Use the newest Layout::array size checks for vec-alloc

Layout::array was updated in https://github.com/rust-lang/rust/commit/a32305a8
to check that array sizes fit in isize::MAX, so I've propogated the
change to vec-alloc here.

src/vec/vec-alloc.md

@@ -170,20 +170,17 @@ use std::alloc::{self, Layout};
 impl<T> Vec<T> {
     fn grow(&mut self) {
         let (new_cap, new_layout) = if self.cap == 0 {
-            (1, Layout::array::<T>(1).unwrap())
+            (1, Layout::array::<T>(1))
         } else {
             // This can't overflow since self.cap <= isize::MAX.
             let new_cap = 2 * self.cap;
-
-            // `Layout::array` checks that the number of bytes is <= usize::MAX,
-            // but this is redundant since old_layout.size() <= isize::MAX,
-            // so the `unwrap` should never fail.
-            let new_layout = Layout::array::<T>(new_cap).unwrap();
-            (new_cap, new_layout)
+            (new_cap, Layout::array::<T>(new_cap))
         };
 
-        // Ensure that the new allocation doesn't exceed `isize::MAX` bytes.
-        assert!(new_layout.size() <= isize::MAX as usize, "Allocation too large");
+        // `Layout::array` checks that the number of bytes allocated is
+        // in 1..=isize::MAX and will error otherwise.  An allocation of
+        // 0 bytes isn't possible thanks to the above condition.
+        let new_layout = new_layout.expect("Allocation too large");
 
         let new_ptr = if self.cap == 0 {
             unsafe { alloc::alloc(new_layout) }

src/vec/vec-final.md

@@ -33,23 +33,17 @@ impl<T> RawVec<T> {
         assert!(mem::size_of::<T>() != 0, "capacity overflow");
 
         let (new_cap, new_layout) = if self.cap == 0 {
-            (1, Layout::array::<T>(1).unwrap())
+            (1, Layout::array::<T>(1))
         } else {
-            // This can't overflow because we ensure self.cap <= isize::MAX.
+            // This can't overflow since self.cap <= isize::MAX.
             let new_cap = 2 * self.cap;
-
-            // `Layout::array` checks that the number of bytes is <= usize::MAX,
-            // but this is redundant since old_layout.size() <= isize::MAX,
-            // so the `unwrap` should never fail.
-            let new_layout = Layout::array::<T>(new_cap).unwrap();
-            (new_cap, new_layout)
+            (new_cap, Layout::array::<T>(new_cap))
         };
 
-        // Ensure that the new allocation doesn't exceed `isize::MAX` bytes.
-        assert!(
-            new_layout.size() <= isize::MAX as usize,
-            "Allocation too large"
-        );
+        // `Layout::array` checks that the number of bytes allocated is
+        // in 1..=isize::MAX and will error otherwise.  An allocation of
+        // 0 bytes isn't possible thanks to the above condition.
+        let new_layout = new_layout.expect("Allocation too large");
 
         let new_ptr = if self.cap == 0 {
             unsafe { alloc::alloc(new_layout) }