Rollup merge of rust-lang#101455 - thomcc:why-is-this-here, r=jyn514

Avoid UB in the Windows filesystem code in... bootstrap?

This basically a subset of the changes from rust-lang#101171. I didn't think to look in src/bootstrap for more windows filesystem API usage, which was apparently a mistake on my part. It's kinda goofy that stuff like this is in here, but what are you gonna do, computers are awful.

I also added `winbase` to the `winapi` dep -- I tested this in a tmp crate but needed to add this to your Cargo.toml -- you `use winapi::stuff::winbase` in this function, but are relying on something else turning on that feature.

Author: Dylan-DPC

src/bootstrap/Cargo.toml

@@ -67,6 +67,7 @@ features = [
     "psapi",
     "impl-default",
     "timezoneapi",
+    "winbase",
 ]
 
 [dev-dependencies]

src/bootstrap/util.rs

@@ -197,9 +197,11 @@ pub fn symlink_dir(config: &Config, src: &Path, dest: &Path) -> io::Result<()> {
                 ptr::null_mut(),
             );
 
-            let mut data = [0u8; MAXIMUM_REPARSE_DATA_BUFFER_SIZE as usize];
-            let db = data.as_mut_ptr() as *mut REPARSE_MOUNTPOINT_DATA_BUFFER;
-            let buf = &mut (*db).ReparseTarget as *mut u16;
+            #[repr(C, align(8))]
+            struct Align8<T>(T);
+            let mut data = Align8([0u8; MAXIMUM_REPARSE_DATA_BUFFER_SIZE as usize]);
+            let db = data.0.as_mut_ptr() as *mut REPARSE_MOUNTPOINT_DATA_BUFFER;
+            let buf = core::ptr::addr_of_mut!((*db).ReparseTarget) as *mut u16;
             let mut i = 0;
             // FIXME: this conversion is very hacky
             let v = br"\??\";
@@ -219,7 +221,7 @@ pub fn symlink_dir(config: &Config, src: &Path, dest: &Path) -> io::Result<()> {
             let res = DeviceIoControl(
                 h as *mut _,
                 FSCTL_SET_REPARSE_POINT,
-                data.as_ptr() as *mut _,
+                db.cast(),
                 (*db).ReparseDataLength + 8,
                 ptr::null_mut(),
                 0,