use triomphe::Arc for InternedObligationCauseCode instead of Arc

Cargo.lock

@@ -3666,6 +3666,7 @@ dependencies = [
  "tempfile",
  "thin-vec",
  "tracing",
+ "triomphe",
  "windows 0.59.0",
 ]
 
@@ -4183,6 +4184,7 @@ dependencies = [
  "smallvec",
  "thin-vec",
  "tracing",
+ "triomphe",
 ]
 
 [[package]]
@@ -4492,6 +4494,7 @@ dependencies = [
  "smallvec",
  "tempfile",
  "thin-vec",
+ "triomphe",
 ]
 
 [[package]]
@@ -4694,6 +4697,7 @@ dependencies = [
  "smallvec",
  "thin-vec",
  "tracing",
+ "triomphe",
 ]
 
 [[package]]
@@ -5630,6 +5634,15 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "triomphe"
+version = "0.1.14"
+source = "git+https://github.com/Manishearth/triomphe.git?rev=6ff0527036632be33a6ed45e5e8a0fb6243dee47#6ff0527036632be33a6ed45e5e8a0fb6243dee47"
+dependencies = [
+ "serde",
+ "stable_deref_trait",
+]
+
 [[package]]
 name = "twox-hash"
 version = "1.6.3"

Cargo.toml

@@ -93,3 +93,6 @@ codegen-units = 1
 # FIXME: LTO cannot be enabled for binaries in a workspace
 # <https://github.com/rust-lang/cargo/issues/9330>
 # lto = true
+
+[patch.crates-io]
+triomphe = { git = 'https://github.com/Manishearth/triomphe.git', rev = '6ff0527036632be33a6ed45e5e8a0fb6243dee47' }

compiler/rustc_data_structures/Cargo.toml

@@ -26,6 +26,7 @@ stacker = "0.1.17"
 tempfile = "3.2"
 thin-vec = "0.2.12"
 tracing = "0.1"
+triomphe = { version = "0.1.14", features = ["unstable_dropck_eyepatch"] }
 # tidy-alphabetical-end
 
 [dependencies.parking_lot]

compiler/rustc_data_structures/src/marker.rs

@@ -65,6 +65,7 @@ impl_dyn_send!(
     [std::sync::Mutex<T> where T: ?Sized+ DynSend]
     [std::sync::mpsc::Sender<T> where T: DynSend]
     [std::sync::Arc<T> where T: ?Sized + DynSync + DynSend]
+    [triomphe::Arc<T> where T: ?Sized + DynSync + DynSend]
     [std::sync::LazyLock<T, F> where T: DynSend, F: DynSend]
     [std::collections::HashSet<K, S> where K: DynSend, S: DynSend]
     [std::collections::HashMap<K, V, S> where K: DynSend, V: DynSend, S: DynSend]
@@ -139,6 +140,7 @@ impl_dyn_sync!(
     [std::sync::OnceLock<T> where T: DynSend + DynSync]
     [std::sync::Mutex<T> where T: ?Sized + DynSend]
     [std::sync::Arc<T> where T: ?Sized + DynSync + DynSend]
+    [triomphe::Arc<T> where T: ?Sized + DynSync + DynSend]
     [std::sync::LazyLock<T, F> where T: DynSend + DynSync, F: DynSend]
     [std::collections::HashSet<K, S> where K: DynSync, S: DynSync]
     [std::collections::HashMap<K, V, S> where K: DynSync, V: DynSync, S: DynSync]

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -395,6 +395,13 @@ impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::std::sync::Arc<T> {
     }
 }
 
+impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::triomphe::Arc<T> {
+    #[inline]
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
+        (**self).hash_stable(ctx, hasher);
+    }
+}
+
 impl<CTX> HashStable<CTX> for str {
     #[inline]
     fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {

compiler/rustc_middle/Cargo.toml

@@ -36,6 +36,7 @@ rustc_type_ir = { path = "../rustc_type_ir" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2.12"
 tracing = "0.1"
+triomphe = "0.1.14"
 # tidy-alphabetical-end
 
 [features]

compiler/rustc_middle/src/traits/mod.rs

@@ -10,10 +10,8 @@ mod structural_impls;
 
 use std::borrow::Cow;
 use std::hash::{Hash, Hasher};
-use std::sync::Arc;
 
 use rustc_errors::{Applicability, Diag, EmissionGuarantee};
-use rustc_hir as hir;
 use rustc_hir::HirId;
 use rustc_hir::def_id::DefId;
 use rustc_macros::{
@@ -23,6 +21,7 @@ use rustc_span::def_id::{CRATE_DEF_ID, LocalDefId};
 use rustc_span::{DUMMY_SP, Span, Symbol};
 use smallvec::{SmallVec, smallvec};
 use thin_vec::ThinVec;
+use {rustc_hir as hir, triomphe};
 
 pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
 use crate::mir::ConstraintCategory;
@@ -157,7 +156,7 @@ pub struct UnifyReceiverContext<'tcx> {
 pub struct InternedObligationCauseCode<'tcx> {
     /// `None` for `ObligationCauseCode::Misc` (a common case, occurs ~60% of
     /// the time). `Some` otherwise.
-    code: Option<Arc<ObligationCauseCode<'tcx>>>,
+    code: Option<triomphe::Arc<ObligationCauseCode<'tcx>>>,
 }
 
 impl<'tcx> std::fmt::Debug for InternedObligationCauseCode<'tcx> {
@@ -171,7 +170,11 @@ impl<'tcx> ObligationCauseCode<'tcx> {
     #[inline(always)]
     fn into(self) -> InternedObligationCauseCode<'tcx> {
         InternedObligationCauseCode {
-            code: if let ObligationCauseCode::Misc = self { None } else { Some(Arc::new(self)) },
+            code: if let ObligationCauseCode::Misc = self {
+                None
+            } else {
+                Some(triomphe::Arc::new(self))
+            },
         }
     }
 }

compiler/rustc_serialize/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2021"
 indexmap = "2.0.0"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2.12"
+triomphe = "0.1.14"
 # tidy-alphabetical-end
 
 [dev-dependencies]

compiler/rustc_serialize/src/serialize.rs

@@ -12,6 +12,7 @@ use std::sync::Arc;
 
 use smallvec::{Array, SmallVec};
 use thin_vec::ThinVec;
+use triomphe;
 
 /// A byte that [cannot occur in UTF8 sequences][utf8]. Used to mark the end of a string.
 /// This way we can skip validation and still be relatively sure that deserialization
@@ -486,6 +487,18 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for Arc<T> {
     }
 }
 
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for triomphe::Arc<T> {
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for triomphe::Arc<T> {
+    fn decode(d: &mut D) -> triomphe::Arc<T> {
+        triomphe::Arc::new(Decodable::decode(d))
+    }
+}
+
 impl<S: Encoder, T: ?Sized + Encodable<S>> Encodable<S> for Box<T> {
     fn encode(&self, s: &mut S) {
         (**self).encode(s)

compiler/rustc_type_ir/Cargo.toml

@@ -19,6 +19,7 @@ rustc_type_ir_macros = { path = "../rustc_type_ir_macros" }
 smallvec = { version = "1.8.1", default-features = false }
 thin-vec = "0.2.12"
 tracing = "0.1"
+triomphe = "0.1.14"
 # tidy-alphabetical-end
 
 [features]

compiler/rustc_type_ir/src/fold.rs

@@ -51,6 +51,7 @@ use std::sync::Arc;
 use rustc_index::{Idx, IndexVec};
 use thin_vec::ThinVec;
 use tracing::{debug, instrument};
+use triomphe;
 
 use crate::inherent::*;
 use crate::visit::{TypeVisitable, TypeVisitableExt as _};
@@ -310,6 +311,43 @@ impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Arc<T> {
     }
 }
 
+impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for triomphe::Arc<T> {
+    fn try_fold_with<F: FallibleTypeFolder<I>>(mut self, folder: &mut F) -> Result<Self, F::Error> {
+        // We merely want to replace the contained `T`, if at all possible,
+        // so that we don't needlessly allocate a new `Arc` or indeed clone
+        // the contained type.
+        unsafe {
+            // First step is to ensure that we have a unique reference to
+            // the contained type, which `Arc::make_mut` will accomplish (by
+            // allocating a new `Arc` and cloning the `T` only if required).
+            // This is done *before* casting to `Arc<ManuallyDrop<T>>` so that
+            // panicking during `make_mut` does not leak the `T`.
+            triomphe::Arc::make_mut(&mut self);
+
+            // Casting to `Arc<ManuallyDrop<T>>` is safe because `ManuallyDrop`
+            // is `repr(transparent)`.
+            let ptr = triomphe::Arc::into_raw(self).cast::<mem::ManuallyDrop<T>>();
+            let mut unique = triomphe::Arc::from_raw(ptr);
+
+            // Call to `Arc::make_mut` above guarantees that `unique` is the
+            // sole reference to the contained value, so we can avoid doing
+            // a checked `get_mut` here.
+            let slot = triomphe::Arc::get_mut(&mut unique).unwrap_unchecked();
+
+            // Semantically move the contained type out from `unique`, fold
+            // it, then move the folded value back into `unique`. Should
+            // folding fail, `ManuallyDrop` ensures that the "moved-out"
+            // value is not re-dropped.
+            let owned = mem::ManuallyDrop::take(slot);
+            let folded = owned.try_fold_with(folder)?;
+            *slot = mem::ManuallyDrop::new(folded);
+
+            // Cast back to `Arc<T>`.
+            Ok(triomphe::Arc::from_raw(triomphe::Arc::into_raw(unique).cast()))
+        }
+    }
+}
+
 impl<I: Interner, T: TypeFoldable<I>> TypeFoldable<I> for Box<T> {
     fn try_fold_with<F: FallibleTypeFolder<I>>(mut self, folder: &mut F) -> Result<Self, F::Error> {
         *self = (*self).try_fold_with(folder)?;

compiler/rustc_type_ir/src/visit.rs

@@ -173,6 +173,12 @@ impl<I: Interner, T: TypeVisitable<I>> TypeVisitable<I> for Arc<T> {
     }
 }
 
+impl<I: Interner, T: TypeVisitable<I>> TypeVisitable<I> for triomphe::Arc<T> {
+    fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> V::Result {
+        (**self).visit_with(visitor)
+    }
+}
+
 impl<I: Interner, T: TypeVisitable<I>> TypeVisitable<I> for Box<T> {
     fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> V::Result {
         (**self).visit_with(visitor)

src/tools/tidy/src/deps.rs

@@ -406,6 +406,7 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "tracing-log",
     "tracing-subscriber",
     "tracing-tree",
+    "triomphe",
     "twox-hash",
     "type-map",
     "typenum",

src/tools/tidy/src/extdeps.rs

@@ -8,6 +8,8 @@ const ALLOWED_SOURCES: &[&str] = &[
     r#""registry+https://github.com/rust-lang/crates.io-index""#,
     // This is `rust_team_data` used by `site` in src/tools/rustc-perf,
     r#""git+https://github.com/rust-lang/team#a5260e76d3aa894c64c56e6ddc8545b9a98043ec""#,
+    // temporary to test
+    r#""git+https://github.com/Manishearth/triomphe.git?rev=6ff0527036632be33a6ed45e5e8a0fb6243dee47#6ff0527036632be33a6ed45e5e8a0fb6243dee47""#,
 ];
 
 /// Checks for external package sources. `root` is the path to the directory that contains the