singleton: forward attributes and visibility

This only works in the long form with a named variable because of
constraints on where the `vis` metavariable can be used.

close #521

Author: jordens

cortex-m/CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added the ability to name the statics generated by `singleton!()` for better debuggability (#364, #380).
 - Added `critical-section-single-core` feature which provides an implementation for the `critical_section` crate for single-core systems, based on disabling all interrupts. (#447)
 - Added support for `embedded-hal` version 1 delay traits, requiring rust 1.60.
+- `singleton!()` now forwards attributes and visibility (#521).
 
 ### Fixed
 - Fixed `singleton!()` statics sometimes ending up in `.data` instead of `.bss` (#364, #380).

cortex-m/src/macros.rs

@@ -64,12 +64,13 @@ macro_rules! iprintln {
 /// ```
 #[macro_export]
 macro_rules! singleton {
-    ($name:ident: $ty:ty = $expr:expr) => {
+    ($(#[$meta:meta])* $vis:vis $name:ident: $ty:ty = $expr:expr) => {
         $crate::_export::critical_section::with(|_| {
             // this is a tuple of a MaybeUninit and a bool because using an Option here is
             // problematic:  Due to niche-optimization, an Option could end up producing a non-zero
             // initializer value which would move the entire static from `.bss` into `.data`...
-            static mut $name: (::core::mem::MaybeUninit<$ty>, bool) =
+            $(#[$meta])*
+            $vis static mut $name: (::core::mem::MaybeUninit<$ty>, bool) =
                 (::core::mem::MaybeUninit::uninit(), false);
 
             #[allow(unsafe_code)]
@@ -115,3 +116,14 @@ const CFAIL: () = ();
 /// ```
 #[allow(dead_code)]
 const CPASS: () = ();
+
+/// ```
+/// use cortex_m::singleton;
+///
+/// fn foo() {
+///     // check that attributes and visibility are forwarded
+///     singleton!(#[link_section = ".bss"] pub(crate) FOO: u8 = 0);
+/// }
+/// ```
+#[allow(dead_code)]
+const CPASS_ATTR: () = ();