Use a separate trait for object safety (#579)

This is relatively major change to the main trait's API. For context,
I always started from the concept of monomorphized traits, but later
several contributors asked about object safety. At that point I made
`Template` object-safe, and then even later added a `SizedTemplate`
to make some things easier for people who don't need object safety.

However, having object-safety in the primary trait is bad for
performance (a substantial number of calls into the virtual `Write`
trait is relatively slow), and I don't think those who don't need
object safety should pay for the cost of having it.

Additionally, I feel using associated consts for the extension and
size hint is more idiomatic than having accessor methods. I don't
know why I didn't use these from the start -- maybe associated
consts didn't exist yet, or I didn't yet know how/when to use them.
Askama is pretty old at this point...

1 files changed, 5 insertions, 17 deletions

diff --git a/askama_shared/src/generator.rs b/askama_shared/src/generator.rs
index 41b81df..56c82aa 100644
--- a/askama_shared/src/generator.rs
+++ b/askama_shared/src/generator.rs
@@ -128,7 +128,7 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
     ) -> Result<(), CompileError> {
         self.write_header(buf, "::askama::Template", None)?;
         buf.writeln(
-            "fn render_into(&self, writer: &mut dyn ::std::fmt::Write) -> \
+            "fn render_into(&self, writer: &mut (impl ::std::fmt::Write + ?Sized)) -> \
              ::askama::Result<()> {",
         )?;
 
@@ -160,25 +160,13 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
         buf.writeln("Ok(())")?;
         buf.writeln("}")?;
 
-        buf.writeln("fn extension(&self) -> Option<&'static str> {")?;
+        buf.writeln("const EXTENSION: ::std::option::Option<&'static ::std::primitive::str> = ")?;
         buf.writeln(&format!("{:?}", self.input.extension()))?;
-        buf.writeln("}")?;
+        buf.writeln(";")?;
 
-        buf.writeln("fn size_hint(&self) -> usize {")?;
+        buf.writeln("const SIZE_HINT: ::std::primitive::usize = ")?;
         buf.writeln(&format!("{}", size_hint))?;
-        buf.writeln("}")?;
-
-        buf.writeln("}")?;
-
-        self.write_header(buf, "::askama::SizedTemplate", None)?;
-
-        buf.writeln("fn size_hint() -> usize {")?;
-        buf.writeln(&format!("{}", size_hint))?;
-        buf.writeln("}")?;
-
-        buf.writeln("fn extension() -> Option<&'static str> {")?;
-        buf.writeln(&format!("{:?}", self.input.extension()))?;
-        buf.writeln("}")?;
+        buf.writeln(";")?;
 
         buf.writeln("}")?;
         Ok(())