Rollup of 8 pull requests

library/core/src/pin/unsafe_pinned.rs

@@ -1,10 +1,13 @@
+use crate::cell::UnsafeCell;
 use crate::marker::{PointerLike, Unpin};
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
 use crate::pin::Pin;
 use crate::{fmt, ptr};
 
-/// This type provides a way to opt-out of typical aliasing rules;
+/// This type provides a way to entirely opt-out of typical aliasing rules;
 /// specifically, `&mut UnsafePinned<T>` is not guaranteed to be a unique pointer.
+/// This also subsumes the effects of `UnsafeCell`, i.e., `&UnsafePinned<T>` may point to data
+/// that is being mutated.
 ///
 /// However, even if you define your type like `pub struct Wrapper(UnsafePinned<...>)`, it is still
 /// very risky to have an `&mut Wrapper` that aliases anything else. Many functions that work
@@ -17,38 +20,24 @@ use crate::{fmt, ptr};
 /// the public API of a library. It is an internal implementation detail of libraries that need to
 /// support aliasing mutable references.
 ///
-/// Further note that this does *not* lift the requirement that shared references must be read-only!
-/// Use `UnsafeCell` for that.
-///
 /// This type blocks niches the same way `UnsafeCell` does.
 #[lang = "unsafe_pinned"]
 #[repr(transparent)]
 #[unstable(feature = "unsafe_pinned", issue = "125735")]
 pub struct UnsafePinned<T: ?Sized> {
-    value: T,
+    value: UnsafeCell<T>,
 }
 
+// Override the manual `!Sync` in `UnsafeCell`.
+#[unstable(feature = "unsafe_pinned", issue = "125735")]
+unsafe impl<T: ?Sized + Sync> Sync for UnsafePinned<T> {}
+
 /// When this type is used, that almost certainly means safe APIs need to use pinning to avoid the
 /// aliases from becoming invalidated. Therefore let's mark this as `!Unpin`. You can always opt
 /// back in to `Unpin` with an `impl` block, provided your API is still sound while unpinned.
 #[unstable(feature = "unsafe_pinned", issue = "125735")]
 impl<T: ?Sized> !Unpin for UnsafePinned<T> {}
 
-/// The type is `Copy` when `T` is to avoid people assuming that `Copy` implies there is no
-/// `UnsafePinned` anywhere. (This is an issue with `UnsafeCell`: people use `Copy` bounds to mean
-/// `Freeze`.) Given that there is no `unsafe impl Copy for ...`, this is also the option that
-/// leaves the user more choices (as they can always wrap this in a `!Copy` type).
-// FIXME(unsafe_pinned): this may be unsound or a footgun?
-#[unstable(feature = "unsafe_pinned", issue = "125735")]
-impl<T: Copy> Copy for UnsafePinned<T> {}
-
-#[unstable(feature = "unsafe_pinned", issue = "125735")]
-impl<T: Copy> Clone for UnsafePinned<T> {
-    fn clone(&self) -> Self {
-        *self
-    }
-}
-
 // `Send` and `Sync` are inherited from `T`. This is similar to `SyncUnsafeCell`, since
 // we eventually concluded that `UnsafeCell` implicitly making things `!Sync` is sometimes
 // unergonomic. A type that needs to be `!Send`/`!Sync` should really have an explicit
@@ -63,7 +52,7 @@ impl<T> UnsafePinned<T> {
     #[must_use]
     #[unstable(feature = "unsafe_pinned", issue = "125735")]
     pub const fn new(value: T) -> Self {
-        UnsafePinned { value }
+        UnsafePinned { value: UnsafeCell::new(value) }
     }
 
     /// Unwraps the value, consuming this `UnsafePinned`.
@@ -72,7 +61,7 @@ impl<T> UnsafePinned<T> {
     #[unstable(feature = "unsafe_pinned", issue = "125735")]
     #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
     pub const fn into_inner(self) -> T {
-        self.value
+        self.value.into_inner()
     }
 }
 

src/bootstrap/src/core/build_steps/check.rs

@@ -77,6 +77,13 @@ impl Step for Std {
     }
 
     fn run(self, builder: &Builder<'_>) {
+        if !builder.download_rustc() && builder.config.skip_std_check_if_no_download_rustc {
+            eprintln!(
+                "WARNING: `--skip-std-check-if-no-download-rustc` flag was passed and `rust.download-rustc` is not available. Skipping."
+            );
+            return;
+        }
+
         builder.require_submodule("library/stdarch", None);
 
         let stage = self.custom_stage.unwrap_or(builder.top_stage);

src/bootstrap/src/core/build_steps/llvm.rs

@@ -443,23 +443,26 @@ impl Step for Llvm {
         // See https://github.com/rust-lang/rust/pull/50104
         cfg.define("LLVM_ENABLE_LIBXML2", "OFF");
 
-        if !enabled_llvm_projects.is_empty() {
-            enabled_llvm_projects.sort();
-            enabled_llvm_projects.dedup();
-            cfg.define("LLVM_ENABLE_PROJECTS", enabled_llvm_projects.join(";"));
-        }
-
         let mut enabled_llvm_runtimes = Vec::new();
 
         if helpers::forcing_clang_based_tests() {
             enabled_llvm_runtimes.push("compiler-rt");
         }
 
+        // This is an experimental flag, which likely builds more than necessary.
+        // We will optimize it when we get closer to releasing it on nightly.
         if builder.config.llvm_offload {
             enabled_llvm_runtimes.push("offload");
             //FIXME(ZuseZ4): LLVM intends to drop the offload dependency on openmp.
             //Remove this line once they achieved it.
             enabled_llvm_runtimes.push("openmp");
+            enabled_llvm_projects.push("compiler-rt");
+        }
+
+        if !enabled_llvm_projects.is_empty() {
+            enabled_llvm_projects.sort();
+            enabled_llvm_projects.dedup();
+            cfg.define("LLVM_ENABLE_PROJECTS", enabled_llvm_projects.join(";"));
         }
 
         if !enabled_llvm_runtimes.is_empty() {