Move MetadataLoader{,Dyn} to rustc_metadata.

They're not used in `rustc_session`, and `rustc_metadata` is a more
obvious location.

`MetadataLoader` was originally put into `rustc_session` in #41565 to
avoid a dependency on LLVM, but things have changed a lot since then and
that's no longer relevant, e.g. `rustc_codegen_llvm` depends on
`rustc_metadata`.

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -12,9 +12,9 @@ use object::{
 
 use rustc_data_structures::memmap::Mmap;
 use rustc_data_structures::owned_slice::{try_slice_owned, OwnedSlice};
+use rustc_metadata::creader::MetadataLoader;
 use rustc_metadata::fs::METADATA_FILENAME;
 use rustc_metadata::EncodedMetadata;
-use rustc_session::cstore::MetadataLoader;
 use rustc_session::Session;
 use rustc_span::sym;
 use rustc_target::abi::Endian;

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -9,6 +9,7 @@ use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::{DynSend, DynSync};
 use rustc_errors::ErrorGuaranteed;
+use rustc_metadata::creader::MetadataLoaderDyn;
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt, LayoutOf, TyAndLayout};
@@ -16,7 +17,6 @@ use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_middle::util::Providers;
 use rustc_session::{
     config::{self, OutputFilenames, PrintRequest},
-    cstore::MetadataLoaderDyn,
     Session,
 };
 use rustc_span::symbol::Symbol;

compiler/rustc_driver_impl/src/lib.rs

@@ -33,10 +33,10 @@ use rustc_feature::find_gated_cfg;
 use rustc_interface::util::{self, collect_crate_types, get_codegen_backend};
 use rustc_interface::{interface, Queries};
 use rustc_lint::unerased_lint_store;
+use rustc_metadata::creader::MetadataLoader;
 use rustc_metadata::locator;
 use rustc_session::config::{nightly_options, CG_OPTIONS, Z_OPTIONS};
 use rustc_session::config::{ErrorOutputType, Input, OutFileName, OutputType, TrimmedDefPaths};
-use rustc_session::cstore::MetadataLoader;
 use rustc_session::getopts::{self, Matches};
 use rustc_session::lint::{Lint, LintId};
 use rustc_session::{config, EarlyErrorHandler, Session};

compiler/rustc_metadata/src/creader.rs

@@ -7,8 +7,9 @@ use crate::rmeta::{CrateDep, CrateMetadata, CrateNumMap, CrateRoot, MetadataBlob
 use rustc_ast::expand::allocator::{alloc_error_handler_name, global_fn_name, AllocatorKind};
 use rustc_ast::{self as ast, *};
 use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::owned_slice::OwnedSlice;
 use rustc_data_structures::svh::Svh;
-use rustc_data_structures::sync::{FreezeReadGuard, FreezeWriteGuard};
+use rustc_data_structures::sync::{self, FreezeReadGuard, FreezeWriteGuard};
 use rustc_expand::base::SyntaxExtension;
 use rustc_fs_util::try_canonicalize;
 use rustc_hir::def_id::{CrateNum, LocalDefId, StableCrateId, StableCrateIdMap, LOCAL_CRATE};
@@ -16,16 +17,14 @@ use rustc_hir::definitions::Definitions;
 use rustc_index::IndexVec;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{self, CrateType, ExternLocation};
-use rustc_session::cstore::{
-    CrateDepKind, CrateSource, ExternCrate, ExternCrateSource, MetadataLoaderDyn,
-};
+use rustc_session::cstore::{CrateDepKind, CrateSource, ExternCrate, ExternCrateSource};
 use rustc_session::lint;
 use rustc_session::output::validate_crate_name;
 use rustc_session::search_paths::PathKind;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
-use rustc_target::spec::{PanicStrategy, TargetTriple};
+use rustc_target::spec::{PanicStrategy, Target, TargetTriple};
 
 use proc_macro::bridge::client::ProcMacro;
 use std::error::Error;
@@ -34,6 +33,17 @@ use std::path::Path;
 use std::time::Duration;
 use std::{cmp, iter};
 
+/// The backend's way to give the crate store access to the metadata in a library.
+/// Note that it returns the raw metadata bytes stored in the library file, whether
+/// it is compressed, uncompressed, some weird mix, etc.
+/// rmeta files are backend independent and not handled here.
+pub trait MetadataLoader {
+    fn get_rlib_metadata(&self, target: &Target, filename: &Path) -> Result<OwnedSlice, String>;
+    fn get_dylib_metadata(&self, target: &Target, filename: &Path) -> Result<OwnedSlice, String>;
+}
+
+pub type MetadataLoaderDyn = dyn MetadataLoader + Send + Sync + sync::DynSend + sync::DynSync;
+
 pub struct CStore {
     metadata_loader: Box<MetadataLoaderDyn>,
 

compiler/rustc_metadata/src/locator.rs

@@ -212,7 +212,7 @@
 //! no means all of the necessary details. Take a look at the rest of
 //! metadata::locator or metadata::creader for all the juicy details!
 
-use crate::creader::Library;
+use crate::creader::{Library, MetadataLoader};
 use crate::errors;
 use crate::rmeta::{rustc_version, MetadataBlob, METADATA_HEADER};
 
@@ -223,7 +223,7 @@ use rustc_data_structures::svh::Svh;
 use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_fs_util::try_canonicalize;
 use rustc_session::config;
-use rustc_session::cstore::{CrateSource, MetadataLoader};
+use rustc_session::cstore::CrateSource;
 use rustc_session::filesearch::FileSearch;
 use rustc_session::search_paths::PathKind;
 use rustc_session::utils::CanonicalizedPath;

compiler/rustc_session/src/cstore.rs

@@ -6,7 +6,6 @@ use crate::search_paths::PathKind;
 use crate::utils::NativeLibKind;
 use crate::Session;
 use rustc_ast as ast;
-use rustc_data_structures::owned_slice::OwnedSlice;
 use rustc_data_structures::sync::{self, AppendOnlyIndexVec, FreezeLock};
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, StableCrateId, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash, Definitions};
@@ -14,10 +13,9 @@ use rustc_span::hygiene::{ExpnHash, ExpnId};
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use rustc_target::spec::abi::Abi;
-use rustc_target::spec::Target;
 
 use std::any::Any;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 // lonely orphan structs and enums looking for a better home
 
@@ -197,21 +195,6 @@ pub enum ExternCrateSource {
     Path,
 }
 
-/// The backend's way to give the crate store access to the metadata in a library.
-/// Note that it returns the raw metadata bytes stored in the library file, whether
-/// it is compressed, uncompressed, some weird mix, etc.
-/// rmeta files are backend independent and not handled here.
-///
-/// At the time of this writing, there is only one backend and one way to store
-/// metadata in library -- this trait just serves to decouple rustc_metadata from
-/// the archive reader, which depends on LLVM.
-pub trait MetadataLoader: std::fmt::Debug {
-    fn get_rlib_metadata(&self, target: &Target, filename: &Path) -> Result<OwnedSlice, String>;
-    fn get_dylib_metadata(&self, target: &Target, filename: &Path) -> Result<OwnedSlice, String>;
-}
-
-pub type MetadataLoaderDyn = dyn MetadataLoader + Send + Sync + sync::DynSend + sync::DynSync;
-
 /// A store of Rust crates, through which their metadata can be accessed.
 ///
 /// Note that this trait should probably not be expanding today. All new