multiple assign

specs.md

@@ -849,6 +849,7 @@ Statements are terminated by `;`.
 
 - **Declaration**: `name :: value;` / `name := value;`
 - **Assignment**: `name = value;` / `name += value;` (and other compound assignments). Also supports field targets: `obj.field = value;`
+- **Multi-target assignment**: `a, b = b, a;` — all RHS values are evaluated before any stores, enabling swaps without temporaries. Target count must equal value count. Only plain `=` is supported (no compound operators). Each target must be a valid lvalue (variable, field, index, dereference).
 - **Expression statement**: `expr;` — evaluates the expression (last in a block = return value)
 - **Return**: `return expr;` — returns from the enclosing function with the given value. `return;` returns void.
 - **Break**: `break;` — exits a match arm or while loop
@@ -1059,7 +1060,7 @@ field_group     = IDENT (',' IDENT)* ':' type ('=' expr)? ';'
 params          = param (',' param)*
 param           = IDENT ':' type
 block           = '{' stmt* '}'
-stmt            = decl | assignment ';' | return_stmt | defer_stmt | insert_stmt
+stmt            = decl | assignment ';' | multi_assign ';' | return_stmt | defer_stmt | insert_stmt
                 | break_stmt | continue_stmt | expr ';'
 return_stmt     = 'return' expr? ';'
 break_stmt      = 'break' ';'
@@ -1067,6 +1068,7 @@ continue_stmt   = 'continue' ';'
 defer_stmt      = 'defer' expr ';'
 insert_stmt     = '#insert' expr ';'
 assignment      = lvalue ('=' | '+=' | '-=' | '*=' | '/=') expr
+multi_assign    = lvalue (',' lvalue)+ '=' expr (',' expr)+
 lvalue          = IDENT | postfix '.' IDENT
 expr            = if_expr | match_expr | while_expr | for_expr | lambda | binary
 while_expr      = 'while' expr block