Fix: add assignment as right-associative expression

`=` was missing from the Pratt table, causing `a = b;` to fail with
"expected `;`, found `=`". Assignment is now BinaryOp::Assign with
binding power (2, 2) — lowest precedence, right-associative — so
`a = b = c` parses as `a = (b = c)`.

GRAMMAR.ebnf

@@ -33,7 +33,23 @@ top_level_def = func_def
 (* Expressions                                                      *)
 (* ================================================================ *)
 
-expr = or_expr ;
+expr = assign_expr ;
+
+
+(* --- Assignment (lowest-precedence binary operator) ---           *)
+(*                                                                  *)
+(* Uses token `=`; right-associative via recursion.                 *)
+(* The optional form encodes at-most-one assignment target: chains  *)
+(* like `a = b = c` parse as `a = (b = c)` thanks to right          *)
+(* recursion.                                                       *)
+(*                                                                  *)
+(* LL(1): after or_expr, peek at next token.                        *)
+(*   "=" → consume and recurse into assign_expr                     *)
+(*   other → return the or_expr as-is                               *)
+(* "=" is not in FIRST(stmt), so expr_stmt can still be             *)
+(* distinguished from other statement kinds.                        *)
+
+assign_expr = or_expr , [ "=" , assign_expr ] ;
 
 
 (* --- Logical OR (lowest-precedence binary operator) ---           *)
@@ -165,7 +181,9 @@ arg_list = [ expr , { "," , expr } ] ;
 (* if_stmt and while_stmt use expr_ns for their condition.          *)
 (* All other expression positions use the full expr.                *)
 
-expr_ns = or_expr_ns ;
+expr_ns = assign_expr_ns ;
+
+assign_expr_ns = or_expr_ns , [ "=" , assign_expr_ns ] ;
 
 or_expr_ns  = and_expr_ns , { "or"  , and_expr_ns } ;
 and_expr_ns = bitor_expr_ns , { "and" , bitor_expr_ns } ;

SYNTAX.md

@@ -37,6 +37,7 @@ appear as UPPERCASE terminals in `GRAMMAR.ebnf`.
 | `BANG`    | `!`    | Logical NOT                            |
 | `TILDE`   | `~`    | Bitwise NOT                            |
 | `DOT`     | `.`    | Member access                          |
+| `EQ`      | `=`    | Assignment                             |
 
 ### Keyword Tokens
 
@@ -128,23 +129,25 @@ tightly).
 
 | Level | Operators                   | Associativity  | Description                      |
 | ----- | --------------------------- | -------------- | -------------------------------- |
-| 1     | `or`                        | left           | Logical OR (lowest)              |
-| 2     | `and`                       | left           | Logical AND                      |
-| 3     | `\|`                        | left           | Bitwise OR                       |
-| 4     | `^`                         | left           | Bitwise XOR                      |
-| 5     | `&`                         | left           | Bitwise AND                      |
-| 6     | `+`  `-`                    | left           | Addition, subtraction            |
-| 7     | `*`  `/`  `%`               | left           | Multiplication, division, modulo |
-| 8     | `!`  `~`  `-`  `*`  `&`     | right (unary)  | Prefix unary operators           |
-| 9     | `.`  `[…]`  `(…)`           | left (postfix) | Member access, index, call       |
-| 10    | literals, identifiers, `()` | —              | Primary expressions (highest)    |
+| 1     | `=`                         | right          | Assignment (lowest)              |
+| 2     | `or`                        | left           | Logical OR                       |
+| 3     | `and`                       | left           | Logical AND                      |
+| 4     | `\|`                        | left           | Bitwise OR                       |
+| 5     | `^`                         | left           | Bitwise XOR                      |
+| 6     | `&`                         | left           | Bitwise AND                      |
+| 7     | `+`  `-`                    | left           | Addition, subtraction            |
+| 8     | `*`  `/`  `%`               | left           | Multiplication, division, modulo |
+| 9     | `!`  `~`  `-`  `*`  `&`     | right (unary)  | Prefix unary operators           |
+| 10    | `.`  `[…]`  `(…)`           | left (postfix) | Member access, index, call       |
+| 11    | literals, identifiers, `()` | —              | Primary expressions (highest)    |
 
 ### Operator Descriptions
 
 #### Binary Operators
 
 | Operator | Name           | Example   | Notes                                          |
-| -------- | -------------- | --------- | -------------------------------------------- |
+| -------- | -------------- | --------- | ---------------------------------------------- |
+| `=`      | Assignment     | `a = b`   | Right-associative; `a = b = c` → `a = (b = c)` |
 | `or`     | Logical OR     | `a or b`  | Short-circuits; both operands must be `bool`   |
 | `and`    | Logical AND    | `a and b` | Short-circuits; both operands must be `bool`   |
 | `\|`     | Bitwise OR     | `a \| b`  | Integer types                                  |

fluxc/src/ast.rs

@@ -33,6 +33,8 @@ pub enum BinaryOp {
     Mul, // `*`
     Div, // `/`
     Rem, // `%`
+    // Assignment (lowest precedence, right-associative)
+    Assign, // `=`
 }
 
 // ── Types ──────────────────────────────────────────────────────────────────────

fluxc/src/parser.rs

@@ -35,6 +35,9 @@ impl fmt::Display for ParseError {
 
 fn infix_bp(kind: TokenKind) -> Option<(u8, u8)> {
     let bp = match kind {
+        // Assignment: lowest precedence, right-associative (left_bp == right_bp).
+        // `a = b = c` → `a = (b = c)`.
+        TokenKind::Eq => (2, 2),
         TokenKind::Or => (10, 11),
         TokenKind::And => (20, 21),
         TokenKind::Pipe => (30, 31),
@@ -97,6 +100,7 @@ fn token_to_binary_op(kind: TokenKind) -> BinaryOp {
         TokenKind::Star => BinaryOp::Mul,
         TokenKind::Slash => BinaryOp::Div,
         TokenKind::Percent => BinaryOp::Rem,
+        TokenKind::Eq => BinaryOp::Assign,
         _ => unreachable!("not a binary op: {:?}", kind),
     }
 }
@@ -1048,6 +1052,55 @@ mod tests {
         }
     }
 
+    #[test]
+    fn assignment_expr() {
+        let expr = parse("a = b");
+        assert!(matches!(
+            expr.kind,
+            ExprKind::Binary {
+                op: BinaryOp::Assign,
+                ..
+            }
+        ));
+    }
+
+    #[test]
+    fn assignment_right_associative() {
+        // `a = b = c` → `a = (b = c)`
+        let expr = parse("a = b = c");
+        match &expr.kind {
+            ExprKind::Binary {
+                op: BinaryOp::Assign,
+                rhs,
+                ..
+            } => {
+                assert!(matches!(
+                    rhs.kind,
+                    ExprKind::Binary {
+                        op: BinaryOp::Assign,
+                        ..
+                    }
+                ));
+            }
+            _ => panic!("expected assignment"),
+        }
+    }
+
+    #[test]
+    fn assignment_stmt() {
+        let s = stmt("a = b + 1;");
+        match &s.kind {
+            StmtKind::Expr(e) => assert!(matches!(
+                e.kind,
+                ExprKind::Binary {
+                    op: BinaryOp::Assign,
+                    ..
+                }
+            )),
+            _ => panic!("expected expr stmt with assignment"),
+        }
+    }
+
     #[test]
     fn deref_and_addrof() {
         assert!(matches!(