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New static mutable storage for the entry function

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Issue 类型
功能
描述清晰度
基本清楚
活跃度
停滞
技术栈
rust
领域
embedded-iot

调研方向

首先检查现有 #[entry] macro 的行为以及提议的 entry_with_context 语法，然后对比 issue 中描述的 RTIC local-resource 示例。追踪当前 static mut expansion 如何到达 entry function，并定义所需的 context API 和 expansion；当设计达成一致，并且其安全性与兼容性影响已记录在案时，即视为完成。

由索引模型根据 Issue 内容生成。

描述

We carefully ensure the #[entry] function cannot be called other than after a reset (and after the start-up code has initialised everything).

Currently we use this knowledge to 'safely' replace static mut FOO: T = T::new() with code that uses an unsafe block to create &mut T reference to the static, which is passed in as an argument.

That is:

#[cortex_m_rt::entry]
fn main() -> ! {
    static mut FOO: u32 = 123;

    loop {}
}

becomes:

#[doc(hidden)]
#[export_name = "main"]
pub unsafe extern "C" fn __cortex_m_rt_main_trampoline() {
    #[allow(static_mut_refs)]
    __cortex_m_rt_main({
        static mut FOO: u32 = 123;
        unsafe { &mut FOO }
    })
}
fn __cortex_m_rt_main(#[allow(non_snake_case)] FOO: &'static mut u32) -> ! {
    loop {}
}

It is widely accepted that this kind of sleight of hand is not good. However, RTIC shows us a syntax that could work:

#[task(local = [state: u32 = 0])]
async fn foo(c: foo::Context) {
    let old_state: u32 = *c.local.state;
    *c.local.state += 1;
}

So, what if we wrote a new macro, called entry_with_context:

#[cortex_m_rt::entry_with_context(context = [state: u32 = 0])]
fn main(c: main::Context) -> ! {
    let old_state: u32 = *c.state;
    *c.state += 1;

    loop {}
}

The advantage here is that is clear some 'magic' is happening to create a static resource and pass it to the main function.

It would expand to something like:

mod main {
    pub(crate) struct Context {
        state: &mut u32
    }
}

#[doc(hidden)]
#[export_name = "main"]
pub unsafe extern "C" fn __cortex_m_rt_main_trampoline() {
    static STATE: RacyCell<u32> = RacyCell::new(123);
    __cortex_m_rt_main(main::Context {
        state: unsafe { STATE.get_mut_ref() },
    })
}

fn main(c: main::Context) -> ! {
    let old_state: u32 = *c.state;
    *c.state += 1;

    loop {}
}

We don't even need to hide the actual fn main() function anymore, because you cannot call it without creating the context object.

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