explain let_base_ptr

src/offset_of.rs

@@ -26,6 +26,10 @@
 macro_rules! let_base_ptr {
     ($name:ident, $type:tt) => {
         // No UB here, and the pointer does not dangle, either.
+        // But we have to make sure that `uninit` lives long enough,
+        // so it has to be in the same scope as `$name`. That's why
+        // `let_base_ptr` declares a variable (several, actually)
+        // instad of returning one.
         let uninit = $crate::mem::MaybeUninit::<$type>::uninit();
         let $name = uninit.as_ptr();
     };