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 }}`

2 files changed, 41 insertions, 1 deletions

diff --git a/askama_shared/src/filters/mod.rs b/askama_shared/src/filters/mod.rs
index 2604ed9..7253e4b 100644
--- a/askama_shared/src/filters/mod.rs
+++ b/askama_shared/src/filters/mod.rs
@@ -58,13 +58,14 @@ const ENCODE_SET: &AsciiSet = &NON_ALPHANUMERIC
 // Askama or should refer to a local `filters` module. It should contain all the
 // filters shipped with Askama, even the optional ones (since optional inclusion
 // in the const vector based on features seems impossible right now).
-pub const BUILT_IN_FILTERS: [&str; 24] = [
+pub const BUILT_IN_FILTERS: [&str; 25] = [
     "abs",
     "capitalize",
     "center",
     "e",
     "escape",
     "filesizeformat",
+    "fmt",
     "format",
     "indent",
     "into_f64",
@@ -139,10 +140,31 @@ pub fn urlencode(s: &dyn fmt::Display) -> Result<String> {
 
 /// Formats arguments according to the specified format
 ///
+/// The *second* argument to this filter must be a string literal (as in normal
+/// Rust). The two arguments are passed through to the `format!()`
+/// [macro](https://doc.rust-lang.org/stable/std/macro.format.html) by
+/// the Askama code generator, but the order is swapped to support filter
+/// composition.
+///
+/// ```ignore
+/// {{ value | fmt("{:?}") }}
+/// ```
+///
+/// Compare with [format](./fn.format.html).
+pub fn fmt() {}
+
+/// Formats arguments according to the specified format
+///
 /// The first argument to this filter must be a string literal (as in normal
 /// Rust). All arguments are passed through to the `format!()`
 /// [macro](https://doc.rust-lang.org/stable/std/macro.format.html) by
 /// the Askama code generator.
+///
+/// ```ignore
+/// {{ "{:?}{:?}" | format(value, other_value) }}
+/// ```
+///
+/// Compare with [fmt](./fn.fmt.html).
 pub fn format() {}
 
 /// Replaces line breaks in plain text with appropriate HTML
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((&");