Auto merge of #72256 - ecstatic-morse:once-cell, r=Mark-Simulacrum

Use `once_cell` crate instead of custom data structure

Internally, we use the [`Once`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sync/struct.Once.html) type for shared data that is initialized exactly once and only read from afterwards. `Once` uses a `parking_lot::Mutex` when the parallel compiler is enabled and a `RefCell` when it is not. This PR switches to the [`once_cell`](https://crates.io/crates/once_cell) crate, which also uses a `parking_lot::Mutex` for its `sync` version (because we enable the `parking_lot` feature) but has zero overhead for its `unsync` one.

This PR adds `once_cell` to the list of whitelisted dependencies. I think this is acceptable because it is already used in `rustc_driver`, is owned by a well-known community member (cc @matklad), and has a stable release. cc @rust-lang/compiler

`once_cell` has a slightly more minimal API than `Once`, which allows for initialization to be either optimistic (evaluate the initializer and then synchronize) or pessimistic (synchronize and then evaluate the initializer). `once_cell`'s `get_or_init` is always pessimistic. The optimistic version is only used once in the current `master`.

r? @Mark-Simulacrum

5 files changed, 10 insertions, 14 deletions

diff --git a/src/librustc_codegen_ssa/back/link.rs b/src/librustc_codegen_ssa/back/link.rs
index 5de60b5b9c8..9a7c4907754 100644
--- a/src/librustc_codegen_ssa/back/link.rs
+++ b/src/librustc_codegen_ssa/back/link.rs
@@ -53,7 +53,7 @@ pub fn link_binary<'a, B: ArchiveBuilder<'a>>(
 ) {
     let _timer = sess.timer("link_binary");
     let output_metadata = sess.opts.output_types.contains_key(&OutputType::Metadata);
-    for &crate_type in sess.crate_types.borrow().iter() {
+    for &crate_type in sess.crate_types().iter() {
         // Ignore executable crates if we have -Z no-codegen, as they will error.
         if (sess.opts.debugging_opts.no_codegen || !sess.opts.output_types.should_codegen())
             && !output_metadata
@@ -875,11 +875,8 @@ fn preserve_objects_for_their_debuginfo(sess: &Session) -> bool {
 
     // If we're only producing artifacts that are archives, no need to preserve
     // the objects as they're losslessly contained inside the archives.
-    let output_linked = sess
-        .crate_types
-        .borrow()
-        .iter()
-        .any(|&x| x != CrateType::Rlib && x != CrateType::Staticlib);
+    let output_linked =
+        sess.crate_types().iter().any(|&x| x != CrateType::Rlib && x != CrateType::Staticlib);
     if !output_linked {
         return false;
     }
diff --git a/src/librustc_codegen_ssa/back/linker.rs b/src/librustc_codegen_ssa/back/linker.rs
index ee5bcf4b9f5..49de8c5e28a 100644
--- a/src/librustc_codegen_ssa/back/linker.rs
+++ b/src/librustc_codegen_ssa/back/linker.rs
@@ -44,8 +44,7 @@ impl LinkerInfo {
         LinkerInfo {
             exports: tcx
                 .sess
-                .crate_types
-                .borrow()
+                .crate_types()
                 .iter()
                 .map(|&c| (c, exported_symbols(tcx, c)))
                 .collect(),
diff --git a/src/librustc_codegen_ssa/back/symbol_export.rs b/src/librustc_codegen_ssa/back/symbol_export.rs
index c0272e1cd2d..970d13b30c0 100644
--- a/src/librustc_codegen_ssa/back/symbol_export.rs
+++ b/src/librustc_codegen_ssa/back/symbol_export.rs
@@ -18,7 +18,7 @@ use rustc_middle::ty::{SymbolName, TyCtxt};
 use rustc_session::config::{CrateType, Sanitizer};
 
 pub fn threshold(tcx: TyCtxt<'_>) -> SymbolExportLevel {
-    crates_export_threshold(&tcx.sess.crate_types.borrow())
+    crates_export_threshold(&tcx.sess.crate_types())
 }
 
 fn crate_export_threshold(crate_type: CrateType) -> SymbolExportLevel {
@@ -212,7 +212,7 @@ fn exported_symbols_provider_local(
         }));
     }
 
-    if tcx.sess.crate_types.borrow().contains(&CrateType::Dylib) {
+    if tcx.sess.crate_types().contains(&CrateType::Dylib) {
         let symbol_name = metadata_symbol_name(tcx);
         let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(&symbol_name));
 
diff --git a/src/librustc_codegen_ssa/back/write.rs b/src/librustc_codegen_ssa/back/write.rs
index f9ee7d8c5de..9e03c283cfb 100644
--- a/src/librustc_codegen_ssa/back/write.rs
+++ b/src/librustc_codegen_ssa/back/write.rs
@@ -382,7 +382,7 @@ pub struct CompiledModules {
 
 fn need_bitcode_in_object(sess: &Session) -> bool {
     let requested_for_rlib = sess.opts.cg.embed_bitcode
-        && sess.crate_types.borrow().contains(&CrateType::Rlib)
+        && sess.crate_types().contains(&CrateType::Rlib)
         && sess.opts.output_types.contains_key(&OutputType::Exe);
     let forced_by_target = sess.target.target.options.forces_embed_bitcode;
     requested_for_rlib || forced_by_target
@@ -991,7 +991,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
     };
     let cgcx = CodegenContext::<B> {
         backend: backend.clone(),
-        crate_types: sess.crate_types.borrow().clone(),
+        crate_types: sess.crate_types().to_vec(),
         each_linked_rlib_for_lto,
         lto: sess.lto(),
         no_landing_pads: sess.panic_strategy() == PanicStrategy::Abort,
@@ -1812,7 +1812,7 @@ fn msvc_imps_needed(tcx: TyCtxt<'_>) -> bool {
     );
 
     tcx.sess.target.target.options.is_like_msvc &&
-        tcx.sess.crate_types.borrow().iter().any(|ct| *ct == CrateType::Rlib) &&
+        tcx.sess.crate_types().iter().any(|ct| *ct == CrateType::Rlib) &&
     // ThinLTO can't handle this workaround in all cases, so we don't
     // emit the `__imp_` symbols. Instead we make them unnecessary by disallowing
     // dynamic linking when linker plugin LTO is enabled.
diff --git a/src/librustc_codegen_ssa/base.rs b/src/librustc_codegen_ssa/base.rs
index ed5448ca8ba..5b14258bd25 100644
--- a/src/librustc_codegen_ssa/base.rs
+++ b/src/librustc_codegen_ssa/base.rs
@@ -948,7 +948,7 @@ fn determine_cgu_reuse<'tcx>(tcx: TyCtxt<'tcx>, cgu: &CodegenUnit<'tcx>) -> CguR
         match compute_per_cgu_lto_type(
             &tcx.sess.lto(),
             &tcx.sess.opts,
-            &tcx.sess.crate_types.borrow(),
+            &tcx.sess.crate_types(),
             ModuleKind::Regular,
         ) {
             ComputedLtoType::No => CguReuse::PostLto,