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, 69 insertions, 0 deletions

diff --git a/testing/tests/try.rs b/testing/tests/try.rs
new file mode 100644
index 0000000..e199f9c
--- /dev/null
+++ b/testing/tests/try.rs
@@ -0,0 +1,69 @@
+use askama::Template;
+
+#[derive(Template)]
+#[template(source = "{% let v = self.parse()? %}{{s}}={{v}}", ext = "txt")]
+struct IntParserTemplate<'a> {
+    s: &'a str,
+}
+
+impl IntParserTemplate<'_> {
+    fn parse(&self) -> Result<i32, std::num::ParseIntError> {
+        self.s.parse()
+    }
+}
+
+#[test]
+fn test_int_parser() {
+    let template = IntParserTemplate { s: "💯" };
+    assert!(matches!(template.render(), Err(askama::Error::Custom(_))));
+    assert_eq!(
+        format!("{}", &template.render().unwrap_err()),
+        "invalid digit found in string"
+    );
+
+    let template = IntParserTemplate { s: "100" };
+    assert_eq!(template.render().unwrap(), "100=100");
+}
+
+#[derive(Template)]
+#[template(source = "{{ value()? }}", ext = "txt")]
+struct FailFmt {
+    value: fn() -> Result<&'static str, std::fmt::Error>,
+}
+
+#[test]
+fn fail_fmt() {
+    let template = FailFmt {
+        value: || Err(std::fmt::Error),
+    };
+    assert!(matches!(template.render(), Err(askama::Error::Custom(_))));
+    assert_eq!(
+        format!("{}", &template.render().unwrap_err()),
+        format!("{}", std::fmt::Error)
+    );
+
+    let template = FailFmt {
+        value: || Ok("hello world"),
+    };
+    assert_eq!(template.render().unwrap(), "hello world");
+}
+
+#[derive(Template)]
+#[template(source = "{{ value()? }}", ext = "txt")]
+struct FailStr {
+    value: fn() -> Result<&'static str, &'static str>,
+}
+
+#[test]
+fn fail_str() {
+    let template = FailStr {
+        value: || Err("FAIL"),
+    };
+    assert!(matches!(template.render(), Err(askama::Error::Custom(_))));
+    assert_eq!(format!("{}", &template.render().unwrap_err()), "FAIL");
+
+    let template = FailStr {
+        value: || Ok("hello world"),
+    };
+    assert_eq!(template.render().unwrap(), "hello world");
+}