Auto merge of #74493 - Manishearth:rollup-ust7yr4, r=Manishearth

Rollup of 7 pull requests

Successful merges:

 - #70817 (Add core::task::ready! macro)
 - #73762 (Document the trait keyword)
 - #74021 (impl Index<RangeFrom> for CStr)
 - #74071 (rustc_metadata: Make crate loading fully speculative)
 - #74445 (add test for #62878)
 - #74459 (Make unreachable_unchecked a const fn)
 - #74478 (Revert "Use an UTF-8 locale for the linker.")

Failed merges:

r? @ghost

5 files changed, 68 insertions, 1 deletions

diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index 9ebcde79b63..3116815f5d6 100644
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -45,7 +45,8 @@ use crate::intrinsics;
 /// ```
 #[inline]
 #[stable(feature = "unreachable", since = "1.27.0")]
-pub unsafe fn unreachable_unchecked() -> ! {
+#[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
+pub const unsafe fn unreachable_unchecked() -> ! {
     // SAFETY: the safety contract for `intrinsics::unreachable` must
     // be upheld by the caller.
     unsafe { intrinsics::unreachable() }
diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 080760aa81f..8f0cf4361e7 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -932,6 +932,7 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::hint::unreachable_unchecked`](../../std/hint/fn.unreachable_unchecked.html).
+    #[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
     pub fn unreachable() -> !;
 
     /// Informs the optimizer that a condition is always true.
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 1621cf79751..2e443064706 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -92,6 +92,7 @@
 #![feature(const_slice_ptr_len)]
 #![feature(const_type_name)]
 #![feature(const_likely)]
+#![feature(const_unreachable_unchecked)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]
diff --git a/src/libcore/task/mod.rs b/src/libcore/task/mod.rs
index 27760749c1d..3d6f4f5971a 100644
--- a/src/libcore/task/mod.rs
+++ b/src/libcore/task/mod.rs
@@ -9,3 +9,7 @@ pub use self::poll::Poll;
 mod wake;
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::wake::{Context, RawWaker, RawWakerVTable, Waker};
+
+mod ready;
+#[unstable(feature = "ready_macro", issue = "70922")]
+pub use ready::ready;
diff --git a/src/libcore/task/ready.rs b/src/libcore/task/ready.rs
new file mode 100644
index 00000000000..d4e733eb2bc
--- /dev/null
+++ b/src/libcore/task/ready.rs
@@ -0,0 +1,60 @@
+/// Extracts the successful type of a `Poll<T>`.
+///
+/// This macro bakes in propagation of `Pending` signals by returning early.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(future_readiness_fns)]
+/// #![feature(ready_macro)]
+///
+/// use core::task::{ready, Context, Poll};
+/// use core::future::{self, Future};
+/// use core::pin::Pin;
+///
+/// pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
+///     let mut fut = future::ready(42);
+///     let fut = Pin::new(&mut fut);
+///
+///     let num = ready!(fut.poll(cx));
+///     # drop(num);
+///     // ... use num
+///
+///     Poll::Ready(())
+/// }
+/// ```
+///
+/// The `ready!` call expands to:
+///
+/// ```
+/// # #![feature(future_readiness_fns)]
+/// # #![feature(ready_macro)]
+/// #
+/// # use core::task::{Context, Poll};
+/// # use core::future::{self, Future};
+/// # use core::pin::Pin;
+/// #
+/// # pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
+///     # let mut fut = future::ready(42);
+///     # let fut = Pin::new(&mut fut);
+///     #
+/// let num = match fut.poll(cx) {
+///     Poll::Ready(t) => t,
+///     Poll::Pending => return Poll::Pending,
+/// };
+///     # drop(num);
+///     # // ... use num
+///     #
+///     # Poll::Ready(())
+/// # }
+/// ```
+#[unstable(feature = "ready_macro", issue = "70922")]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro ready($e:expr) {
+    match $e {
+        $crate::task::Poll::Ready(t) => t,
+        $crate::task::Poll::Pending => {
+            return $crate::task::Poll::Pending;
+        }
+    }
+}