Merge #767

767: allow #[init] and #[idle] to be externed r=korken89 a=wiktorwieclaw

I updated `rtic-macros` to a allow init and idle to be externally defined.

## Design notes
* Updated `extern_binds` example to include external #[init] and #[idle] functions.
* Added docs to Local and Shared structs. The `extern_binds` example has a `#![deny(missing_docs)]` which caused some issues.

## Testing
Apart from building the example, I also used this feature in one of my projects and ran it on a MCU [here](https://github.com/grupacosmo/cansat/blob/98ca7bd42e737adfb140a548da1ade01beb495da/crates/cansat-stm32f4/src/main.rs#L59-L74)

## Related issues
* https://github.com/rtic-rs/rtic/issues/505

## Related PRs
* https://github.com/rtic-rs/rtic-syntax/pull/71

Co-authored-by: Vixu <lonevixu@gmail.com>

1 files changed, 39 insertions, 1 deletions

diff --git a/rtic-macros/src/syntax/parse/init.rs b/rtic-macros/src/syntax/parse/init.rs
index 0aea20b..59d0093 100644
--- a/rtic-macros/src/syntax/parse/init.rs
+++ b/rtic-macros/src/syntax/parse/init.rs
@@ -1,6 +1,6 @@
 use proc_macro2::TokenStream as TokenStream2;
 
-use syn::{parse, ItemFn};
+use syn::{parse, ForeignItemFn, ItemFn, Stmt};
 
 use crate::syntax::{
     ast::{Init, InitArgs},
@@ -35,6 +35,44 @@ impl Init {
                             stmts: item.block.stmts,
                             user_shared_struct,
                             user_local_struct,
+                            is_extern: false,
+                        });
+                    }
+                }
+            }
+        }
+
+        Err(parse::Error::new(
+            span,
+            format!(
+                "the `#[init]` function must have signature `fn({name}::Context) -> (Shared resources struct, Local resources struct)`"
+            ),
+        ))
+    }
+
+    pub(crate) fn parse_foreign(args: InitArgs, item: ForeignItemFn) -> parse::Result<Self> {
+        let valid_signature =
+            util::check_foreign_fn_signature(&item, false) && item.sig.inputs.len() == 1;
+
+        let span = item.sig.ident.span();
+
+        let name = item.sig.ident.to_string();
+
+        if valid_signature {
+            if let Ok((user_shared_struct, user_local_struct)) =
+                util::type_is_init_return(&item.sig.output)
+            {
+                if let Some((context, Ok(rest))) = util::parse_inputs(item.sig.inputs, &name) {
+                    if rest.is_empty() {
+                        return Ok(Init {
+                            args,
+                            attrs: item.attrs,
+                            context,
+                            name: item.sig.ident,
+                            stmts: Vec::<Stmt>::new(),
+                            user_shared_struct,
+                            user_local_struct,
+                            is_extern: true,
                         });
                     }
                 }