Add write_to helper

src/main.rs

@@ -26,6 +26,7 @@ extern crate rlibc;
 pub mod arch;
 pub use arch::*;
 pub mod platform;
+mod write_to;
 
 use core::fmt::Write;
 use platform::{

src/platform/uart.rs

@@ -1,5 +1,5 @@
 use arch::*;
-use core::ops;
+use core::{fmt, ops};
 use platform::{gpio, rpi3::PERIPHERAL_BASE};
 use register::mmio::*;
 
@@ -232,8 +232,8 @@ impl MiniUart {
     }
 }
 
-impl core::fmt::Write for MiniUart {
-    fn write_str(&mut self, s: &str) -> core::fmt::Result {
+impl fmt::Write for MiniUart {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
         self.puts(s);
         Ok(())
     }

src/write_to.rs

@@ -0,0 +1,69 @@
+// No-alloc write!() implementation from https://stackoverflow.com/a/50201632/145434
+// Requires you to allocate a buffer somewhere manually.
+//
+
+use core::cmp::min;
+use core::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> {
+    pub fn new(buffer: &'a mut [u8]) -> Self {
+        WriteTo { buffer, used: 0 }
+    }
+
+    pub fn as_str(self) -> Option<&'a str> {
+        if self.used <= self.buffer.len() {
+            // only successful concats of str - must be a valid str.
+            use core::str::from_utf8_unchecked;
+            Some(unsafe { 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(())
+        }
+    }
+}
+
+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.as_str().ok_or(fmt::Error)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    pub fn test() {
+        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");
+    }
+}