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

Author: bors

Cargo.lock

@@ -2327,6 +2327,9 @@ name = "once_cell"
 version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d6a04cb71e910d0034815600180f62a95bf6e67942d7ab52a166a68c7d7e9cd0"
+dependencies = [
+ "parking_lot 0.9.0",
+]
 
 [[package]]
 name = "opaque-debug"
@@ -3791,6 +3794,7 @@ dependencies = [
  "libc",
  "log",
  "measureme",
+ "once_cell",
  "parking_lot 0.10.2",
  "rustc-hash",
  "rustc-rayon",

src/librustc_ast_lowering/lib.rs

@@ -269,7 +269,7 @@ pub fn lower_crate<'a, 'hir>(
     let _prof_timer = sess.prof.verbose_generic_activity("hir_lowering");
 
     LoweringContext {
-        crate_root: sess.parse_sess.injected_crate_name.try_get().copied(),
+        crate_root: sess.parse_sess.injected_crate_name.get().copied(),
         sess,
         resolver,
         nt_to_tokenstream,

src/librustc_codegen_llvm/context.rs

@@ -174,7 +174,7 @@ pub unsafe fn create_module(
         llvm::LLVMRustSetModulePICLevel(llmod);
         // PIE is potentially more effective than PIC, but can only be used in executables.
         // If all our outputs are executables, then we can relax PIC to PIE.
-        if sess.crate_types.get().iter().all(|ty| *ty == CrateType::Executable) {
+        if sess.crate_types().iter().all(|ty| *ty == CrateType::Executable) {
             llvm::LLVMRustSetModulePIELevel(llmod);
         }
     }

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;
     }

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(),

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));
 

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.

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,

src/librustc_data_structures/Cargo.toml

@@ -15,6 +15,7 @@ indexmap = "1"
 log = "0.4"
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
 lazy_static = "1"
+once_cell = { version = "1", features = ["parking_lot"] }
 rustc_serialize = { path = "../libserialize", package = "serialize" }
 graphviz = { path = "../libgraphviz" }
 cfg-if = "0.1.2"

src/librustc_data_structures/sync.rs

@@ -20,7 +20,6 @@
 use crate::owning_ref::{Erased, OwningRef};
 use std::collections::HashMap;
 use std::hash::{BuildHasher, Hash};
-use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
 
 pub use std::sync::atomic::Ordering;
@@ -230,6 +229,8 @@ cfg_if! {
         pub use std::cell::RefMut as LockGuard;
         pub use std::cell::RefMut as MappedLockGuard;
 
+        pub use once_cell::unsync::OnceCell;
+
         use std::cell::RefCell as InnerRwLock;
         use std::cell::RefCell as InnerLock;
 
@@ -313,6 +314,8 @@ cfg_if! {
         pub use parking_lot::MutexGuard as LockGuard;
         pub use parking_lot::MappedMutexGuard as MappedLockGuard;
 
+        pub use once_cell::sync::OnceCell;
+
         pub use std::sync::atomic::{AtomicBool, AtomicUsize, AtomicU32, AtomicU64};
 
         pub use crossbeam_utils::atomic::AtomicCell;
@@ -432,134 +435,6 @@ impl<K: Eq + Hash, V: Eq, S: BuildHasher> HashMapExt<K, V> for HashMap<K, V, S>
     }
 }
 
-/// A type whose inner value can be written once and then will stay read-only
-// This contains a PhantomData<T> since this type conceptually owns a T outside the Mutex once
-// initialized. This ensures that Once<T> is Sync only if T is. If we did not have PhantomData<T>
-// we could send a &Once<Cell<bool>> to multiple threads and call `get` on it to get access
-// to &Cell<bool> on those threads.
-pub struct Once<T>(Lock<Option<T>>, PhantomData<T>);
-
-impl<T> Once<T> {
-    /// Creates an Once value which is uninitialized
-    #[inline(always)]
-    pub fn new() -> Self {
-        Once(Lock::new(None), PhantomData)
-    }
-
-    /// Consumes the value and returns Some(T) if it was initialized
-    #[inline(always)]
-    pub fn into_inner(self) -> Option<T> {
-        self.0.into_inner()
-    }
-
-    /// Tries to initialize the inner value to `value`.
-    /// Returns `None` if the inner value was uninitialized and `value` was consumed setting it
-    /// otherwise if the inner value was already set it returns `value` back to the caller
-    #[inline]
-    pub fn try_set(&self, value: T) -> Option<T> {
-        let mut lock = self.0.lock();
-        if lock.is_some() {
-            return Some(value);
-        }
-        *lock = Some(value);
-        None
-    }
-
-    /// Tries to initialize the inner value to `value`.
-    /// Returns `None` if the inner value was uninitialized and `value` was consumed setting it
-    /// otherwise if the inner value was already set it asserts that `value` is equal to the inner
-    /// value and then returns `value` back to the caller
-    #[inline]
-    pub fn try_set_same(&self, value: T) -> Option<T>
-    where
-        T: Eq,
-    {
-        let mut lock = self.0.lock();
-        if let Some(ref inner) = *lock {
-            assert!(*inner == value);
-            return Some(value);
-        }
-        *lock = Some(value);
-        None
-    }
-
-    /// Tries to initialize the inner value to `value` and panics if it was already initialized
-    #[inline]
-    pub fn set(&self, value: T) {
-        assert!(self.try_set(value).is_none());
-    }
-
-    /// Initializes the inner value if it wasn't already done by calling the provided closure. It
-    /// ensures that no-one else can access the value in the mean time by holding a lock for the
-    /// duration of the closure.
-    /// A reference to the inner value is returned.
-    #[inline]
-    pub fn init_locking<F: FnOnce() -> T>(&self, f: F) -> &T {
-        {
-            let mut lock = self.0.lock();
-            if lock.is_none() {
-                *lock = Some(f());
-            }
-        }
-
-        self.borrow()
-    }
-
-    /// Tries to initialize the inner value by calling the closure without ensuring that no-one
-    /// else can access it. This mean when this is called from multiple threads, multiple
-    /// closures may concurrently be computing a value which the inner value should take.
-    /// Only one of these closures are used to actually initialize the value.
-    /// If some other closure already set the value,
-    /// we return the value our closure computed wrapped in a `Option`.
-    /// If our closure set the value, `None` is returned.
-    /// If the value is already initialized, the closure is not called and `None` is returned.
-    #[inline]
-    pub fn init_nonlocking<F: FnOnce() -> T>(&self, f: F) -> Option<T> {
-        if self.0.lock().is_some() { None } else { self.try_set(f()) }
-    }
-
-    /// Tries to initialize the inner value by calling the closure without ensuring that no-one
-    /// else can access it. This mean when this is called from multiple threads, multiple
-    /// closures may concurrently be computing a value which the inner value should take.
-    /// Only one of these closures are used to actually initialize the value.
-    /// If some other closure already set the value, we assert that it our closure computed
-    /// a value equal to the value already set and then
-    /// we return the value our closure computed wrapped in a `Option`.
-    /// If our closure set the value, `None` is returned.
-    /// If the value is already initialized, the closure is not called and `None` is returned.
-    #[inline]
-    pub fn init_nonlocking_same<F: FnOnce() -> T>(&self, f: F) -> Option<T>
-    where
-        T: Eq,
-    {
-        if self.0.lock().is_some() { None } else { self.try_set_same(f()) }
-    }
-
-    /// Tries to get a reference to the inner value, returns `None` if it is not yet initialized
-    #[inline(always)]
-    pub fn try_get(&self) -> Option<&T> {
-        let lock = &*self.0.lock();
-        if let Some(ref inner) = *lock {
-            // This is safe since we won't mutate the inner value
-            unsafe { Some(&*(inner as *const T)) }
-        } else {
-            None
-        }
-    }
-
-    /// Gets reference to the inner value, panics if it is not yet initialized
-    #[inline(always)]
-    pub fn get(&self) -> &T {
-        self.try_get().expect("value was not set")
-    }
-
-    /// Gets reference to the inner value, panics if it is not yet initialized
-    #[inline(always)]
-    pub fn borrow(&self) -> &T {
-        self.get()
-    }
-}
-
 #[derive(Debug)]
 pub struct Lock<T>(InnerLock<T>);