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, 8 insertions, 1 deletions

diff --git a/askama_shared/src/parser.rs b/askama_shared/src/parser.rs
index f429628..63cea0c 100644
--- a/askama_shared/src/parser.rs
+++ b/askama_shared/src/parser.rs
@@ -63,6 +63,7 @@ pub enum Expr<'a> {
     Group(Box<Expr<'a>>),
     Call(Box<Expr<'a>>, Vec<Expr<'a>>),
     RustMacro(&'a str, &'a str),
+    Try(Box<Expr<'a>>),
 }
 
 impl Expr<'_> {
@@ -510,6 +511,7 @@ enum Suffix<'a> {
     Attr(&'a str),
     Index(Expr<'a>),
     Call(Vec<Expr<'a>>),
+    Try,
 }
 
 fn expr_attr(i: &str) -> IResult<&str, Suffix<'_>> {
@@ -533,6 +535,10 @@ fn expr_call(i: &str) -> IResult<&str, Suffix<'_>> {
     map(arguments, Suffix::Call)(i)
 }
 
+fn expr_try(i: &str) -> IResult<&str, Suffix<'_>> {
+    map(preceded(take_till(not_ws), char('?')), |_| Suffix::Try)(i)
+}
+
 fn filter(i: &str) -> IResult<&str, (&str, Option<Vec<Expr<'_>>>)> {
     let (i, (_, fname, args)) = tuple((char('|'), ws(identifier), opt(arguments)))(i)?;
     Ok((i, (fname, args)))
@@ -567,12 +573,13 @@ fn expr_prefix(i: &str) -> IResult<&str, Expr<'_>> {
 fn expr_suffix(i: &str) -> IResult<&str, Expr<'_>> {
     let (mut i, mut expr) = expr_single(i)?;
     loop {
-        let (j, suffix) = opt(alt((expr_attr, expr_index, expr_call)))(i)?;
+        let (j, suffix) = opt(alt((expr_attr, expr_index, expr_call, expr_try)))(i)?;
         i = j;
         match suffix {
             Some(Suffix::Attr(attr)) => expr = Expr::Attr(expr.into(), attr),
             Some(Suffix::Index(index)) => expr = Expr::Index(expr.into(), index.into()),
             Some(Suffix::Call(args)) => expr = Expr::Call(expr.into(), args),
+            Some(Suffix::Try) => expr = Expr::Try(expr.into()),
             None => break,
         }
     }