Add non-allocating formatted write implementation

nucleus/src/write_to.rs

@@ -0,0 +1,107 @@
+/*
+ * SPDX-License-Identifier: BlueOak-1.0.0
+ */
+// No-alloc write!() implementation from https://stackoverflow.com/a/50201632/145434
+// Requires you to allocate a buffer somewhere manually.
+// @todo Try to use arrayvec::ArrayString here instead?
+
+use core::{cmp::min, fmt};
+
+pub struct WriteTo<'a> {
+    buffer: &'a mut [u8],
+    // on write error (i.e. not enough space in buffer) this grows beyond
+    // `buffer.len()`.
+    used: usize,
+}
+
+impl<'a> WriteTo<'a> {
+    #[allow(unused)]
+    pub fn new(buffer: &'a mut [u8]) -> Self {
+        WriteTo { buffer, used: 0 }
+    }
+
+    #[allow(unused)]
+    pub fn into_str(self) -> Option<&'a str> {
+        if self.used <= self.buffer.len() {
+            // only successful concats of str - must be a valid str.
+            Some(unsafe { core::str::from_utf8_unchecked(&self.buffer[..self.used]) })
+        } else {
+            None
+        }
+    }
+
+    #[allow(unused)]
+    pub fn into_cstr(self) -> Option<&'a str> {
+        if self.used < self.buffer.len() {
+            self.buffer[self.used] = 0; // Terminate the string
+            Some(unsafe { core::str::from_utf8_unchecked(&self.buffer[..=self.used]) })
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a> fmt::Write for WriteTo<'a> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        if self.used > self.buffer.len() {
+            return Err(fmt::Error);
+        }
+        let remaining_buf = &mut self.buffer[self.used..];
+        let raw_s = s.as_bytes();
+        let write_num = min(raw_s.len(), remaining_buf.len());
+        remaining_buf[..write_num].copy_from_slice(&raw_s[..write_num]);
+        self.used += raw_s.len();
+        if write_num < raw_s.len() {
+            Err(fmt::Error)
+        } else {
+            Ok(())
+        }
+    }
+}
+
+#[allow(unused)]
+#[inline]
+pub fn show<'a>(buffer: &'a mut [u8], args: fmt::Arguments) -> Result<&'a str, fmt::Error> {
+    let mut w = WriteTo::new(buffer);
+    fmt::write(&mut w, args)?;
+    w.into_str().ok_or(fmt::Error)
+}
+
+// Return a zero-terminated str
+#[allow(unused)]
+#[inline]
+// Crazy bug - result of this call disappears if it's not inlined (https://github.com/rust-lang/rust/issues/68812)
+pub fn c_show<'a>(buffer: &'a mut [u8], args: fmt::Arguments) -> Result<&'a str, fmt::Error> {
+    let mut w = WriteTo::new(buffer);
+    fmt::write(&mut w, args)?;
+    w.into_cstr().ok_or(fmt::Error)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test_case]
+    pub fn write_to_works() {
+        let mut buf = [0u8; 64];
+        let s: &str = show(
+            &mut buf,
+            format_args!("write some stuff {:?}: {}", "foo", 42),
+        )
+        .unwrap();
+        assert_eq!(s, "write some stuff \"foo\": 42");
+        assert_eq!(s.as_ptr(), buf.as_ptr());
+    }
+
+    #[test_case]
+    pub fn zero_terminated_write_to_works() {
+        let mut buf = [0u8; 64];
+        let s: &str = c_show(
+            &mut buf,
+            format_args!("write some stuff {:?}: {}", "foo", 42),
+        )
+        .unwrap();
+        assert_eq!(s, "write some stuff \"foo\": 42\0");
+        assert_eq!(s.as_ptr(), buf.as_ptr());
+    }
+}