Implement error propagation expression: `?` (#590)

This change allows using the operator `?` in askama expressions. It
works like the same operator in Rust: if a `Result` is `Ok`, it is
unwrapped. If it is an error, then the `render()` method fails with this
error value.

1 files changed, 12 insertions, 0 deletions

diff --git a/askama_shared/src/generator.rs b/askama_shared/src/generator.rs
index fabe277..de2fdcb 100644
--- a/askama_shared/src/generator.rs
+++ b/askama_shared/src/generator.rs
@@ -1060,9 +1060,21 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
             Expr::Group(ref inner) => self.visit_group(buf, inner)?,
             Expr::Call(ref obj, ref args) => self.visit_call(buf, obj, args)?,
             Expr::RustMacro(name, args) => self.visit_rust_macro(buf, name, args),
+            Expr::Try(ref expr) => self.visit_try(buf, expr.as_ref())?,
         })
     }
 
+    fn visit_try(
+        &mut self,
+        buf: &mut Buffer,
+        expr: &Expr<'_>,
+    ) -> Result<DisplayWrap, CompileError> {
+        buf.write("::core::result::Result::map_err(");
+        self.visit_expr(buf, expr)?;
+        buf.write(", |err| ::askama::shared::Error::Custom(::core::convert::Into::into(err)))?");
+        Ok(DisplayWrap::Unwrapped)
+    }
+
     fn visit_rust_macro(&mut self, buf: &mut Buffer, name: &str, args: &str) -> DisplayWrap {
         buf.write(name);
         buf.write("!(");