Require buf reference to implement Write, not buf itself

Allows one level of indirection less

src/template.rs

@@ -41,7 +41,7 @@ impl Template {
         writeln!(
             out,
             "\n\
-             pub fn {name}<W: Write>(mut out: W{args}) -> io::Result<()> {{\n\
+             pub fn {name}<W>(out: &mut W{args}) -> io::Result<()> where W: ?Sized, for<'a> &'a mut W: Write {{\n\
              {body}\
              Ok(())\n\
              }}",

src/template_utils.rs

@@ -6,7 +6,7 @@
 /// formats the value using Display and then html-encodes the result.
 pub trait ToHtml {
     /// Write self to `out`, which is in html representation.
-    fn to_html<W: Write + ?Sized>(&self, out: &mut W) -> io::Result<()>;
+    fn to_html<W>(&self, out: &mut W) -> io::Result<()> where W: ?Sized, for<'a> &'a mut W: Write;
 }
 
 /// Wrapper object for data that should be outputted as raw html
@@ -16,21 +16,21 @@ pub struct Html<T>(pub T);
 
 impl<T: Display> ToHtml for Html<T> {
     #[inline]
-    fn to_html<W: Write + ?Sized>(&self, out: &mut W) -> io::Result<()> {
+    fn to_html<W>(&self, mut out: &mut W) -> io::Result<()>  where W: ?Sized, for<'a> &'a mut W: Write {
         write!(out, "{}", self.0)
     }
 }
 
 impl<T: Display> ToHtml for T {
     #[inline]
-    fn to_html<W: Write + ?Sized>(&self, out: &mut W) -> io::Result<()> {
+    fn to_html<W>(&self, out: &mut W) -> io::Result<()>  where W: ?Sized, for<'a> &'a mut W: Write {
         write!(ToHtmlEscapingWriter(out), "{}", self)
     }
 }
 
-struct ToHtmlEscapingWriter<'a, W: Write + ?Sized>(&'a mut W);
+struct ToHtmlEscapingWriter<'w, W: ?Sized>(&'w mut W);
 
-impl<'a, W: Write + ?Sized> Write for ToHtmlEscapingWriter<'a, W> {
+impl<'w, W> Write for ToHtmlEscapingWriter<'w, W> where W: ?Sized, for<'a> &'a mut W: Write {
     #[inline]
     // This takes advantage of the fact that `write` doesn't have to write everything,
     // and the call will be retried with the rest of the data
@@ -56,10 +56,10 @@ impl<'a, W: Write + ?Sized> Write for ToHtmlEscapingWriter<'a, W> {
     }
 }
 
-impl<'a, W: Write + ?Sized> ToHtmlEscapingWriter<'a, W> {
+impl<'w, W> ToHtmlEscapingWriter<'w, W> where W: ?Sized, for<'a> &'a mut W: Write {
     #[inline(never)]
     fn write_one_byte_escaped(
-        out: &mut W,
+        mut out: &mut W,
         data: &[u8],
     ) -> io::Result<usize> {
         let next = data.get(0);