use strict provenance api

stage_3/src/vga.rs

@@ -2,6 +2,7 @@ use core::{fmt, ptr};
 
 use crate::spin::Spinlock;
 
+const VGA_ADDR: usize = 0xb8000;
 const VGA_WIDTH: usize = 80;
 const VGA_HEIGHT: usize = 25;
 
@@ -28,7 +29,7 @@ macro_rules! vga_println {
 }
 
 pub fn vga_init() {
-  let vga_buf = unsafe { VgaBuf::new(0xb8000 as *mut u16) };
+  let vga_buf = unsafe { VgaBuf::new(ptr::with_exposed_provenance_mut::<u16>(VGA_ADDR)) };
   let mut guard = VGA.lock();
   *guard = Some(vga_buf);
 }