Robustly read remaining bytes in a character

src/libstd/io/buffered.rs

@@ -322,7 +322,7 @@ mod test {
     use io;
     use prelude::*;
     use super::*;
-    use super::super::mem::{MemReader, MemWriter};
+    use super::super::mem::{MemReader, MemWriter, BufReader};
     use Harness = extra::test::BenchHarness;
 
     /// A type, free to create, primarily intended for benchmarking creation of wrappers that, just
@@ -526,6 +526,12 @@ mod test {
         assert_eq!(reader.read(buf), None);
     }
 
+    #[test]
+    fn read_char_buffered() {
+        let buf = [195u8, 159u8];
+        let mut reader = BufferedReader::with_capacity(1, BufReader::new(buf));
+        assert_eq!(reader.read_char(), Some('ß'));
+    }
 
     #[bench]
     fn bench_buffered_reader(bh: &mut Harness) {

src/libstd/io/mod.rs

@@ -1212,9 +1212,15 @@ pub trait Buffer: Reader {
         };
         if width == 0 { return None } // not uf8
         let mut buf = [0, ..4];
-        match self.read(buf.mut_slice_to(width)) {
-            Some(n) if n == width => {}
-            Some(..) | None => return None // read error
+        {
+            let mut start = 0;
+            loop {
+                match self.read(buf.mut_slice(start, width)) {
+                    Some(n) if n == width - start => break,
+                    Some(n) if n < width - start => { start += n; }
+                    Some(..) | None => return None // read error
+                }
+            }
         }
         match str::from_utf8_opt(buf.slice_to(width)) {
             Some(s) => Some(s.char_at(0)),