Improve core::alloc coverage

Author: pvdrz

library/coretests/tests/alloc.rs

@@ -55,6 +55,23 @@ fn layout_array_edge_cases() {
     }
 }
 
+#[test]
+fn layout_errors() {
+    let layout = Layout::new::<[u8; 2]>();
+    // Should error if the alignment is not a power of two.
+    assert!(layout.align_to(3).is_err());
+
+    // The remaining assertions ensure that the methods error on arithmetic overflow as the
+    // alignment cannot overflow `isize`.
+    assert!(layout.align_to(isize::MAX as usize + 1).is_err());
+    assert!(layout.repeat(usize::MAX).is_err());
+
+    assert!(layout.repeat_packed(usize::MAX).is_err());
+
+    let next = Layout::from_size_align(isize::MAX as usize, 1).unwrap();
+    assert!(layout.extend(next).is_err());
+}
+
 #[test]
 fn layout_debug_shows_log2_of_alignment() {
     // `Debug` is not stable, but here's what it does right now