Improve compilation error message for invalid usage of rest pattern

rinja_parser/src/target.rs

@@ -238,7 +238,7 @@ fn only_one_rest_pattern<'a>(
         if let Target::Rest(s) = target {
             if !allow_named_rest && s.inner.is_some() {
                 return Err(nom::Err::Failure(ErrorContext::new(
-                    format!("`@ ..` cannot be used in {type_kind}"),
+                    "`@ ..` is only allowed in slice patterns",
                     s.span,
                 )));
             }

testing/tests/ui/rest_pattern.stderr

@@ -35,7 +35,7 @@ error: unexpected `,` character after `..`
 31 | | "#,
    | |__^
 
-error: `@ ..` cannot be used in tuple
+error: `@ ..` is only allowed in slice patterns
  --> <source attribute>:2:18
        "who @ .., 4) = (1, 2, 3, 4) -%}\n{%- endif -%}\n"
   --> tests/ui/rest_pattern.rs:39:14