More IS_ZST in library

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

d19b64fb54/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.

library/alloc/src/boxed/thin.rs

@@ -7,7 +7,7 @@
 use core::marker::PhantomData;
 #[cfg(not(no_global_oom_handling))]
 use core::marker::Unsize;
-use core::mem;
+use core::mem::{self, SizedTypeProperties};
 use core::ops::{Deref, DerefMut};
 use core::ptr::Pointee;
 use core::ptr::{self, NonNull};
@@ -202,9 +202,7 @@ fn new<T>(header: H, value: T) -> WithHeader<H> {
             let ptr = if layout.size() == 0 {
                 // Some paranoia checking, mostly so that the ThinBox tests are
                 // more able to catch issues.
-                debug_assert!(
-                    value_offset == 0 && mem::size_of::<T>() == 0 && mem::size_of::<H>() == 0
-                );
+                debug_assert!(value_offset == 0 && T::IS_ZST && H::IS_ZST);
                 layout.dangling()
             } else {
                 let ptr = alloc::alloc(layout);
@@ -249,9 +247,7 @@ fn drop(&mut self) {
                         alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
                     } else {
                         debug_assert!(
-                            value_offset == 0
-                                && mem::size_of::<H>() == 0
-                                && self.value_layout.size() == 0
+                            value_offset == 0 && H::IS_ZST && self.value_layout.size() == 0
                         );
                     }
                 }

library/alloc/src/vec/drain.rs

@@ -112,9 +112,7 @@ pub fn keep_rest(self) {
             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

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 @@ pub fn keep_rest(self) {
 
         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);

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

@@ -73,7 +73,7 @@ fn make_slice(&self) -> &'a [T] {
             // Unsafe because the offset must not exceed `self.len()`.
             #[inline(always)]
             unsafe fn post_inc_start(&mut self, offset: usize) -> * $raw_mut T {
-                if mem::size_of::<T>() == 0 {
+                if T::IS_ZST {
                     zst_shrink!(self, offset);
                     self.ptr.as_ptr()
                 } else {