From a7bec568626f5f5296b965d7c392185a0c1b1ea2 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 8 Jun 2024 11:26:10 +0200
Subject: [PATCH] size_of_val_raw: for length 0 this is safe to call

---
 core/src/alloc/layout.rs | 2 ++
 core/src/mem/mod.rs      | 8 ++++++++
 2 files changed, 10 insertions(+)

diff --git a/core/src/alloc/layout.rs b/core/src/alloc/layout.rs
index 0b92767c93205..e96a41422a2af 100644
--- a/core/src/alloc/layout.rs
+++ b/core/src/alloc/layout.rs
@@ -183,6 +183,8 @@ impl Layout {
     ///     - a [slice], then the length of the slice tail must be an initialized
     ///       integer, and the size of the *entire value*
     ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
+    ///       For the special case where the dynamic tail length is 0, this function
+    ///       is safe to call.
     ///     - a [trait object], then the vtable part of the pointer must point
     ///       to a valid vtable for the type `T` acquired by an unsizing coercion,
     ///       and the size of the *entire value*
diff --git a/core/src/mem/mod.rs b/core/src/mem/mod.rs
index dd4b6e823434e..9bb4ba922cd44 100644
--- a/core/src/mem/mod.rs
+++ b/core/src/mem/mod.rs
@@ -359,6 +359,12 @@ pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
 ///     - a [slice], then the length of the slice tail must be an initialized
 ///       integer, and the size of the *entire value*
 ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
+///       For the special case where the dynamic tail length is 0, this function
+///       is safe to call.
+//        NOTE: the reason this is safe is that if an overflow were to occur already with size 0,
+//        then we would stop compilation as even the "statically known" part of the type would
+//        already be too big (or the call may be in dead code and optimized away, but then it
+//        doesn't matter).
 ///     - a [trait object], then the vtable part of the pointer must point
 ///       to a valid vtable acquired by an unsizing coercion, and the size
 ///       of the *entire value* (dynamic tail length + statically sized prefix)
@@ -506,6 +512,8 @@ pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
 ///     - a [slice], then the length of the slice tail must be an initialized
 ///       integer, and the size of the *entire value*
 ///       (dynamic tail length + statically sized prefix) must fit in `isize`.
+///       For the special case where the dynamic tail length is 0, this function
+///       is safe to call.
 ///     - a [trait object], then the vtable part of the pointer must point
 ///       to a valid vtable acquired by an unsizing coercion, and the size
 ///       of the *entire value* (dynamic tail length + statically sized prefix)