Auto merge of #55012 - kennytm:rollup, r=kennytm

Rollup of 16 pull requests

Successful merges:

 - #54755 (Documents reference equality by address (#54197))
 - #54811 (During rustc bootstrap, make default for `optimize` independent of `debug`)
 - #54825 (NLL says "borrowed content" instead of more precise "dereference of raw pointer")
 - #54860 (Add doc comments about safest way to initialize a vector of zeros)
 - #54869 (Fix mobile docs)
 - #54891 (Fix tracking issue for Once::is_completed)
 - #54913 (doc fix: it's auto traits that make for automatic implementations)
 - #54920 (Fix handling of #[must_use] on unit and uninhabited types)
 - #54932 (A handful of random string-related improvements)
 - #54936 (impl Eq+Hash for TyLayout)
 - #54950 (std: Synchronize global allocator on wasm32)
 - #54956 ("(using ..." doesn't have the matching ")")
 - #54958 (add a macro for static (compile-time) assertions)
 - #54967 (Remove incorrect span for second label inner macro invocation)
 - #54983 (Fix slice's benchmarks)
 - #54989 (Fix spelling in the documentation to htmldocck.py)

Failed merges:

r? @ghost

4 files changed, 28 insertions, 3 deletions

diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index f14d55cb2d3..017949291bc 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -726,7 +726,7 @@ impl OpenOptions {
     /// If a file is opened with both read and append access, beware that after
     /// opening, and after every write, the position for reading may be set at the
     /// end of the file. So, before writing, save the current position (using
-    /// [`seek`]`(`[`SeekFrom`]`::`[`Current`]`(0))`, and restore it before the next read.
+    /// [`seek`]`(`[`SeekFrom`]`::`[`Current`]`(0))`), and restore it before the next read.
     ///
     /// ## Note
     ///
diff --git a/src/libstd/panic.rs b/src/libstd/panic.rs
index 48a9b2f4a93..5c87035d8e9 100644
--- a/src/libstd/panic.rs
+++ b/src/libstd/panic.rs
@@ -79,7 +79,7 @@ pub use core::panic::{PanicInfo, Location};
 ///
 /// Simply put, a type `T` implements `UnwindSafe` if it cannot easily allow
 /// witnessing a broken invariant through the use of `catch_unwind` (catching a
-/// panic). This trait is a marker trait, so it is automatically implemented for
+/// panic). This trait is an auto trait, so it is automatically implemented for
 /// many types, and it is also structurally composed (e.g. a struct is unwind
 /// safe if all of its components are unwind safe).
 ///
diff --git a/src/libstd/primitive_docs.rs b/src/libstd/primitive_docs.rs
index 8d54728a75f..3b432d05132 100644
--- a/src/libstd/primitive_docs.rs
+++ b/src/libstd/primitive_docs.rs
@@ -908,11 +908,36 @@ mod prim_usize { }
 /// `&mut T` references can be freely coerced into `&T` references with the same referent type, and
 /// references with longer lifetimes can be freely coerced into references with shorter ones.
 ///
+/// Reference equality by address, instead of comparing the values pointed to, is accomplished via
+/// implicit reference-pointer coercion and raw pointer equality via [`ptr::eq`], while
+/// [`PartialEq`] compares values.
+///
+/// [`ptr::eq`]: ptr/fn.eq.html
+/// [`PartialEq`]: cmp/trait.PartialEq.html
+///
+/// ```
+/// use std::ptr;
+///
+/// let five = 5;
+/// let other_five = 5;
+/// let five_ref = &five;
+/// let same_five_ref = &five;
+/// let other_five_ref = &other_five;
+///
+/// assert!(five_ref == same_five_ref);
+/// assert!(five_ref == other_five_ref);
+///
+/// assert!(ptr::eq(five_ref, same_five_ref));
+/// assert!(!ptr::eq(five_ref, other_five_ref));
+/// ```
+///
 /// For more information on how to use references, see [the book's section on "References and
 /// Borrowing"][book-refs].
 ///
 /// [book-refs]: ../book/second-edition/ch04-02-references-and-borrowing.html
 ///
+/// # Trait implementations
+///
 /// The following traits are implemented for all `&T`, regardless of the type of its referent:
 ///
 /// * [`Copy`]
diff --git a/src/libstd/sync/once.rs b/src/libstd/sync/once.rs
index 17cb614ba11..98845e457b2 100644
--- a/src/libstd/sync/once.rs
+++ b/src/libstd/sync/once.rs
@@ -329,7 +329,7 @@ impl Once {
     /// assert!(handle.join().is_err());
     /// assert_eq!(INIT.is_completed(), false);
     /// ```
-    #[unstable(feature = "once_is_completed", issue = "42")]
+    #[unstable(feature = "once_is_completed", issue = "54890")]
     #[inline]
     pub fn is_completed(&self) -> bool {
         // An `Acquire` load is enough because that makes all the initialization