Add fmt filter that swaps the first two arguments to format!().

This allows a more natural filter usage: `{{ val | fmt("{:?}") }}`
as well as enabling convenient filter composition:
`{{ price | to_f64 | fmt("${:.2}") | center }}`

1 files changed, 18 insertions, 0 deletions

diff --git a/askama_shared/src/generator.rs b/askama_shared/src/generator.rs
index 82f68ff..e45acb6 100644
--- a/askama_shared/src/generator.rs
+++ b/askama_shared/src/generator.rs
@@ -999,6 +999,9 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
         if name == "format" {
             self._visit_format_filter(buf, args);
             return DisplayWrap::Unwrapped;
+        } else if name == "fmt" {
+            self._visit_fmt_filter(buf, args);
+            return DisplayWrap::Unwrapped;
         } else if name == "join" {
             self._visit_join_filter(buf, args);
             return DisplayWrap::Unwrapped;
@@ -1038,6 +1041,21 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
         buf.write(")");
     }
 
+    fn _visit_fmt_filter(&mut self, buf: &mut Buffer, args: &[Expr]) {
+        buf.write("format!(");
+        if let Some(Expr::StrLit(v)) = args.get(1) {
+            self.visit_str_lit(buf, v);
+            buf.write(", ");
+        } else {
+            panic!("invalid expression type for fmt filter");
+        }
+        self._visit_args(buf, &args[0..1]);
+        if args.len() > 2 {
+            panic!("only two arguments allowed to fmt filter");
+        }
+        buf.write(")");
+    }
+
     // Force type coercion on first argument to `join` filter (see #39).
     fn _visit_join_filter(&mut self, buf: &mut Buffer, args: &[Expr]) {
         buf.write("::askama::filters::join((&");