     use snap::write::FrameEncoder;