From 3d0ced0bd110550162db2c2292aaf082d1033648 Mon Sep 17 00:00:00 2001
From: Oleksandr Babak <alexanderbabak@proton.me>
Date: Tue, 28 Oct 2025 13:58:18 +0100
Subject: [PATCH 1/2] fix: make mutexes `!Send`

Each mutex is generated uniquely for each task, it is unsound to send
them between tasks. But they are `Send`. Before, it wasn't an issue,
because you couldn't share non-`'static` data between them, but with
\#1043 you can make the mutex `'static`. Thus we need to use actual
tools that Rust provides and out out from `Send`.

Currently, mutexes are simple ZSTs with `PhantomData<&'a ()>`, which is `Send`.
We replace it with `PhantomData<(&'a (), *const u8)>`, and return `Sync`
back via `unsafe` implementation. It is trivially sound, because mutexes have
no method methods that accept `&self`.

See https://doc.rust-lang.org/std/sync/struct.Exclusive.html for details.
---
 rtic-macros/src/codegen/shared_resources.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/rtic-macros/src/codegen/shared_resources.rs b/rtic-macros/src/codegen/shared_resources.rs
index 5e8ca9ae77ca..92657c8fe697 100644
--- a/rtic-macros/src/codegen/shared_resources.rs
+++ b/rtic-macros/src/codegen/shared_resources.rs
@@ -62,9 +62,14 @@ pub fn codegen(app: &App, analysis: &Analysis) -> TokenStream2 {
                 #[allow(non_camel_case_types)]
                 #(#cfgs)*
                 pub struct #shared_name<'a> {
-                    __rtic_internal_p: ::core::marker::PhantomData<&'a ()>,
+                    __rtic_internal_p: ::core::marker::PhantomData<(&'a (), *const u8)>,
                 }
 
+                // Opt out from `Send`.
+                // `#shared_name` is trivially `Sync` because there are no `&self` methods.
+                // See https://doc.rust-lang.org/std/sync/struct.Exclusive.html .
+                unsafe impl<'a> Sync for #shared_name<'a> {}
+
                 #(#cfgs)*
                 impl<'a> #shared_name<'a> {
                     #[inline(always)]

From 7b758e524eb70205222b4bfb1473de1b5b8000b0 Mon Sep 17 00:00:00 2001
From: Oleksandr Babak <alexanderbabak@proton.me>
Date: Tue, 28 Oct 2025 14:05:17 +0100
Subject: [PATCH 2/2] chore: changelog entry

---
 rtic-macros/CHANGELOG.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/rtic-macros/CHANGELOG.md b/rtic-macros/CHANGELOG.md
index f9a9445e5d07..2a2f44274894 100644
--- a/rtic-macros/CHANGELOG.md
+++ b/rtic-macros/CHANGELOG.md
@@ -7,6 +7,10 @@ For each category, *Added*, *Changed*, *Fixed* add new entries at the top!
 
 ## [Unreleased]
 
+### Fixed
+
+- Generated mutexes for shared resourses no longer implement `Send`. They are meant to be used inside a task locally.
+
 ### Added
 
 - Outer attributes applied to RTIC app module are now forwarded to the generated code.