Emit a warning for let without a value

Author: GuillaumeGomez

askama_derive/src/generator/node.rs

@@ -7,7 +7,8 @@ use std::str::FromStr;
 
 use parser::expr::BinOp;
 use parser::node::{
-    Call, Comment, Cond, CondTest, Create, FilterBlock, If, Include, Let, Lit, Loop, Match, Whitespace, Ws,
+    Call, Comment, Cond, CondTest, Create, FilterBlock, If, Include, Let, Lit, Loop, Match,
+    Whitespace, Ws,
 };
 use parser::{Expr, Node, Span, Target, WithSpan};
 use proc_macro2::TokenStream;
@@ -861,6 +862,14 @@ impl<'a> Generator<'a, '_> {
         let span = ctx.span_for_node(l.span());
 
         let Some(val) = &l.val else {
+            let file_info = ctx
+                .file_info_of(l.span())
+                .map(|info| format!(" {}:", info.to_string()))
+                .unwrap_or(String::new());
+            eprintln!(
+                "⚠️{file_info} `let` tag will stop supporting declaring variables without value. \
+                 Use `create` instead for this case",
+            );
             self.write_buf_writable(ctx, buf)?;
             buf.write_token(Token![let], span);
             if l.is_mutable {