initial version

src/lib.rs

@@ -1,7 +1,98 @@
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
+#![no_std]
+
+use core::fmt::Write;
+use core::marker::PhantomPinned;
+use core::mem::{transmute, MaybeUninit};
+use core::ops::DerefMut;
+use core::panic::PanicInfo;
+use core::pin::Pin;
+use core::ptr::null_mut;
+
+static mut PANIC_HANDLER_GETTER: Option<unsafe fn(handler: *mut (), info: &PanicInfo)> = None;
+static mut PANIC_HANDLER: *mut () = null_mut();
+
+/// Use monomorphization to "save" the type parameter of the static pointer
+unsafe fn trampoline<W: Write>(ptr: *mut (), info: &PanicInfo) {
+    let handler: &mut PanicHandler<W> = transmute(ptr);
+
+    let _ = write!(handler.deref_mut(), "{}", info);
+}
+
+pub struct PanicHandler<W: Write> {
+    writer: MaybeUninit<W>,
+    _pin: PhantomPinned,
+}
+
+impl<W: Write> PanicHandler<W> {
+    /// Create a panic handler from a `core::fmt::Write`
+    ///
+    /// Note that the returned handler is detached when it goes out of scope so in most cases it's
+    /// desired to keep the handler in scope for the full duration of the program.
+    ///
+    /// Additionally, the panic handler implements `Deref` for the provided `Write` and can be used
+    /// in place of the original `Write` throughout the app.
+    #[must_use = "the panic handler must be kept in scope"]
+    pub fn new(writer: W) -> Pin<Self> {
+        let handler = unsafe {
+            Pin::new_unchecked(PanicHandler {
+                writer: MaybeUninit::new(writer),
+                _pin: PhantomPinned,
+            })
+        };
+        unsafe {
+            PANIC_HANDLER_GETTER = Some(trampoline::<W>);
+            PANIC_HANDLER = transmute(&handler);
+        }
+        handler
+    }
+
+    /// Detach this panic handler and return the underlying writer
+    pub fn detach(handler: Pin<Self>) -> W {
+        unsafe {
+            PANIC_HANDLER_GETTER = None;
+            PANIC_HANDLER = null_mut();
+
+            // unpin is safe because the pointer to the handler is removed
+            let mut handler = Pin::into_inner_unchecked(handler);
+            let writer = core::mem::replace(&mut handler.writer, MaybeUninit::uninit());
+
+            // safe because self.writer is only uninit during drop
+            writer.assume_init()
+        }
     }
 }
+
+impl<W: Write> Drop for PanicHandler<W> {
+    fn drop(&mut self) {
+        unsafe {
+            PANIC_HANDLER_GETTER = None;
+            PANIC_HANDLER = null_mut();
+        }
+    }
+}
+
+impl<W: Write> core::ops::Deref for PanicHandler<W> {
+    type Target = W;
+
+    fn deref(&self) -> &Self::Target {
+        // safe because self.writer is only uninit during drop
+        unsafe { &*self.writer.as_ptr() }
+    }
+}
+
+impl<W: Write> core::ops::DerefMut for PanicHandler<W> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // safe because self.writer is only uninit during drop
+        unsafe { &mut *self.writer.as_mut_ptr() }
+    }
+}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    unsafe {
+        if let Some(trampoline) = PANIC_HANDLER_GETTER {
+            trampoline(PANIC_HANDLER, info);
+        }
+    }
+    loop {}
+}