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    /// Macro for sending a formatted string through an ITM channel
    #[macro_export]
    macro_rules! iprint {
        ($channel:expr, $s:expr) => {
            $crate::itm::write_str($channel, $s);
        };
        ($channel:expr, $($arg:tt)*) => {
            $crate::itm::write_fmt($channel, format_args!($($arg)*));
        };
    }

    /// Macro for sending a formatted string through an ITM channel, with a newline.
    #[macro_export]
    macro_rules! iprintln {
        ($channel:expr) => {
            iprint!($channel, "\n");
        };
        ($channel:expr, $fmt:expr) => {
            iprint!($channel, concat!($fmt, "\n"));
        };
        ($channel:expr, $fmt:expr, $($arg:tt)*) => {
            iprint!($channel, concat!($fmt, "\n"), $($arg)*);
        };
    }

    /// Macro to create a mutable reference to a statically allocated value
    ///
    /// This macro returns a value with type `Option<&'static mut $ty>`. `Some($expr)` will be returned
    /// the first time the macro is executed; further calls will return `None`. To avoid `unwrap`ping a
    /// `None` variant the caller must ensure that the macro is called from a function that's executed
    /// at most once in the whole lifetime of the program.
    ///
    /// # Example
    ///
    /// ``` no_run
    /// #[macro_use(singleton)]
    /// extern crate cortex_m;
    ///
    /// fn main() {
    ///     // OK if `main` is executed only once
    ///     let x: &'static mut bool = singleton!(: bool = false).unwrap();
    ///
    ///     let y = alias();
    ///     // BAD this second call to `alias` will definitively `panic!`
    ///     let y_alias = alias();
    /// }
    ///
    /// fn alias() -> &'static mut bool {
    ///     singleton!(: bool = false).unwrap()
    /// }
    /// ```
    #[macro_export]
    macro_rules! singleton {
        (: $ty:ty = $expr:expr) => {
            $crate::interrupt::free(|_| {
                static mut VAR: Option<$ty> = None;

                #[allow(unsafe_code)]
                let used = unsafe { VAR.is_some() };
                if used {
                    None
                } else {
                    let expr = $expr;

                    #[allow(unsafe_code)]
                    unsafe {
                        VAR = Some(expr)
                    }

                    #[allow(unsafe_code)]
                    unsafe {
                        VAR.as_mut()
                    }
                }
            })
        };
    }

    /// ``` compile_fail
    /// #[macro_use(singleton)]
    /// extern crate cortex_m;
    ///
    /// fn main() {}
    ///
    /// fn foo() {
    ///     // check that the call to `uninitialized` requires unsafe
    ///     singleton!(: u8 = std::mem::uninitialized());
    /// }
    /// ```
    #[allow(dead_code)]
    const CFAIL: () = ();

    /// ```
    /// #![deny(unsafe_code)]
    /// #[macro_use(singleton)]
    /// extern crate cortex_m;
    ///
    /// fn main() {}
    ///
    /// fn foo() {
    ///     // check that calls to `singleton!` don't trip the `unsafe_code` lint
    ///     singleton!(: u8 = 0);
    /// }
    /// ```
    #[allow(dead_code)]
    const CPASS: () = ();

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