goodby static mut, welcome back to UnsafeCell

examples/minimal_early_resource.rs

@@ -0,0 +1,25 @@
+//! examples/idle.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965)]
+mod app {
+
+    #[resources]
+    struct Resources {
+        #[init(0)]
+        resource_x: u32,
+    }
+
+    #[idle(resources = [resource_x])]
+    fn idle(_: idle::Context) -> ! {
+        loop {
+            cortex_m::asm::nop();
+        }
+    }
+}

examples/minimal_late_resource.rs

@@ -0,0 +1,29 @@
+//! examples/minimal_late_resource.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965)]
+mod app {
+
+    #[resources]
+    struct Resources {
+        resource_x: u32,
+    }
+
+    #[init]
+    fn init(_: init::Context) -> (init::LateResources, init::Monotonics) {
+        (init::LateResources { resource_x: 0 }, init::Monotonics {})
+    }
+
+    #[idle(resources = [resource_x])]
+    fn idle(_: idle::Context) -> ! {
+        loop {
+            cortex_m::asm::nop();
+        }
+    }
+}

macros/src/codegen/post_init.rs

@@ -17,10 +17,14 @@ 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!(
+                    // We include the cfgs
                     #(#cfgs)*
-                #mangled_name.as_mut_ptr().write(late.#name);
+                    // 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);
                 ));
             }
         }

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,
@@ -24,18 +31,26 @@ 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())),
                 )
             };
 
@@ -46,7 +61,7 @@ pub fn codegen(
                 #(#attrs)*
                 #(#cfgs)*
                 #section
-                static mut #mangled_name: #ty = #expr;
+                static #mangled_name: #ty = #expr;
             ));
         }
 
@@ -74,14 +89,16 @@ pub fn codegen(
             ));
 
             let ptr = if expr.is_none() {
+                // late resource
                 quote!(
                     #(#cfgs)*
-                    #mangled_name.as_mut_ptr()
+                    &mut #mangled_name.get_mut_unchecked().assume_init()
                 )
             } else {
+                // early resource
                 quote!(
                     #(#cfgs)*
-                    &mut #mangled_name
+                    unsafe { #mangled_name.get_mut_unchecked() }
                 )
             };
 

src/lib.rs

@@ -55,3 +55,24 @@ where
 {
     NVIC::pend(interrupt)
 }
+
+use core::cell::UnsafeCell;
+
+/// Internal replacement for `static mut T`
+// TODO: Decide name and location.
+pub struct RacyCell<T>(UnsafeCell<T>);
+
+impl<T> RacyCell<T> {
+    /// Create a RacyCell
+    pub const fn new(value: T) -> Self {
+        RacyCell(UnsafeCell::new(value))
+    }
+
+    /// Get &mut T
+    pub unsafe fn get_mut_unchecked(&self) -> &mut T {
+        &mut *self.0.get()
+    }
+}
+
+// The type wrapped need to be Sync for RacyCell<T> to be Sync
+unsafe impl<T> Sync for RacyCell<T> where T: Sync {}