Rollup merge of #110635 - scottmcm:zst-checks, r=the8472

More `IS_ZST` in `library`

I noticed that `post_inc_start` and `pre_dec_end` were doing this check in different ways

https://github.com/rust-lang/rust/blob/d19b64fb54391b64ce99981577c67c93ac2a9ffa/library/core/src/slice/iter/macros.rs#L76-L93

so started making this PR, then added a few more I found since I was already making changes anyway.

2 files changed, 3 insertions, 7 deletions

diff --git a/library/alloc/src/vec/drain.rs b/library/alloc/src/vec/drain.rs
index e3ca6eb7833..3091efabd68 100644
--- a/library/alloc/src/vec/drain.rs
+++ b/library/alloc/src/vec/drain.rs
@@ -112,9 +112,7 @@ impl<'a, T, A: Allocator> Drain<'a, T, A> {
             let unyielded_ptr = this.iter.as_slice().as_ptr();
 
             // ZSTs have no identity, so we don't need to move them around.
-            let needs_move = mem::size_of::<T>() != 0;
-
-            if needs_move {
+            if !T::IS_ZST {
                 let start_ptr = source_vec.as_mut_ptr().add(start);
 
                 // memmove back unyielded elements
diff --git a/library/alloc/src/vec/drain_filter.rs b/library/alloc/src/vec/drain_filter.rs
index 8c03f1692d9..650f9213890 100644
--- a/library/alloc/src/vec/drain_filter.rs
+++ b/library/alloc/src/vec/drain_filter.rs
@@ -1,5 +1,5 @@
 use crate::alloc::{Allocator, Global};
-use core::mem::{self, ManuallyDrop};
+use core::mem::{ManuallyDrop, SizedTypeProperties};
 use core::ptr;
 use core::slice;
 
@@ -96,9 +96,7 @@ where
 
         unsafe {
             // ZSTs have no identity, so we don't need to move them around.
-            let needs_move = mem::size_of::<T>() != 0;
-
-            if needs_move && this.idx < this.old_len && this.del > 0 {
+            if !T::IS_ZST && this.idx < this.old_len && this.del > 0 {
                 let ptr = this.vec.as_mut_ptr();
                 let src = ptr.add(this.idx);
                 let dst = src.sub(this.del);