From 9ee2afecfadbc36ec0e8cf7b69d779b753c82fc4 Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume1.gomez@gmail.com>
Date: Sun, 25 May 2025 15:46:44 +0200
Subject: [PATCH] Add support for mutable variables

---
 askama_derive/src/generator/node.rs      |  6 ++++++
 askama_parser/src/node.rs                | 11 ++++++++++-
 testing/tests/ui/assign-to-askama.rs     |  2 +-
 testing/tests/ui/assign-to-askama.stderr |  6 +++---
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/askama_derive/src/generator/node.rs b/askama_derive/src/generator/node.rs
index 760409d8..a4ccbd12 100644
--- a/askama_derive/src/generator/node.rs
+++ b/askama_derive/src/generator/node.rs
@@ -961,6 +961,9 @@ impl<'a> Generator<'a, '_> {
         let Some(val) = &l.val else {
             self.write_buf_writable(ctx, buf)?;
             buf.write("let ");
+            if l.is_mutable {
+                buf.write("mut ");
+            }
             self.visit_target(buf, false, true, &l.var);
             buf.write(';');
             return Ok(());
@@ -980,6 +983,9 @@ impl<'a> Generator<'a, '_> {
             || matches!(&l.var, Target::Name(name) if self.locals.get(name).is_none())
         {
             buf.write("let ");
+            if l.is_mutable {
+                buf.write("mut ");
+            }
         }
 
         self.visit_target(buf, true, true, &l.var);
diff --git a/askama_parser/src/node.rs b/askama_parser/src/node.rs
index c006dc3e..d6352fbd 100644
--- a/askama_parser/src/node.rs
+++ b/askama_parser/src/node.rs
@@ -1204,6 +1204,7 @@ pub struct Let<'a> {
     pub ws: Ws,
     pub var: Target<'a>,
     pub val: Option<WithSpan<'a, Expr<'a>>>,
+    pub is_mutable: bool,
 }
 
 impl<'a> Let<'a> {
@@ -1212,6 +1213,7 @@ impl<'a> Let<'a> {
         let mut p = (
             opt(Whitespace::parse),
             ws(alt((keyword("let"), keyword("set")))),
+            ws(opt(keyword("mut"))),
             cut_node(
                 Some("let"),
                 (
@@ -1224,7 +1226,7 @@ impl<'a> Let<'a> {
                 ),
             ),
         );
-        let (pws, _, (var, val, nws)) = p.parse_next(i)?;
+        let (pws, _, is_mut, (var, val, nws)) = p.parse_next(i)?;
         if val.is_none() {
             let kind = match &var {
                 Target::Name(_) => None,
@@ -1250,12 +1252,19 @@ impl<'a> Let<'a> {
                 )));
             }
         }
+        if is_mut.is_some() && !matches!(var, Target::Name(_)) {
+            return Err(winnow::error::ErrMode::Cut(ErrorContext::new(
+                "you can only use the `mut` keyword with a variable name",
+                start,
+            )));
+        }
 
         Ok(WithSpan::new(
             Let {
                 ws: Ws(pws, nws),
                 var,
                 val,
+                is_mutable: is_mut.is_some(),
             },
             start,
         ))
diff --git a/testing/tests/ui/assign-to-askama.rs b/testing/tests/ui/assign-to-askama.rs
index f1b5e4ee..5842a150 100644
--- a/testing/tests/ui/assign-to-askama.rs
+++ b/testing/tests/ui/assign-to-askama.rs
@@ -45,7 +45,7 @@ test_kw!(Macro "{% let macro %}");
 test_kw!(Match "{% let match %}");
 test_kw!(Mod "{% let mod %}");
 test_kw!(Move "{% let move %}");
-test_kw!(Mut "{% let mut %}");
+test_kw!(Mut "{% let mut mut %}");
 test_kw!(Override "{% let override %}");
 test_kw!(Priv "{% let priv %}");
 test_kw!(Pub "{% let pub %}");
diff --git a/testing/tests/ui/assign-to-askama.stderr b/testing/tests/ui/assign-to-askama.stderr
index 20f9b83e..f8344abe 100644
--- a/testing/tests/ui/assign-to-askama.stderr
+++ b/testing/tests/ui/assign-to-askama.stderr
@@ -247,12 +247,12 @@ error: cannot use `move` as a name: it is a rust keyword
    |               ^^^^^^^^^^^^^^^^
 
 error: cannot use `mut` as a name: it is a rust keyword
- --> <source attribute>:1:7
+ --> <source attribute>:1:11
        "mut %}"
   --> tests/ui/assign-to-askama.rs:48:14
    |
-48 | test_kw!(Mut "{% let mut %}");
-   |              ^^^^^^^^^^^^^^^
+48 | test_kw!(Mut "{% let mut mut %}");
+   |              ^^^^^^^^^^^^^^^^^^^
 
 error: cannot use `override` as a name: it is a rust keyword
  --> <source attribute>:1:7