Merge #390

390: Spawn and schedule from anywhere r=AfoHT a=korken89

This PR moves RTIC to the spawn and schedule from anywhere syntax.

Notable changes:
 * We do no longer support non-`Send` types.
 * Some extra code is generated as any task may spawn/schedule any task. However Rust/LLVM does a great job optimizing away non used instantiations (no real code-size difference observed).
 * Worst case priority inversion has increased, but it is now predictable.

Upsides:
 * With this we should be able to support async/await.
 * RTIC tasks can now be callbacks (spawned and scheduled).
 * RTIC tasks can be stored.

Needs the following PR to land first: https://github.com/rtic-rs/rtic-syntax/pull/34

The following now works:

```rust
#[rtic::app(device = lm3s6965, monotonic = rtic::cyccnt::CYCCNT)]
mod app {
    #[init]
    fn init(mut cx: init::Context) -> init::LateResources {
        // Init stuff...

        // New spawn syntax
        foo::spawn().unwrap();

        // New schedule syntax
        bar::schedule(now + 4_000_000.cycles()).unwrap();

        init::LateResources {}
    }

    #[task]
    fn foo(_: foo::Context) {}

    #[task]
    fn bar(_: bar::Context) {}

    extern "C" {
        fn SSI0();
    }
}

```

Co-authored-by: Per Lindgren <per.lindgren@ltu.se>
Co-authored-by: Emil Fresk <emil.fresk@gmail.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/examples/spawn2.rs b/examples/spawn2.rs
new file mode 100644
index 00000000..b50a3ee6
--- /dev/null
+++ b/examples/spawn2.rs
@@ -0,0 +1,41 @@
+//! examples/spawn2.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::{debug, hprintln};
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965)]
+mod app {
+    #[init]
+    fn init(_c: init::Context) -> init::LateResources {
+        foo::spawn(1, 2).unwrap();
+
+        init::LateResources {}
+    }
+
+    #[task]
+    fn foo(_c: foo::Context, x: i32, y: u32) {
+        hprintln!("foo {}, {}", x, y).unwrap();
+        if x == 2 {
+            debug::exit(debug::EXIT_SUCCESS);
+        }
+        foo2::spawn(2).unwrap();
+    }
+
+    #[task]
+    fn foo2(_c: foo2::Context, x: i32) {
+        hprintln!("foo2 {}", x).unwrap();
+        foo::spawn(x, 0).unwrap();
+    }
+
+    // RTIC requires that unused interrupts are declared in an extern block when
+    // using software tasks; these free interrupts will be used to dispatch the
+    // software tasks.
+    extern "C" {
+        fn SSI0();
+    }
+}