From b307ad614c723d8d4967b8c5eabd64f6d947f616 Mon Sep 17 00:00:00 2001
From: Amos Wenger <amos@bearcove.net>
Date: Sun, 20 Oct 2024 17:10:43 +0200
Subject: [PATCH] Don't allow trivial UB via FieldSlot in safe code

cf. https://x.com/qdoit_/status/1848004580115726710, thanks qdoit_ and kupiakos
on twitter!
---
 merde/src/lib.rs              | 6 ++----
 merde_core/src/deserialize.rs | 5 +++--
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/merde/src/lib.rs b/merde/src/lib.rs
index f7d53e8..9e2e8fd 100644
--- a/merde/src/lib.rs
+++ b/merde/src/lib.rs
@@ -100,8 +100,7 @@ macro_rules! impl_deserialize {
                 Ok($struct_name {
                     $($field: {
                         if $field.is_none() {
-                            let __field_type_name = std::any::type_name_of_val(&$field);
-                            let __slot = $crate::FieldSlot::new(&mut $field, __field_type_name);
+                            let __slot = $crate::FieldSlot::new(&mut $field);
                             __opinions.default_field_value(stringify!($field), __slot);
                         }
                         $crate::Deserialize::from_option($field, stringify!($field).into())?
@@ -163,8 +162,7 @@ macro_rules! impl_deserialize {
                 Ok($struct_name {
                     $($field: {
                         if $field.is_none() {
-                            let __field_type_name = std::any::type_name_of_val(&$field);
-                            let __slot = $crate::FieldSlot::new(&mut $field, __field_type_name);
+                            let __slot = $crate::FieldSlot::new(&mut $field);
                             __opinions.default_field_value(stringify!($field), __slot);
                         }
                         $crate::Deserialize::from_option($field, stringify!($field).into())?
diff --git a/merde_core/src/deserialize.rs b/merde_core/src/deserialize.rs
index ec9c359..bb101cf 100644
--- a/merde_core/src/deserialize.rs
+++ b/merde_core/src/deserialize.rs
@@ -317,12 +317,13 @@ pub struct FieldSlot<'s, 'borrow> {
 impl<'s, 'borrow> FieldSlot<'s, 'borrow> {
     /// Construct a new `FieldSlot`, ready to be filled
     #[inline(always)]
-    pub fn new<T: 's>(option: &'borrow mut Option<T>, type_name_of_slot: &'static str) -> Self {
+    #[doc(hidden)]
+    pub fn new<T: 's>(option: &'borrow mut Option<T>) -> Self {
         Self {
             option: unsafe {
                 std::mem::transmute::<*mut Option<T>, *mut Option<()>>(option as *mut _)
             },
-            type_name_of_option_field: type_name_of_slot,
+            type_name_of_option_field: std::any::type_name::<Option<T>>(),
             _phantom: PhantomData,
         }
     }