fix some nits

2 files changed, 27 insertions, 39 deletions

diff --git a/compiler/rustc_data_structures/src/marker.rs b/compiler/rustc_data_structures/src/marker.rs
index fef8177214b..2af7bc917c2 100644
--- a/compiler/rustc_data_structures/src/marker.rs
+++ b/compiler/rustc_data_structures/src/marker.rs
@@ -8,31 +8,20 @@ cfg_if!(
     } else {
         #[rustc_on_unimplemented(
             message = "`{Self}` doesn't implement `DynSend`. \
-            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Send`",
-            label = "`{Self}` doesn't implement `DynSend`. \
             Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Send`"
         )]
-        // Ensure data structures is `Send` if `sync::active()` is true.
-        // `sync::active()` should be checked before using these data structures.
-        // Note: Ensure that the data structure **will not break**
-        // thread safety after being created.
-        //
-        // `sync::active()` should be checked when downcasting these data structures
-        // to `Send` via `FromDyn`.
+        // This is an auto trait for types which can be sent across threads if `sync::active()`
+        // is true. These types can be wrapped in a `FromDyn` to get a `Send` type. Wrapping a
+        // `Send` type in `IntoDyn` will create a `DynSend` type.
         pub unsafe auto trait DynSend {}
 
         #[rustc_on_unimplemented(
             message = "`{Self}` doesn't implement `DynSync`. \
-            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Sync`",
-            label = "`{Self}` doesn't implement `DynSync`. \
             Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Sync`"
         )]
-        // Ensure data structures is `Sync` if `sync::active()` is true.
-        // Note: Ensure that the data structure **will not break**
-        // thread safety after being checked.
-        //
-        // `sync::active()` should be checked when downcasting these data structures
-        // to `Send` via `FromDyn`.
+        // This is an auto trait for types which can be shared across threads if `sync::active()`
+        // is true. These types can be wrapped in a `FromDyn` to get a `Sync` type. Wrapping a
+        // `Sync` type in `IntoDyn` will create a `DynSync` type.
         pub unsafe auto trait DynSync {}
 
         // Same with `Sync` and `Send`.
@@ -110,8 +99,8 @@ cfg_if!(
             [thin_vec::ThinVec<T> where T: DynSend]
             [smallvec::SmallVec<A> where A: smallvec::Array + DynSend]
 
-            // We use `Send` here to omit some extra code, since they are only
-            // used in `Send` situations now.
+            // We use `Send` here, since they are only used in `Send` situations now.
+            // In this case we don't need copy or change the codes in `crate::owning_ref`.
             [crate::owning_ref::OwningRef<O, T> where O: Send, T: ?Sized + Send]
             [crate::owning_ref::OwningRefMut<O, T> where O: Send, T: ?Sized + Send]
         );
@@ -196,8 +185,8 @@ cfg_if!(
             [smallvec::SmallVec<A> where A: smallvec::Array + DynSync]
             [thin_vec::ThinVec<T> where T: DynSync]
 
-            // We use `Sync` here to omit some extra code, since they are only
-            // used in `Sync` situations now.
+            // We use `Sync` here, since they are only used in `Sync` situations now.
+            // In this case we don't need copy or change the codes in `crate::owning_ref`.
             [crate::owning_ref::OwningRef<O, T> where O: Sync, T: ?Sized + Sync]
             [crate::owning_ref::OwningRefMut<O, T> where O: Sync, T: ?Sized + Sync]
         );
@@ -213,11 +202,11 @@ pub fn assert_dyn_send_sync_val<T: ?Sized + DynSync + DynSend>(_t: &T) {}
 pub struct FromDyn<T>(T);
 
 impl<T> FromDyn<T> {
-    // Check `sync::active()` when creating this structure
-    // and downcasting to `Send`. So we can ensure it is
-    // thread-safe.
     #[inline(always)]
     pub fn from(val: T) -> Self {
+        // Check that `sync::active()` is true on creation so we can
+        // implement `Send` and `Sync` for this structure when `T`
+        // implements `DynSend` and `DynSync` respectively.
         #[cfg(parallel_compiler)]
         assert!(crate::sync::active());
         FromDyn(val)
@@ -229,11 +218,11 @@ impl<T> FromDyn<T> {
     }
 }
 
-// `FromDyn` is `Send` if `T` is `DynSend`, since it check when created.
+// `FromDyn` is `Send` if `T` is `DynSend`, since it ensures that sync::active() is true.
 #[cfg(parallel_compiler)]
 unsafe impl<T: DynSend> Send for FromDyn<T> {}
 
-// `FromDyn` is `Sync` if `T` is `DynSync`, since it check when created.
+// `FromDyn` is `Sync` if `T` is `DynSync`, since it ensures that sync::active() is true.
 #[cfg(parallel_compiler)]
 unsafe impl<T: DynSync> Sync for FromDyn<T> {}
 
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 5f865486880..bffb98630c8 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -181,7 +181,7 @@ cfg_if! {
 
         #[macro_export]
         macro_rules! parallel {
-            ($($blocks:block),*) => {{
+            ($($blocks:block),*) => {
                 // We catch panics here ensuring that all the blocks execute.
                 // This makes behavior consistent with the parallel compiler.
                 let mut panic = None;
@@ -197,7 +197,7 @@ cfg_if! {
                 if let Some(panic) = panic {
                     ::std::panic::resume_unwind(panic);
                 }
-            }}
+            }
         }
 
         pub fn par_for_each_in<T: IntoIterator>(t: T, mut for_each: impl FnMut(T::Item) + Sync + Send) {
@@ -368,6 +368,7 @@ cfg_if! {
             }
         }
 
+        // This function only works when `mode::active()`.
         pub fn scope<'scope, OP, R>(op: OP) -> R
         where
             OP: FnOnce(&rayon::Scope<'scope>) -> R + DynSend,
@@ -381,24 +382,22 @@ cfg_if! {
         /// the current thread. Use that for the longest running block.
         #[macro_export]
         macro_rules! parallel {
-            ($fblock:block [$($c:expr,)*] [$block:expr $(, $rest:expr)*]) => {
-                parallel!($fblock [$block, $($c,)*] [$($rest),*])
+            (impl $fblock:block [$($c:expr,)*] [$block:expr $(, $rest:expr)*]) => {
+                parallel!(impl $fblock [$block, $($c,)*] [$($rest),*])
             };
-            ($fblock:block [$($blocks:expr,)*] []) => {
-                {
-                    ::rustc_data_structures::sync::scope(|s| {
-                        $(let block = rustc_data_structures::sync::FromDyn::from(|| $blocks);
-                        s.spawn(move |_| block.into_inner()());)*
-                        (|| $fblock)();
-                    });
-                }
+            (impl $fblock:block [$($blocks:expr,)*] []) => {
+                ::rustc_data_structures::sync::scope(|s| {
+                    $(let block = rustc_data_structures::sync::FromDyn::from(|| $blocks);
+                    s.spawn(move |_| block.into_inner()());)*
+                    (|| $fblock)();
+                });
             };
             ($fblock:block, $($blocks:block),*) => {
                 if rustc_data_structures::sync::active() {
                     // Reverse the order of the later blocks since Rayon executes them in reverse order
                     // when using a single thread. This ensures the execution order matches that
                     // of a single threaded rustc
-                    parallel!($fblock [] [$($blocks),*]);
+                    parallel!(impl $fblock [] [$($blocks),*]);
                 } else {
                     // We catch panics here ensuring that all the blocks execute.
                     // This makes behavior consistent with the parallel compiler.