Auto merge of #135344 - scottmcm:safe-dangling, r=joboet

Less unsafe in `dangling`/`without_provenance`

This PR was inspired by the new `NonNull::without_provenance` (cc https://github.com/rust-lang/rust/issues/135243#issuecomment-2583913562) since it made me realize that we could write `NonNull::dangling` in completely-safe code using other existing things.

Then doing that led me to a few more places that could be simplified, like now that GVN will optimize Transmute-then-PtrToPtr, we can just implement `ptr::without_provenance` by calling `ptr::without_provenance_mut` since the shipped rlib of `core` ends up with the same single statement as the implementation (thanks to GVN merging the steps) and thus there's no need to duplicate the `transmute` -- and more importantly, no need to repeat a long safety comment.

There did end up being a couple of other changes needed to avoid exploding certain bits of MIR, though -- like `<Box<[i32]>>::default()`'s MIR originally got way worse as certain things didn't inline, or had a bunch of extraneous UbChecks -- so there's a couple of other changes to solve that.

1 files changed, 8 insertions, 6 deletions

diff --git a/tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir
index e7ddacf3144..6ed8ef9715b 100644
--- a/tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir
+++ b/tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir
@@ -18,7 +18,7 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
         scope 2 {
             debug x => _17;
         }
-        scope 18 (inlined <Rev<std::slice::Iter<'_, T>> as Iterator>::next) {
+        scope 19 (inlined <Rev<std::slice::Iter<'_, T>> as Iterator>::next) {
             let mut _14: &mut std::slice::Iter<'_, T>;
         }
     }
@@ -35,10 +35,12 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
                     scope 7 {
                     }
                     scope 12 (inlined without_provenance::<T>) {
+                        scope 13 (inlined without_provenance_mut::<T>) {
+                        }
                     }
-                    scope 13 (inlined NonNull::<T>::as_ptr) {
+                    scope 14 (inlined NonNull::<T>::as_ptr) {
                     }
-                    scope 14 (inlined std::ptr::mut_ptr::<impl *mut T>::add) {
+                    scope 15 (inlined std::ptr::mut_ptr::<impl *mut T>::add) {
                     }
                 }
                 scope 8 (inlined <NonNull<[T]> as From<&[T]>>::from) {
@@ -54,11 +56,11 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
             }
         }
     }
-    scope 15 (inlined <std::slice::Iter<'_, T> as Iterator>::rev) {
-        scope 16 (inlined Rev::<std::slice::Iter<'_, T>>::new) {
+    scope 16 (inlined <std::slice::Iter<'_, T> as Iterator>::rev) {
+        scope 17 (inlined Rev::<std::slice::Iter<'_, T>>::new) {
         }
     }
-    scope 17 (inlined <Rev<std::slice::Iter<'_, T>> as IntoIterator>::into_iter) {
+    scope 18 (inlined <Rev<std::slice::Iter<'_, T>> as IntoIterator>::into_iter) {
     }
 
     bb0: {