Auto merge of rust-lang#111395 - scottmcm:slice-iter-zst-experiment, r=the8472

Simplify the implementation of iterators over slices of ZSTs

Currently, slice iterators over ZSTs store `end = start.wrapping_byte_add(len)`.

That's slightly convenient for `is_empty`, but kinda annoying for pretty much everything else -- see bugs like rust-lang#42789, for example.

This PR instead changes it to just `end = ptr::invalid(len)` instead.

That's easier to think about (IMHO, at least) as well as easier to represent.

`next` is still to big to get inlined into the mir-opt/pre-codegen/ tests, but if I bump the inline threshold to force it to show the whole thing, this implementation is also less MIR:
```
> git diff --numstat
241     370     tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.mir
255     329     tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.mir
184     216     tests/mir-opt/pre-codegen/slice_iter.slice_iter_mut_next_back.PreCodegen.after.mir
182     254     tests/mir-opt/pre-codegen/slice_iter.slice_iter_next.PreCodegen.after.mir
```
(That's ≈70 lines less for `Iter::next`, for example.)

r? `@ghost`

~~Built atop rust-lang#111282, so draft until that lands.~~

Author: bors

library/core/src/ptr/non_null.rs

@@ -449,6 +449,19 @@ impl<T: ?Sized> NonNull<T> {
         // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null
         unsafe { NonNull::new_unchecked(self.as_ptr() as *mut U) }
     }
+
+    /// See [`pointer::add`] for semantics and safety requirements.
+    #[inline]
+    pub(crate) const unsafe fn add(self, delta: usize) -> Self
+    where
+        T: Sized,
+    {
+        // SAFETY: We require that the delta stays in-bounds of the object, and
+        // thus it cannot become null, as that would require wrapping the
+        // address space, which no legal objects are allowed to do.
+        // And the caller promised the `delta` is sound to add.
+        unsafe { NonNull { pointer: self.pointer.add(delta) } }
+    }
 }
 
 impl<T> NonNull<[T]> {

library/core/src/slice/iter.rs

@@ -13,7 +13,7 @@ use crate::iter::{
 use crate::marker::{PhantomData, Send, Sized, Sync};
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZeroUsize;
-use crate::ptr::NonNull;
+use crate::ptr::{invalid, invalid_mut, NonNull};
 
 use super::{from_raw_parts, from_raw_parts_mut};
 
@@ -67,9 +67,7 @@ pub struct Iter<'a, T: 'a> {
     ptr: NonNull<T>,
     /// For non-ZSTs, the non-null pointer to the past-the-end element.
     ///
-    /// For ZSTs, this is `ptr.wrapping_byte_add(len)`.
-    ///
-    /// For all types, `ptr == end` tests whether the iterator is empty.
+    /// For ZSTs, this is `ptr::invalid(len)`.
     end: *const T,
     _marker: PhantomData<&'a T>,
 }
@@ -94,8 +92,7 @@ impl<'a, T> Iter<'a, T> {
         unsafe {
             assume(!ptr.is_null());
 
-            let end =
-                if T::IS_ZST { ptr.wrapping_byte_add(slice.len()) } else { ptr.add(slice.len()) };
+            let end = if T::IS_ZST { invalid(slice.len()) } else { ptr.add(slice.len()) };
 
             Self { ptr: NonNull::new_unchecked(ptr as *mut T), end, _marker: PhantomData }
         }
@@ -193,9 +190,7 @@ pub struct IterMut<'a, T: 'a> {
     ptr: NonNull<T>,
     /// For non-ZSTs, the non-null pointer to the past-the-end element.
     ///
-    /// For ZSTs, this is `ptr.wrapping_byte_add(len)`.
-    ///
-    /// For all types, `ptr == end` tests whether the iterator is empty.
+    /// For ZSTs, this is `ptr::invalid_mut(len)`.
     end: *mut T,
     _marker: PhantomData<&'a mut T>,
 }
@@ -235,8 +230,7 @@ impl<'a, T> IterMut<'a, T> {
         unsafe {
             assume(!ptr.is_null());
 
-            let end =
-                if T::IS_ZST { ptr.wrapping_byte_add(slice.len()) } else { ptr.add(slice.len()) };
+            let end = if T::IS_ZST { invalid_mut(slice.len()) } else { ptr.add(slice.len()) };
 
             Self { ptr: NonNull::new_unchecked(ptr), end, _marker: PhantomData }
         }

library/core/src/slice/iter/macros.rs

@@ -1,28 +1,44 @@
 //! Macros used by iterators of slice.
 
+// Shrinks the iterator when T is a ZST, setting the length to `new_len`.
+// `new_len` must not exceed `self.len()`.
+macro_rules! zst_set_len {
+    ($self: ident, $new_len: expr) => {{
+        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
+
+        // SAFETY: same as `invalid(_mut)`, but the macro doesn't know
+        // which versions of that function to call, so open-code it.
+        $self.end = unsafe { mem::transmute::<usize, _>($new_len) };
+    }};
+}
+
+// Shrinks the iterator when T is a ZST, reducing the length by `n`.
+// `n` must not exceed `self.len()`.
+macro_rules! zst_shrink {
+    ($self: ident, $n: ident) => {
+        let new_len = $self.end.addr() - $n;
+        zst_set_len!($self, new_len);
+    };
+}
+
 // Inlining is_empty and len makes a huge performance difference
 macro_rules! is_empty {
-    // The way we encode the length of a ZST iterator, this works both for ZST
-    // and non-ZST.
     ($self: ident) => {
-        $self.ptr.as_ptr() as *const T == $self.end
+        if T::IS_ZST { $self.end.addr() == 0 } else { $self.ptr.as_ptr() as *const _ == $self.end }
     };
 }
 
 macro_rules! len {
     ($self: ident) => {{
         #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
 
-        let start = $self.ptr;
         if T::IS_ZST {
-            // This _cannot_ use `ptr_sub` because we depend on wrapping
-            // to represent the length of long ZST slice iterators.
-            $self.end.addr().wrapping_sub(start.as_ptr().addr())
+            $self.end.addr()
         } else {
             // To get rid of some bounds checks (see `position`), we use ptr_sub instead of
             // offset_from (Tested by `codegen/slice-position-bounds-check`.)
             // SAFETY: by the type invariant pointers are aligned and `start <= end`
-            unsafe { $self.end.sub_ptr(start.as_ptr()) }
+            unsafe { $self.end.sub_ptr($self.ptr.as_ptr()) }
         }
     }};
 }
@@ -50,14 +66,6 @@ macro_rules! iterator {
             ($self: ident) => {& $( $mut_ )? *$self.pre_dec_end(1)}
         }
 
-        // Shrinks the iterator when T is a ZST, by moving the end of the iterator
-        // backwards by `n`. `n` must not exceed `self.len()`.
-        macro_rules! zst_shrink {
-            ($self: ident, $n: ident) => {
-                $self.end = $self.end.wrapping_byte_sub($n);
-            }
-        }
-
         impl<'a, T> $name<'a, T> {
             // Helper function for creating a slice from the iterator.
             #[inline(always)]
@@ -73,16 +81,15 @@ macro_rules! iterator {
             // Unsafe because the offset must not exceed `self.len()`.
             #[inline(always)]
             unsafe fn post_inc_start(&mut self, offset: usize) -> * $raw_mut T {
+                let old = self.ptr;
                 if T::IS_ZST {
                     zst_shrink!(self, offset);
-                    self.ptr.as_ptr()
                 } else {
-                    let old = self.ptr.as_ptr();
                     // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                     // so this new pointer is inside `self` and thus guaranteed to be non-null.
-                    self.ptr = unsafe { NonNull::new_unchecked(self.ptr.as_ptr().add(offset)) };
-                    old
+                    self.ptr = unsafe { self.ptr.add(offset) };
                 }
+                old.as_ptr()
             }
 
             // Helper function for moving the end of the iterator backwards by `offset` elements,
@@ -155,9 +162,7 @@ macro_rules! iterator {
                 if n >= len!(self) {
                     // This iterator is now empty.
                     if T::IS_ZST {
-                        // We have to do it this way as `ptr` may never be 0, but `end`
-                        // could be (due to wrapping).
-                        self.end = self.ptr.as_ptr();
+                        zst_set_len!(self, 0);
                     } else {
                         // SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
                         unsafe {
@@ -356,7 +361,11 @@ macro_rules! iterator {
             fn nth_back(&mut self, n: usize) -> Option<$elem> {
                 if n >= len!(self) {
                     // This iterator is now empty.
-                    self.end = self.ptr.as_ptr();
+                    if T::IS_ZST {
+                        zst_set_len!(self, 0);
+                    } else {
+                        self.end = self.ptr.as_ptr();
+                    }
                     return None;
                 }
                 // SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.