From 6aa0fb450f417ce899b43f4539eb226b391a0f2e Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Thu, 8 Apr 2021 18:25:09 +0200
Subject: [PATCH 1/2] Goodbye static mut

---
 macros/src/codegen.rs                  |  2 +-
 macros/src/codegen/dispatchers.rs      | 12 ++++--
 macros/src/codegen/locals.rs           |  6 +--
 macros/src/codegen/module.rs           | 38 ++++++++++------
 macros/src/codegen/post_init.rs        | 15 +++++--
 macros/src/codegen/pre_init.rs         |  6 ++-
 macros/src/codegen/resources.rs        | 60 ++++++++++++++++++++------
 macros/src/codegen/resources_struct.rs |  6 +--
 macros/src/codegen/software_tasks.rs   | 14 +++---
 macros/src/codegen/timer_queue.rs      | 17 ++++----
 src/lib.rs                             | 28 ++++++++++++
 11 files changed, 145 insertions(+), 59 deletions(-)

diff --git a/macros/src/codegen.rs b/macros/src/codegen.rs
index cf728a7d3f..167ba500a6 100644
--- a/macros/src/codegen.rs
+++ b/macros/src/codegen.rs
@@ -125,7 +125,7 @@ pub fn app(app: &App, analysis: &Analysis, extra: &Extra) -> TokenStream2 {
                         rtic::export::interrupt::free(|_| {
                             use rtic::Monotonic as _;
                             use rtic::time::Clock as _;
-                            if let Some(m) = unsafe{ #app_path::#ident.as_ref() } {
+                            if let Some(m) = unsafe{ #app_path::#ident.get_mut_unchecked() } {
                                 if let Ok(v) = m.try_now() {
                                     v
                                 } else {
diff --git a/macros/src/codegen/dispatchers.rs b/macros/src/codegen/dispatchers.rs
index dc33b1af60..6aca901aea 100644
--- a/macros/src/codegen/dispatchers.rs
+++ b/macros/src/codegen/dispatchers.rs
@@ -59,7 +59,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
         // );
         items.push(quote!(
             #[doc(hidden)]
-            static mut #rq: #rq_ty = #rq_expr;
+            static #rq: rtic::RacyCell<#rq_ty> = rtic::RacyCell::new(#rq_expr);
         ));
 
         let arms = channel
@@ -86,8 +86,12 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
                     #(#cfgs)*
                     #t::#name => {
                         let #tupled =
-                            #inputs.get_unchecked(usize::from(index)).as_ptr().read();
-                        #fq.split().0.enqueue_unchecked(index);
+                            #inputs
+                            .get_unchecked()
+                            .get_unchecked(usize::from(index))
+                            .as_ptr()
+                            .read();
+                        #fq.get_mut_unchecked().split().0.enqueue_unchecked(index);
                         let priority = &rtic::export::Priority::new(PRIORITY);
                         #app_path::#name(
                             #locals_new
@@ -100,7 +104,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             .collect::<Vec<_>>();
 
         stmts.push(quote!(
-            while let Some((task, index)) = #rq.split().1.dequeue() {
+            while let Some((task, index)) = #rq.get_mut_unchecked().split().1.dequeue() {
                 match task {
                     #(#arms)*
                 }
diff --git a/macros/src/codegen/locals.rs b/macros/src/codegen/locals.rs
index 5725a1513f..0fb8c6d298 100644
--- a/macros/src/codegen/locals.rs
+++ b/macros/src/codegen/locals.rs
@@ -50,11 +50,11 @@ pub fn codegen(
         items.push(quote!(
             #(#cfgs)*
             #[doc(hidden)]
-            static mut #name: #ty = #expr
+            static #name: rtic::RacyCell<#ty> = rtic::RacyCell::new(#expr)
         ));
         values.push(quote!(
             #(#cfgs)*
-            #name: &mut #name
+            #name: #name.get_mut_unchecked()
         ));
         names.push(name);
         pats.push(quote!(
@@ -64,7 +64,7 @@ pub fn codegen(
     }
 
     if lt.is_some() && has_cfgs {
-        fields.push(quote!(__marker__: core::marker::PhantomData<&'a mut ()>));
+        fields.push(quote!(__marker__: core::marker::PhantomData<&'a ()>));
         values.push(quote!(__marker__: core::marker::PhantomData));
     }
 
diff --git a/macros/src/codegen/module.rs b/macros/src/codegen/module.rs
index 50146c0214..8c148ccb14 100644
--- a/macros/src/codegen/module.rs
+++ b/macros/src/codegen/module.rs
@@ -245,14 +245,15 @@ pub fn codegen(
             let input = #tupled;
 
             unsafe {
-                if let Some(index) = rtic::export::interrupt::free(|_| #app_path::#fq.dequeue()) {
+                if let Some(index) = rtic::export::interrupt::free(|_| #app_path::#fq.get_mut_unchecked().dequeue()) {
                     #app_path::#inputs
+                        .get_mut_unchecked()
                         .get_unchecked_mut(usize::from(index))
                         .as_mut_ptr()
                         .write(input);
 
                     rtic::export::interrupt::free(|_| {
-                        #app_path::#rq.enqueue_unchecked((#app_path::#t::#name, index));
+                        #app_path::#rq.get_mut_unchecked().enqueue_unchecked((#app_path::#t::#name, index));
                     });
 
                     rtic::pend(#device::#enum_::#interrupt);
@@ -304,6 +305,9 @@ pub fn codegen(
             let user_imports = &app.user_imports;
             let tq_marker = util::mark_internal_ident(&util::timer_queue_marker_ident());
 
+            // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
+            // items.push(quote!(#[doc = #doc]));
+
             items.push(quote!(
             /// Holds methods related to this monotonic
             pub mod #m {
@@ -325,12 +329,16 @@ pub fn codegen(
                 impl SpawnHandle {
                     pub fn cancel(self) -> Result<#ty, ()> {
                         rtic::export::interrupt::free(|_| unsafe {
-                            let tq = &mut *#app_path::#tq.as_mut_ptr();
+                            let tq = &mut *#app_path::#tq.get_mut_unchecked().as_mut_ptr();
                             if let Some((_task, index)) = tq.cancel_marker(self.marker) {
                                 // Get the message
-                                let msg = #app_path::#inputs.get_unchecked(usize::from(index)).as_ptr().read();
+                                let msg = #app_path::#inputs
+                                    .get_unchecked()
+                                    .get_unchecked(usize::from(index))
+                                    .as_ptr()
+                                    .read();
                                 // Return the index to the free queue
-                                #app_path::#fq.split().0.enqueue_unchecked(index);
+                                #app_path::#fq.get_mut_unchecked().split().0.enqueue_unchecked(index);
 
                                 Ok(msg)
                             } else {
@@ -350,10 +358,10 @@ pub fn codegen(
                     pub fn reschedule_at(self, instant: rtic::time::Instant<#app_path::#mono_type>) -> Result<Self, ()>
                     {
                         rtic::export::interrupt::free(|_| unsafe {
-                            let marker = #tq_marker;
-                            #tq_marker = #tq_marker.wrapping_add(1);
+                            let marker = *#tq_marker.get_mut_unchecked();
+                            *#tq_marker.get_mut_unchecked() = #tq_marker.get_mut_unchecked().wrapping_add(1);
 
-                            let tq = &mut *#app_path::#tq.as_mut_ptr();
+                            let tq = &mut *#app_path::#tq.get_mut_unchecked().as_mut_ptr();
 
                             tq.update_marker(self.marker, marker, instant, || #pend).map(|_| SpawnHandle { marker })
                         })
@@ -373,7 +381,7 @@ pub fn codegen(
                         D::T: Into<<#app_path::#mono_type as rtic::time::Clock>::T>,
                 {
 
-                    let instant = if rtic::export::interrupt::free(|_| unsafe { #app_path::#m_ident.is_none() }) {
+                    let instant = if rtic::export::interrupt::free(|_| unsafe { #app_path::#m_ident.get_mut_unchecked().is_none() }) {
                         rtic::time::Instant::new(0)
                     } else {
                         #app_path::monotonics::#m::now()
@@ -390,19 +398,21 @@ pub fn codegen(
                 ) -> Result<SpawnHandle, #ty> {
                     unsafe {
                         let input = #tupled;
-                        if let Some(index) = rtic::export::interrupt::free(|_| #app_path::#fq.dequeue()) {
+                        if let Some(index) = rtic::export::interrupt::free(|_| #app_path::#fq.get_mut_unchecked().dequeue()) {
                             #app_path::#inputs
+                                .get_mut_unchecked()
                                 .get_unchecked_mut(usize::from(index))
                                 .as_mut_ptr()
                                 .write(input);
 
                             #app_path::#instants
+                                .get_mut_unchecked()
                                 .get_unchecked_mut(usize::from(index))
                                 .as_mut_ptr()
                                 .write(instant);
 
                             rtic::export::interrupt::free(|_| {
-                                let marker = #tq_marker;
+                                let marker = *#tq_marker.get_mut_unchecked();
                                 let nr = rtic::export::NotReady {
                                     instant,
                                     index,
@@ -410,15 +420,15 @@ pub fn codegen(
                                     marker,
                                 };
 
-                                #tq_marker = #tq_marker.wrapping_add(1);
+                                *#tq_marker.get_mut_unchecked() = #tq_marker.get_mut_unchecked().wrapping_add(1);
 
-                                let tq = unsafe { &mut *#app_path::#tq.as_mut_ptr() };
+                                let tq = &mut *#app_path::#tq.get_mut_unchecked().as_mut_ptr();
 
                                 tq.enqueue_unchecked(
                                     nr,
                                     || #enable_interrupt,
                                     || #pend,
-                                    #app_path::#m_ident.as_mut());
+                                    #app_path::#m_ident.get_mut_unchecked().as_mut());
 
                                 Ok(SpawnHandle { marker })
                             })
diff --git a/macros/src/codegen/post_init.rs b/macros/src/codegen/post_init.rs
index 96c5df80f9..7dd1590338 100644
--- a/macros/src/codegen/post_init.rs
+++ b/macros/src/codegen/post_init.rs
@@ -17,16 +17,23 @@ pub fn codegen(app: &App, analysis: &Analysis) -> Vec<TokenStream2> {
             // If it's live
             let cfgs = app.late_resources[name].cfgs.clone();
             if analysis.locations.get(name).is_some() {
-                // Need to also include the cfgs
                 stmts.push(quote!(
-                #(#cfgs)*
-                #mangled_name.as_mut_ptr().write(late.#name);
+                    // We include the cfgs
+                    #(#cfgs)*
+                    // Late resource is a RacyCell<MaybeUninit<T>>
+                    // - `get_mut_unchecked` to obtain `MaybeUninit<T>`
+                    // - `as_mut_ptr` to obtain a raw pointer to `MaybeUninit<T>`
+                    // - `write` the defined value for the late resource T
+                    #mangled_name.get_mut_unchecked().as_mut_ptr().write(late.#name);
                 ));
             }
         }
     }
 
     for (i, (monotonic, _)) in app.monotonics.iter().enumerate() {
+        // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
+        // stmts.push(quote!(#[doc = #doc]));
+
         let idx = Index {
             index: i as u32,
             span: Span::call_site(),
@@ -36,7 +43,7 @@ pub fn codegen(app: &App, analysis: &Analysis) -> Vec<TokenStream2> {
         // Store the monotonic
         let name = util::monotonic_ident(&monotonic.to_string());
         let name = util::mark_internal_ident(&name);
-        stmts.push(quote!(#name = Some(monotonics.#idx);));
+        stmts.push(quote!(*#name.get_mut_unchecked() = Some(monotonics.#idx);));
     }
 
     // Enable the interrupts -- this completes the `init`-ialization phase
diff --git a/macros/src/codegen/pre_init.rs b/macros/src/codegen/pre_init.rs
index 287f41a43e..3a62da504e 100644
--- a/macros/src/codegen/pre_init.rs
+++ b/macros/src/codegen/pre_init.rs
@@ -20,7 +20,7 @@ pub fn codegen(app: &App, analysis: &Analysis, extra: &Extra) -> Vec<TokenStream
         let fq_ident = util::mark_internal_ident(&fq_ident);
 
         stmts.push(quote!(
-            (0..#cap).for_each(|i| #fq_ident.enqueue_unchecked(i));
+            (0..#cap).for_each(|i| #fq_ident.get_mut_unchecked().enqueue_unchecked(i));
         ));
     }
 
@@ -86,7 +86,9 @@ pub fn codegen(app: &App, analysis: &Analysis, extra: &Extra) -> Vec<TokenStream
         let tq = util::mark_internal_ident(&tq);
 
         // Initialize timer queues
-        stmts.push(quote!(#tq.as_mut_ptr().write(rtic::export::TimerQueue::new());));
+        stmts.push(
+            quote!(#tq.get_mut_unchecked().as_mut_ptr().write(rtic::export::TimerQueue::new());),
+        );
 
         // Compile time assert that this priority is supported by the device
         stmts.push(quote!(let _ = [(); ((1 << #nvic_prio_bits) - #priority as usize)];));
diff --git a/macros/src/codegen/resources.rs b/macros/src/codegen/resources.rs
index fa52b86db6..5352369685 100644
--- a/macros/src/codegen/resources.rs
+++ b/macros/src/codegen/resources.rs
@@ -4,13 +4,20 @@ use rtic_syntax::{analyze::Ownership, ast::App};
 
 use crate::{analyze::Analysis, check::Extra, codegen::util};
 
-/// Generates `static [mut]` variables and resource proxies
+/// Generates `static` variables and resource proxies
+/// Early resources are stored in `RacyCell<T>`
+/// Late resource are stored in `RacyCell<MaybeUninit<T>>`
+///
+/// Safety:
+/// - RacyCell<T> access is `unsafe`.
+/// - RacyCell<MaybeUninit> is always written to before user access, thus
+//    the generated code for user access can safely `assume_init`.
 pub fn codegen(
     app: &App,
     analysis: &Analysis,
     extra: &Extra,
 ) -> (
-    // mod_app -- the `static [mut]` variables behind the proxies
+    // mod_app -- the `static` variables behind the proxies
     Vec<TokenStream2>,
     // mod_resources -- the `resources` module
     TokenStream2,
@@ -24,35 +31,50 @@ pub fn codegen(
         let mangled_name = util::mark_internal_ident(&name);
 
         {
+            // TODO: do we really need this in the single core case
+            // late resources in `util::link_section_uninit`
             let section = if expr.is_none() {
                 util::link_section_uninit(true)
             } else {
                 None
             };
 
+            // resource type and assigned value
             let (ty, expr) = if let Some(expr) = expr {
-                (quote!(#ty), quote!(#expr))
-            } else {
+                // early resource
                 (
-                    quote!(core::mem::MaybeUninit<#ty>),
-                    quote!(core::mem::MaybeUninit::uninit()),
+                    quote!(rtic::RacyCell<#ty>),
+                    quote!(rtic::RacyCell::new(#expr)),
+                )
+            } else {
+                // late resource
+                (
+                    quote!(rtic::RacyCell<core::mem::MaybeUninit<#ty>>),
+                    quote!(rtic::RacyCell::new(core::mem::MaybeUninit::uninit())),
                 )
             };
 
             let attrs = &res.attrs;
+
+            // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             mod_app.push(quote!(
                 #[allow(non_upper_case_globals)]
+                // #[doc = #doc]
                 #[doc(hidden)]
                 #(#attrs)*
                 #(#cfgs)*
                 #section
-                static mut #mangled_name: #ty = #expr;
+                static #mangled_name: #ty = #expr;
             ));
         }
 
         let r_prop = &res.properties;
+        // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
+
         if !r_prop.task_local && !r_prop.lock_free {
             mod_resources.push(quote!(
+                // #[doc = #doc]
+                #[doc(hidden)]
                 #[allow(non_camel_case_types)]
                 #(#cfgs)*
                 pub struct #name<'a> {
@@ -73,15 +95,23 @@ pub fn codegen(
                 }
             ));
 
-            let ptr = if expr.is_none() {
-                quote!(
-                    #(#cfgs)*
-                    #mangled_name.as_mut_ptr()
+            let (ptr, _doc) = if expr.is_none() {
+                // late resource
+                (
+                    quote!(
+                        #(#cfgs)*
+                        #mangled_name.get_mut_unchecked().as_mut_ptr()
+                    ),
+                    "late",
                 )
             } else {
-                quote!(
-                    #(#cfgs)*
-                    &mut #mangled_name
+                // early resource
+                (
+                    quote!(
+                        #(#cfgs)*
+                        #mangled_name.get_mut_unchecked()
+                    ),
+                    "early",
                 )
             };
 
@@ -92,6 +122,8 @@ pub fn codegen(
                 None => 0,
             };
 
+            // let doc = format!(" RTIC internal ({} resource): {}:{}", doc, file!(), line!());
+
             mod_app.push(util::impl_mutex(
                 extra,
                 cfgs,
diff --git a/macros/src/codegen/resources_struct.rs b/macros/src/codegen/resources_struct.rs
index 8ed8a2912c..6fe4678a62 100644
--- a/macros/src/codegen/resources_struct.rs
+++ b/macros/src/codegen/resources_struct.rs
@@ -79,9 +79,9 @@ pub fn codegen(ctxt: Context, needs_lt: &mut bool, app: &App) -> (TokenStream2,
         let is_late = expr.is_none();
         if is_late {
             let expr = if access.is_exclusive() {
-                quote!(&mut *#mangled_name.as_mut_ptr())
+                quote!(&mut *#mangled_name.get_mut_unchecked().as_mut_ptr())
             } else {
-                quote!(&*#mangled_name.as_ptr())
+                quote!(&*#mangled_name.get_unchecked().as_ptr())
             };
 
             values.push(quote!(
@@ -91,7 +91,7 @@ pub fn codegen(ctxt: Context, needs_lt: &mut bool, app: &App) -> (TokenStream2,
         } else {
             values.push(quote!(
                 #(#cfgs)*
-                #name: &#mut_ #mangled_name
+                #name: #mangled_name.get_mut_unchecked()
             ));
         }
     }
diff --git a/macros/src/codegen/software_tasks.rs b/macros/src/codegen/software_tasks.rs
index a39fe4cc2e..e42fb88d41 100644
--- a/macros/src/codegen/software_tasks.rs
+++ b/macros/src/codegen/software_tasks.rs
@@ -52,7 +52,7 @@ pub fn codegen(
             // /// Queue version of a free-list that keeps track of empty slots in
             // /// the following buffers
             #[doc(hidden)]
-            static mut #fq: #fq_ty = #fq_expr;
+            static #fq: rtic::RacyCell<#fq_ty> = rtic::RacyCell::new(#fq_expr);
         ));
 
         let elems = &(0..cap)
@@ -65,13 +65,15 @@ pub fn codegen(
             let mono_type = &monotonic.ty;
 
             let uninit = mk_uninit();
+            // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             mod_app.push(quote!(
                 #uninit
                 // /// Buffer that holds the instants associated to the inputs of a task
+                // #[doc = #doc]
                 #[doc(hidden)]
-                static mut #instants:
-                    [core::mem::MaybeUninit<rtic::time::Instant<#mono_type>>; #cap_lit] =
-                    [#(#elems,)*];
+                static #instants:
+                    rtic::RacyCell<[core::mem::MaybeUninit<rtic::time::Instant<#mono_type>>; #cap_lit]> =
+                    rtic::RacyCell::new([#(#elems,)*]);
             ));
         }
 
@@ -82,8 +84,8 @@ pub fn codegen(
             #uninit
             // /// Buffer that holds the inputs of a task
             #[doc(hidden)]
-            static mut #inputs_ident: [core::mem::MaybeUninit<#input_ty>; #cap_lit] =
-                [#(#elems,)*];
+            static #inputs_ident: rtic::RacyCell<[core::mem::MaybeUninit<#input_ty>; #cap_lit]> =
+                rtic::RacyCell::new([#(#elems,)*]);
         ));
 
         // `${task}Resources`
diff --git a/macros/src/codegen/timer_queue.rs b/macros/src/codegen/timer_queue.rs
index 0d2c51ead9..ed89af660e 100644
--- a/macros/src/codegen/timer_queue.rs
+++ b/macros/src/codegen/timer_queue.rs
@@ -15,7 +15,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             // #[doc = #doc]
             #[doc(hidden)]
             #[allow(non_camel_case_types)]
-            static mut #tq_marker: u32 = 0;
+            static #tq_marker: rtic::RacyCell<u32> = rtic::RacyCell::new(0);
         ));
 
         let t = util::schedule_t_ident();
@@ -71,9 +71,11 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             let tq_ty =
                 quote!(core::mem::MaybeUninit<rtic::export::TimerQueue<#mono_type, #t, #n>>);
 
+            // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             items.push(quote!(
                 #[doc(hidden)]
-                static mut #tq: #tq_ty = core::mem::MaybeUninit::uninit();
+                static #tq: rtic::RacyCell<#tq_ty> =
+                    rtic::RacyCell::new(core::mem::MaybeUninit::uninit());
             ));
 
             let mono = util::monotonic_ident(&monotonic_name);
@@ -82,7 +84,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
 
             items.push(quote!(
                 #[doc(hidden)]
-                static mut #mono: Option<#mono_type> = None;
+                static #mono: rtic::RacyCell<Option<#mono_type>> = rtic::RacyCell::new(None);
             ));
         }
 
@@ -113,7 +115,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
                     quote!(
                         #(#cfgs)*
                         #t::#name => {
-                            rtic::export::interrupt::free(|_| #rq.split().0.enqueue_unchecked((#rqt::#name, index)));
+                            rtic::export::interrupt::free(|_| #rq.get_mut_unchecked().split().0.enqueue_unchecked((#rqt::#name, index)));
 
                             #pend
                         }
@@ -132,10 +134,9 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
                 #[no_mangle]
                 #[allow(non_snake_case)]
                 unsafe fn #bound_interrupt() {
-
                     while let Some((task, index)) = rtic::export::interrupt::free(|_|
-                        if let Some(mono) = #app_path::#m_ident.as_mut() {
-                            (&mut *#tq.as_mut_ptr()).dequeue(|| #disable_isr, mono)
+                        if let Some(mono) = #app_path::#m_ident.get_mut_unchecked().as_mut() {
+                            (&mut *#tq.get_mut_unchecked().as_mut_ptr()).dequeue(|| #disable_isr, mono)
                         } else {
                             // We can only use the timer queue if `init` has returned, and it
                             // writes the `Some(monotonic)` we are accessing here.
@@ -147,7 +148,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
                         }
                     }
 
-                    rtic::export::interrupt::free(|_| if let Some(mono) = #app_path::#m_ident.as_mut() {
+                    rtic::export::interrupt::free(|_| if let Some(mono) = #app_path::#m_ident.get_mut_unchecked().as_mut() {
                         mono.on_interrupt();
                     });
                 }
diff --git a/src/lib.rs b/src/lib.rs
index a4abc4c87b..cd51199613 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -57,3 +57,31 @@ where
 {
     NVIC::pend(interrupt)
 }
+
+use core::cell::UnsafeCell;
+
+/// Internal replacement for `static mut T`
+#[repr(transparent)]
+pub struct RacyCell<T>(UnsafeCell<T>);
+
+impl<T> RacyCell<T> {
+    /// Create a RacyCell
+    #[inline(always)]
+    pub const fn new(value: T) -> Self {
+        RacyCell(UnsafeCell::new(value))
+    }
+
+    /// Get `&mut T`
+    #[inline(always)]
+    pub unsafe fn get_mut_unchecked(&self) -> &mut T {
+        &mut *self.0.get()
+    }
+
+    /// Get `&T`
+    #[inline(always)]
+    pub unsafe fn get_unchecked(&self) -> &T {
+        &*self.0.get()
+    }
+}
+
+unsafe impl<T> Sync for RacyCell<T> {}

From 50f26e78eda57cb1f9f815aa93b357c2cd1a0205 Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Thu, 22 Apr 2021 18:38:42 +0200
Subject: [PATCH 2/2] Keep comments

---
 macros/src/codegen/dispatchers.rs    | 2 ++
 macros/src/codegen/module.rs         | 1 +
 macros/src/codegen/post_init.rs      | 1 +
 macros/src/codegen/resources.rs      | 4 +++-
 macros/src/codegen/software_tasks.rs | 1 +
 macros/src/codegen/timer_queue.rs    | 4 ++++
 6 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/macros/src/codegen/dispatchers.rs b/macros/src/codegen/dispatchers.rs
index 6aca901aea..382e33fe40 100644
--- a/macros/src/codegen/dispatchers.rs
+++ b/macros/src/codegen/dispatchers.rs
@@ -26,6 +26,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             })
             .collect::<Vec<_>>();
 
+        // For future use
         // let doc = format!(
         //     "Software tasks to be dispatched at priority level {}",
         //     level,
@@ -53,6 +54,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             )
         };
 
+        // For future use
         // let doc = format!(
         //     "Queue of tasks ready to be dispatched at priority level {}",
         //     level
diff --git a/macros/src/codegen/module.rs b/macros/src/codegen/module.rs
index 8c148ccb14..9bcc8f640d 100644
--- a/macros/src/codegen/module.rs
+++ b/macros/src/codegen/module.rs
@@ -305,6 +305,7 @@ pub fn codegen(
             let user_imports = &app.user_imports;
             let tq_marker = util::mark_internal_ident(&util::timer_queue_marker_ident());
 
+            // For future use
             // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             // items.push(quote!(#[doc = #doc]));
 
diff --git a/macros/src/codegen/post_init.rs b/macros/src/codegen/post_init.rs
index 7dd1590338..78548bcdaf 100644
--- a/macros/src/codegen/post_init.rs
+++ b/macros/src/codegen/post_init.rs
@@ -31,6 +31,7 @@ pub fn codegen(app: &App, analysis: &Analysis) -> Vec<TokenStream2> {
     }
 
     for (i, (monotonic, _)) in app.monotonics.iter().enumerate() {
+        // For future use
         // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
         // stmts.push(quote!(#[doc = #doc]));
 
diff --git a/macros/src/codegen/resources.rs b/macros/src/codegen/resources.rs
index 5352369685..a623ea6037 100644
--- a/macros/src/codegen/resources.rs
+++ b/macros/src/codegen/resources.rs
@@ -31,7 +31,6 @@ pub fn codegen(
         let mangled_name = util::mark_internal_ident(&name);
 
         {
-            // TODO: do we really need this in the single core case
             // late resources in `util::link_section_uninit`
             let section = if expr.is_none() {
                 util::link_section_uninit(true)
@@ -56,6 +55,7 @@ pub fn codegen(
 
             let attrs = &res.attrs;
 
+            // For future use
             // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             mod_app.push(quote!(
                 #[allow(non_upper_case_globals)]
@@ -69,6 +69,7 @@ pub fn codegen(
         }
 
         let r_prop = &res.properties;
+        // For future use
         // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
 
         if !r_prop.task_local && !r_prop.lock_free {
@@ -122,6 +123,7 @@ pub fn codegen(
                 None => 0,
             };
 
+            // For future use
             // let doc = format!(" RTIC internal ({} resource): {}:{}", doc, file!(), line!());
 
             mod_app.push(util::impl_mutex(
diff --git a/macros/src/codegen/software_tasks.rs b/macros/src/codegen/software_tasks.rs
index e42fb88d41..0372e8ec83 100644
--- a/macros/src/codegen/software_tasks.rs
+++ b/macros/src/codegen/software_tasks.rs
@@ -65,6 +65,7 @@ pub fn codegen(
             let mono_type = &monotonic.ty;
 
             let uninit = mk_uninit();
+            // For future use
             // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             mod_app.push(quote!(
                 #uninit
diff --git a/macros/src/codegen/timer_queue.rs b/macros/src/codegen/timer_queue.rs
index ed89af660e..14e9105453 100644
--- a/macros/src/codegen/timer_queue.rs
+++ b/macros/src/codegen/timer_queue.rs
@@ -35,6 +35,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
                 })
                 .collect::<Vec<_>>();
 
+            // For future use
             // let doc = "Tasks that can be scheduled".to_string();
             items.push(quote!(
                 // #[doc = #doc]
@@ -61,6 +62,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
 
         // Static variables and resource proxy
         {
+            // For future use
             // let doc = &format!("Timer queue for {}", monotonic_name);
             let cap = app
                 .software_tasks
@@ -71,6 +73,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
             let tq_ty =
                 quote!(core::mem::MaybeUninit<rtic::export::TimerQueue<#mono_type, #t, #n>>);
 
+            // For future use
             // let doc = format!(" RTIC internal: {}:{}", file!(), line!());
             items.push(quote!(
                 #[doc(hidden)]
@@ -80,6 +83,7 @@ pub fn codegen(app: &App, analysis: &Analysis, _extra: &Extra) -> Vec<TokenStrea
 
             let mono = util::monotonic_ident(&monotonic_name);
             let mono = util::mark_internal_ident(&mono);
+            // For future use
             // let doc = &format!("Storage for {}", monotonic_name);
 
             items.push(quote!(