rust: init: make initializer values inaccessible after initializing

Previously the init macros would create a local variable with the name
and hygiene of the field that is being initialized to store the value of
the field. This would override any user defined variables. For example:
```
struct Foo {
    a: usize,
    b: usize,
}
let a = 10;
let foo = init!(Foo{
    a: a + 1, // This creates a local variable named `a`.
    b: a, // This refers to that variable!
});
let foo = Box::init!(foo)?;
assert_eq!(foo.a, 11);
assert_eq!(foo.b, 11);
```

This patch changes this behavior, so the above code would panic at the
last assertion, since `b` would have value 10.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-7-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: BennoLossin

rust/kernel/init/macros.rs

@@ -1075,13 +1075,13 @@ macro_rules! __init_internal {
         // In-place initialization syntax.
         @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
     ) => {
-        let $field = $val;
+        let init = $val;
         // Call the initializer.
         //
         // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
         // return when an error/panic occurs.
         // We also use the `data` to require the correct trait (`Init` or `PinInit`) for `$field`.
-        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), $field)? };
+        unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), init)? };
         // Create the drop guard:
         //
         // We rely on macro hygiene to make it impossible for users to access this local variable.
@@ -1107,12 +1107,12 @@ macro_rules! __init_internal {
         // In-place initialization syntax.
         @munch_fields($field:ident <- $val:expr, $($rest:tt)*),
     ) => {
-        let $field = $val;
+        let init = $val;
         // Call the initializer.
         //
         // SAFETY: `slot` is valid, because we are inside of an initializer closure, we
         // return when an error/panic occurs.
-        unsafe { $crate::init::Init::__init($field, ::core::ptr::addr_of_mut!((*$slot).$field))? };
+        unsafe { $crate::init::Init::__init(init, ::core::ptr::addr_of_mut!((*$slot).$field))? };
         // Create the drop guard:
         //
         // We rely on macro hygiene to make it impossible for users to access this local variable.
@@ -1138,11 +1138,13 @@ macro_rules! __init_internal {
         // Init by-value.
         @munch_fields($field:ident $(: $val:expr)?, $($rest:tt)*),
     ) => {
-        $(let $field = $val;)?
-        // Initialize the field.
-        //
-        // SAFETY: The memory at `slot` is uninitialized.
-        unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
+        {
+            $(let $field = $val;)?
+            // Initialize the field.
+            //
+            // SAFETY: The memory at `slot` is uninitialized.
+            unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
+        }
         // Create the drop guard:
         //
         // We rely on macro hygiene to make it impossible for users to access this local variable.