Rollup merge of #111282 - scottmcm:remove-unneeded-assumes, r=workingjubilee

Remove some `assume`s from slice iterators that don't do anything

Because the start pointer is iterators is already a `NonNull`, we emit the appropriate `!nonnull` metadata when loading the pointer to tell LLVM that it's non-null.

Probably the best way to see that it's the metadata that's important (and not the `assume`) is to observe that LLVM actually *removes* the `assume` from the optimized IR: <https://rust.godbolt.org/z/KhE6G963n>.

(I also checked that, yes, the if-not-ZST `assume` on `end` is still doing something: it's how there's a `!nonnull` metadata on its load, even though it's an ordinary raw pointer.  The codegen test added in this PR fails if the other `assume` is  removed.)

Author: matthiaskrgr

core/src/slice/iter/macros.rs

@@ -124,12 +124,10 @@ macro_rules! iterator {
             fn next(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
 
-                // SAFETY: `assume` calls are safe since a slice's start pointer
-                // must be non-null, and slices over non-ZSTs must also have a
-                // non-null end pointer. The call to `next_unchecked!` is safe
-                // since we check if the iterator is empty first.
+                // SAFETY: `assume` call is safe because slices over non-ZSTs must
+                // have a non-null end pointer. The call to `next_unchecked!` is
+                // safe since we check if the iterator is empty first.
                 unsafe {
-                    assume(!self.ptr.as_ptr().is_null());
                     if !<T>::IS_ZST {
                         assume(!self.end.is_null());
                     }
@@ -339,12 +337,10 @@ macro_rules! iterator {
             fn next_back(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
 
-                // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null,
-                // and slices over non-ZSTs must also have a non-null end pointer.
-                // The call to `next_back_unchecked!` is safe since we check if the iterator is
-                // empty first.
+                // SAFETY: `assume` call is safe because slices over non-ZSTs must
+                // have a non-null end pointer. The call to `next_back_unchecked!`
+                // is safe since we check if the iterator is empty first.
                 unsafe {
-                    assume(!self.ptr.as_ptr().is_null());
                     if !<T>::IS_ZST {
                         assume(!self.end.is_null());
                     }