From c8e7c6261ea5bc581a3e10323677e720bf1dc42b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20K=C3=A5re=20Alsaker?= <john.kare.alsaker@gmail.com>
Date: Fri, 25 Apr 2025 07:14:27 +0200
Subject: [PATCH] Zero the buffer passed from `write_with`

---
 compiler/rustc_serialize/src/opaque/mem_encoder.rs | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_serialize/src/opaque/mem_encoder.rs b/compiler/rustc_serialize/src/opaque/mem_encoder.rs
index 5104df042157..56beebb49af7 100644
--- a/compiler/rustc_serialize/src/opaque/mem_encoder.rs
+++ b/compiler/rustc_serialize/src/opaque/mem_encoder.rs
@@ -35,8 +35,15 @@ impl MemEncoder {
         self.data.reserve(N);
 
         let old_len = self.data.len();
-        // SAFETY: fix
-        let buf = unsafe { &mut *(self.data.as_mut_ptr().add(old_len) as *mut [u8; N]) };
+
+        // SAFETY: The above `reserve` ensures that there is enough
+        // room to write the encoded value to the vector's internal buffer.
+        // The memory is also initialized as 0.
+        let buf = unsafe {
+            let buf = self.data.as_mut_ptr().add(old_len) as *mut [u8; N];
+            *buf = [0; N];
+            &mut *buf
+        };
         let written = visitor(buf);
         if written > N {
             Self::panic_invalid_write::<N>(written);