This implements the "rooting" mechanism proposed in #47. However, it implements a `root` constructor function instead of list of `roots` values as originally proposed. In a nutshell: - There's a new field, `root`, which takes a path to the "root" constructor function. - This constructor has signature `fn() -> T` - When the `root` field is used the signature of `init` changes to accommodate a `&'static mut T` argument at the end. The `T` in that argument type matches the type returned by the "root" constructor. - The "root"-ed value is stack allocated. This enables the safe creation of `&'static mut` references. Example below: ``` rust //#![feature(proc_macro)] //#![no_std] extern crate blue_pill; extern crate cortex_m_rt; extern crate cortex_m_rtfm as rtfm; extern crate heapless; use blue_pill::stm32f103xx; use heapless::RingBuffer; use heapless::ring_buffer::{Consumer, Producer}; use rtfm::{app, Threshold}; use stm32f103xx::Interrupt; app! { device: stm32f103xx, resources: { static CONSUMER: Consumer<'static, u32, [u32; 8]>; static PRODUCER: Producer<'static, u32, [u32; 8]>; }, root: root, idle: { resources: [CONSUMER], }, tasks: { EXTI0: { path: exti0, resources: [PRODUCER], }, } } struct Root { rb: RingBuffer<u32, [u32; 8]>, } fn root() -> Root { Root { rb: RingBuffer::new(), } } fn init(_p: init::Peripherals, root: &'static mut Root) -> init::LateResourceValues { let (p, c) = root.rb.split(); init::LateResourceValues { CONSUMER: c, PRODUCER: p, } } fn idle(_t: &mut Threshold, r: idle::Resources) -> ! { rtfm::set_pending(Interrupt::EXTI0); loop { if r.CONSUMER.dequeue().is_some() { rtfm::bkpt(); } else { rtfm::wfi(); } } } fn exti0(_t: &mut Threshold, r: EXTI0::Resources) { r.PRODUCER.enqueue(42).ok(); rtfm::bkpt(); } ``` This produces the following machine code: ``` armasm 0800019c <EXTI0>: 800019c: f240 0000 movw r0, #0 80001a0: f2c2 0000 movt r0, #8192 ; 0x2000 80001a4: 6800 ldr r0, [r0, #0] 80001a6: 6803 ldr r3, [r0, #0] 80001a8: 6842 ldr r2, [r0, #4] 80001aa: 1c51 adds r1, r2, #1 80001ac: f001 0107 and.w r1, r1, #7 80001b0: 4299 cmp r1, r3 80001b2: d006 beq.n 80001c2 <EXTI0+0x26> 80001b4: eb00 0282 add.w r2, r0, r2, lsl #2 80001b8: 232a movs r3, #42 ; 0x2a 80001ba: 6093 str r3, [r2, #8] 80001bc: f3bf 8f5f dmb sy 80001c0: 6041 str r1, [r0, #4] 80001c2: be00 bkpt 0x0000 80001c4: 4770 bx lr 080001c6 <main>: 80001c6: b08a sub sp, #40 ; 0x28 ; Root allocation 80001c8: f240 1030 movw r0, #304 ; 0x130 80001cc: 4669 mov r1, sp 80001ce: 22f0 movs r2, #240 ; 0xf0 80001d0: f6c0 0000 movt r0, #2048 ; 0x800 80001d4: 7800 ldrb r0, [r0, #0] 80001d6: 2000 movs r0, #0 80001d8: e9cd 0000 strd r0, r0, [sp] 80001dc: f240 0000 movw r0, #0 80001e0: f2c2 0000 movt r0, #8192 ; 0x2000 80001e4: b672 cpsid i 80001e6: 6001 str r1, [r0, #0] ; PRODUCER = .. 80001e8: f240 0004 movw r0, #4 80001ec: f2c2 0000 movt r0, #8192 ; 0x2000 80001f0: 6001 str r1, [r0, #0] ; CONSUMER = .. 80001f2: f24e 4106 movw r1, #58374 ; 0xe406 80001f6: f2ce 0100 movt r1, #57344 ; 0xe000 80001fa: 700a strb r2, [r1, #0] 80001fc: f24e 1100 movw r1, #57600 ; 0xe100 8000200: 2240 movs r2, #64 ; 0x40 8000202: f2ce 0100 movt r1, #57344 ; 0xe000 8000206: 600a str r2, [r1, #0] 8000208: b662 cpsie i 800020a: f8c1 2100 str.w r2, [r1, #256] ; 0x100 800020e: e006 b.n 800021e <main+0x58> 8000210: f3bf 8f5f dmb sy 8000214: 3201 adds r2, #1 8000216: f002 0207 and.w r2, r2, #7 800021a: 600a str r2, [r1, #0] 800021c: be00 bkpt 0x0000 800021e: 6801 ldr r1, [r0, #0] 8000220: 684b ldr r3, [r1, #4] 8000222: 680a ldr r2, [r1, #0] 8000224: 429a cmp r2, r3 8000226: d1f3 bne.n 8000210 <main+0x4a> 8000228: bf30 wfi 800022a: e7f8 b.n 800021e <main+0x58> ``` Unresolved questions: - Is this mechanism memory safe in presence of `panic!` unwinding? - If not, can we generate a compile error if `panic = abort` is *not* used? - How does this affect the DMA API proposed in rust-embedded/embedded-hal#14 cc @pftbest

Cortex-M0(+) support #1

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