Auto merge of #83864 - Dylan-DPC:rollup-78an86n, r=Dylan-DPC

Rollup of 7 pull requests

Successful merges:

 - #80525 (wasm64 support)
 - #83019 (core: disable `ptr::swap_nonoverlapping_one`'s block optimization on SPIR-V.)
 - #83717 (rustdoc: Separate filter-empty-string out into its own function)
 - #83807 (Tests: Remove redundant `ignore-tidy-linelength` annotations)
 - #83815 (ptr::addr_of documentation improvements)
 - #83820 (Remove attribute `#[link_args]`)
 - #83841 (Allow clobbering unsupported registers in asm!)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

2 files changed, 59 insertions, 13 deletions

diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs
index 337f0e847bb..64342de6341 100644
--- a/library/core/src/mem/maybe_uninit.rs
+++ b/library/core/src/mem/maybe_uninit.rs
@@ -190,6 +190,8 @@ use crate::ptr;
 ///     let ptr = uninit.as_mut_ptr();
 ///
 ///     // Initializing the `name` field
+///     // Using `write` instead of assignment via `=` to not call `drop` on the
+///     // old, uninitialized value.
 ///     unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }
 ///
 ///     // Initializing the `list` field
diff --git a/library/core/src/ptr/mod.rs b/library/core/src/ptr/mod.rs
index 52660116026..6e207156b55 100644
--- a/library/core/src/ptr/mod.rs
+++ b/library/core/src/ptr/mod.rs
@@ -428,19 +428,32 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
 #[inline]
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
 pub(crate) const unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
-    // For types smaller than the block optimization below,
-    // just swap directly to avoid pessimizing codegen.
-    if mem::size_of::<T>() < 32 {
-        // SAFETY: the caller must guarantee that `x` and `y` are valid
-        // for writes, properly aligned, and non-overlapping.
-        unsafe {
-            let z = read(x);
-            copy_nonoverlapping(y, x, 1);
-            write(y, z);
+    // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
+    // reinterpretation of values as (chunkable) byte arrays, and the loop in the
+    // block optimization in `swap_nonoverlapping_bytes` is hard to rewrite back
+    // into the (unoptimized) direct swapping implementation, so we disable it.
+    // FIXME(eddyb) the block optimization also prevents MIR optimizations from
+    // understanding `mem::replace`, `Option::take`, etc. - a better overall
+    // solution might be to make `swap_nonoverlapping` into an intrinsic, which
+    // a backend can choose to implement using the block optimization, or not.
+    #[cfg(not(target_arch = "spirv"))]
+    {
+        // Only apply the block optimization in `swap_nonoverlapping_bytes` for types
+        // at least as large as the block size, to avoid pessimizing codegen.
+        if mem::size_of::<T>() >= 32 {
+            // SAFETY: the caller must uphold the safety contract for `swap_nonoverlapping`.
+            unsafe { swap_nonoverlapping(x, y, 1) };
+            return;
         }
-    } else {
-        // SAFETY: the caller must uphold the safety contract for `swap_nonoverlapping`.
-        unsafe { swap_nonoverlapping(x, y, 1) };
+    }
+
+    // Direct swapping, for the cases not going through the block optimization.
+    // SAFETY: the caller must guarantee that `x` and `y` are valid
+    // for writes, properly aligned, and non-overlapping.
+    unsafe {
+        let z = read(x);
+        copy_nonoverlapping(y, x, 1);
+        write(y, z);
     }
 }
 
@@ -1479,6 +1492,10 @@ fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }
 /// as all other references. This macro can create a raw pointer *without* creating
 /// a reference first.
 ///
+/// Note, however, that the `expr` in `addr_of!(expr)` is still subject to all
+/// the usual rules. In particular, `addr_of!(*ptr::null())` is Undefined
+/// Behavior because it dereferences a NULL pointer.
+///
 /// # Example
 ///
 /// ```
@@ -1495,6 +1512,10 @@ fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }
 /// let raw_f2 = ptr::addr_of!(packed.f2);
 /// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
 /// ```
+///
+/// See [`addr_of_mut`] for how to create a pointer to unininitialized data.
+/// Doing that with `addr_of` would not make much sense since one could only
+/// read the data, and that would be Undefined Behavior.
 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
 #[rustc_macro_transparency = "semitransparent"]
 #[allow_internal_unstable(raw_ref_op)]
@@ -1511,7 +1532,13 @@ pub macro addr_of($place:expr) {
 /// as all other references. This macro can create a raw pointer *without* creating
 /// a reference first.
 ///
-/// # Example
+/// Note, however, that the `expr` in `addr_of_mut!(expr)` is still subject to all
+/// the usual rules. In particular, `addr_of_mut!(*ptr::null_mut())` is Undefined
+/// Behavior because it dereferences a NULL pointer.
+///
+/// # Examples
+///
+/// **Creating a pointer to unaligned data:**
 ///
 /// ```
 /// use std::ptr;
@@ -1528,6 +1555,23 @@ pub macro addr_of($place:expr) {
 /// unsafe { raw_f2.write_unaligned(42); }
 /// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
 /// ```
+///
+/// **Creating a pointer to uninitialized data:**
+///
+/// ```rust
+/// use std::{ptr, mem::MaybeUninit};
+///
+/// struct Demo {
+///     field: bool,
+/// }
+///
+/// let mut uninit = MaybeUninit::<Demo>::uninit();
+/// // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
+/// // and thus be Undefined Behavior!
+/// let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
+/// unsafe { f1_ptr.write(true); }
+/// let init = unsafe { uninit.assume_init() };
+/// ```
 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
 #[rustc_macro_transparency = "semitransparent"]
 #[allow_internal_unstable(raw_ref_op)]