Merge #469

469: Goodbye static mut r=AfoHT a=korken89

Squashed and updated version of @perlindgren branch.

In release there are 0 bytes difference for all examples :tada: 
For debug we see a bloat of ~1-2% worst case, commonly less.

Complete diff of sizes for all examples (first is goodbye static mut, second is master):

```

<   27220	      0	     28	  27248	   6a70	spawn
---
>   27092	      0	     28	  27120	   69f0	spawn

<   16916	      0	     44	  16960	   4240	late
---
>   16836	      0	     44	  16880	   41f0	late

<   15952	      0	     12	  15964	   3e5c	type-usage
---
>   15872	      0	     12	  15884	   3e0c	type-usage

<   22068	      0	     44	  22112	   5660	cfg
---
>   21812	      0	     44	  21856	   5560	cfg

<   34784	      0	     40	  34824	   8808	periodic
---
>   34496	      0	     40	  34536	   86e8	periodic

<   22308	      0	     32	  22340	   5744	task
---
>   21972	      0	     32	  22004	   55f4	task

<   15076	      8	     24	  15108	   3b04	task-local
---
>   14980	      8	     24	  15012	   3aa4	task-local

<   12884	      0	     24	  12908	   326c	destructure
---
>   12820	      0	     24	  12844	   322c	destructure

<   10128	      0	     16	  10144	   27a0	init
---
>   10112	      0	     16	  10128	   2790	init

<   19044	      0	     20	  19064	   4a78	task_named_main
---
>   18916	      0	     20	  18936	   49f8	task_named_main

<   27252	      0	     28	  27280	   6a90	extern_spawn
---
>   27124	      0	     28	  27152	   6a10	extern_spawn

<   10176	      0	     16	  10192	   27d0	idle
---
>   10160	      0	     16	  10176	   27c0	idle

<   13972	      0	     16	  13988	   36a4	resource
---
>   13940	      0	     16	  13956	   3684	resource

<   16228	      0	     24	  16252	   3f7c	multilock
---
>   16116	      0	     24	  16140	   3f0c	multilock

<   14660	      0	     16	  14676	   3954	lock
---
>   14628	      0	     16	  14644	   3934	lock

<   10416	      0	     16	  10432	   28c0	task-local-minimal
---
>   10400	      0	     16	  10416	   28b0	task-local-minimal

<   14164	      0	     24	  14188	   376c	generics
---
>   14148	      0	     24	  14172	   375c	generics

<   30644	      0	     48	  30692	   77e4	message
---
>   30308	      0	     48	  30356	   7694	message

<   28964	      0	     36	  29000	   7148	spawn2
---
>   28724	      0	     36	  28760	   7058	spawn2

<   15952	      0	      8	  15960	   3e58	t-schedule-core-stable
---
>   15872	      0	      8	  15880	   3e08	t-schedule-core-stable

<   17408	      0	     20	  17428	   4414	t-cfg
---
>   17248	      0	     20	  17268	   4374	t-cfg

<   12948	      0	     16	  12964	   32a4	hardware
---
>   12932	      0	     16	  12948	   3294	hardware

<   54640	      0	    104	  54744	   d5d8	t-schedule
---
>   53696	      0	    104	  53800	   d228	t-schedule

<   26132	      0	    548	  26680	   6838	pool
---
>   25876	      0	    548	  26424	   6738	pool

<   22372	      0	     56	  22428	   579c	cfg-whole-task
---
>   22100	      0	     56	  22156	   568c	cfg-whole-task

<   38292	      0	     76	  38368	   95e0	schedule
---
>   37828	      0	     76	  37904	   9410	schedule

<   21216	      0	     32	  21248	   5300	t-spawn
---
>   20880	      0	     32	  20912	   51b0	t-spawn

<   22820	      0	     56	  22876	   595c	capacity
---
>   22580	      0	     56	  22636	   586c	capacity

<   17060	      0	     48	  17108	   42d4	static
---
>   16980	      0	     48	  17028	   4284	static

<   20288	      0	     24	  20312	   4f58	ramfunc
---
>   20096	      0	     24	  20120	   4e98	ramfunc

<   11760	      0	     20	  11780	   2e04	t-resource
---
>   11664	      0	     20	  11684	   2da4	t-resource

<   13028	      0	     16	  13044	   32f4	only-shared-access
---
>   13012	      0	     16	  13028	   32e4	only-shared-access

<   16576	      0	     16	  16592	   40d0	not-sync
---
>   16432	      0	     16	  16448	   4040	not-sync

<   13892	      0	     16	  13908	   3654	resource-user-struct
---
>   13860	      0	     16	  13876	   3634	resource-user-struct

<   37472	      0	     64	  37536	   92a0	double_schedule
---
>   36960	      0	     64	  37024	   90a0	double_schedule

<   17648	      0	      8	  17656	   44f8	t-stask-main
---
>   17520	      0	      8	  17528	   4478	t-stask-main

<    8816	      0	      4	   8820	   2274	t-late-not-send
---
>    8800	      0	      4	   8804	   2264	t-late-not-send

<   23280	      0	     32	  23312	   5b10	types
---
>   23120	      0	     32	  23152	   5a70	types
```

Co-authored-by: Emil Fresk <emil.fresk@gmail.com>

1 files changed, 47 insertions, 13 deletions

diff --git a/macros/src/codegen/resources.rs b/macros/src/codegen/resources.rs
index fa52b86d..a623ea60 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 +31,51 @@ pub fn codegen(
         let mangled_name = util::mark_internal_ident(&name);
 
         {
+            // 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))
+                // early resource
+                (
+                    quote!(rtic::RacyCell<#ty>),
+                    quote!(rtic::RacyCell::new(#expr)),
+                )
             } else {
+                // late resource
                 (
-                    quote!(core::mem::MaybeUninit<#ty>),
-                    quote!(core::mem::MaybeUninit::uninit()),
+                    quote!(rtic::RacyCell<core::mem::MaybeUninit<#ty>>),
+                    quote!(rtic::RacyCell::new(core::mem::MaybeUninit::uninit())),
                 )
             };
 
             let attrs = &res.attrs;
+
+            // For future use
+            // 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;
+        // For future use
+        // 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 +96,23 @@ 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 +123,9 @@ pub fn codegen(
                 None => 0,
             };
 
+            // For future use
+            // let doc = format!(" RTIC internal ({} resource): {}:{}", doc, file!(), line!());
+
             mod_app.push(util::impl_mutex(
                 extra,
                 cfgs,