feat(compiler): adding reassignable to if let (if let var) (#3840)

I found the 'TODO: support if let var' in tree-sitter-wing and had some free time 🤷‍♂️

## Checklist

- [x] Title matches [Winglang's style guide](https://www.winglang.io/contributing/start-here/pull_requests#how-are-pull-request-titles-formatted)
- [x] Description explains motivation and solution
- [x] Tests added (always)
- [ ] Docs updated (only required for features)
- [ ] Added `pr/e2e-full` label if this feature requires end-to-end testing

*By submitting this pull request, I confirm that my contribution is made under the terms of the [Wing Cloud Contribution License](https://github.com/winglang/wing/blob/main/CONTRIBUTION_LICENSE.md)*.

examples/tests/valid/optionals.w

@@ -58,6 +58,13 @@ let tryParseName = (fullName: str): Name? => {
   };
 };
 
+// if lets reassignable
+let a: num? = 1;
+if let var z = a {
+  assert(z == 1);
+  z = 2;
+  assert(z == 2);
+}
 
 // Nested if lets
 if let parsedName = tryParseName("Good Name") {

libs/tree-sitter-wing/grammar.js

@@ -274,8 +274,8 @@ module.exports = grammar({
 
     if_let_statement: ($) =>
       seq(
-        // TODO: support "if let var"
         "if let",
+        optional(field("reassignable", $.reassignable)),
         field("name", $.identifier),
         "=",
         field("value", $.expression),

libs/tree-sitter-wing/test/corpus/statements/statements.txt

@@ -401,6 +401,22 @@ if let x = y {} else {}
     block: (block)
     else_block: (block)))
 
+================================================================================
+If Let Var
+================================================================================
+
+if let var x = y {}
+
+--------------------------------------------------------------------------------
+
+(source
+  (if_let_statement
+    reassignable: (reassignable)
+    name: (identifier)
+    value: (reference
+      (reference_identifier))
+    block: (block)))
+
 ================================================================================
 Debug symbol env
 ================================================================================

libs/wingc/src/ast.rs

@@ -451,6 +451,7 @@ pub enum StmtKind {
 		statements: Scope,
 	},
 	IfLet {
+		reassignable: bool,
 		var_name: Symbol,
 		value: Expr,
 		statements: Scope,

libs/wingc/src/fold.rs

@@ -116,11 +116,13 @@ where
 		StmtKind::IfLet {
 			value,
 			statements,
+			reassignable,
 			var_name,
 			else_statements,
 		} => StmtKind::IfLet {
 			value: f.fold_expr(value),
 			statements: f.fold_scope(statements),
+			reassignable,
 			var_name: f.fold_symbol(var_name),
 			else_statements: else_statements.map(|statements| f.fold_scope(statements)),
 		},

libs/wingc/src/jsify.rs

@@ -853,6 +853,7 @@ impl<'a> JSifier<'a> {
 			StmtKind::Break => CodeMaker::one_line("break;"),
 			StmtKind::Continue => CodeMaker::one_line("continue;"),
 			StmtKind::IfLet {
+				reassignable,
 				value,
 				statements,
 				var_name,
@@ -894,7 +895,11 @@ impl<'a> JSifier<'a> {
 					self.jsify_expression(value, ctx)
 				));
 				code.open(format!("if ({if_let_value} != undefined) {{"));
-				code.line(format!("const {} = {};", var_name, if_let_value));
+				if *reassignable {
+					code.line(format!("let {} = {};", var_name, if_let_value));
+				} else {
+					code.line(format!("const {} = {};", var_name, if_let_value));
+				}
 				code.add_code(self.jsify_scope_body(statements, ctx));
 				code.close("}");
 

libs/wingc/src/parser.rs

@@ -568,6 +568,7 @@ impl<'s> Parser<'s> {
 
 	fn build_if_let_statement(&self, statement_node: &Node, phase: Phase) -> DiagnosticResult<StmtKind> {
 		let if_block = self.build_scope(&statement_node.child_by_field_name("block").unwrap(), phase);
+		let reassignable = statement_node.child_by_field_name("reassignable").is_some();
 		let value = self.build_expression(&statement_node.child_by_field_name("value").unwrap(), phase)?;
 		let name = self.check_reserved_symbol(&statement_node.child_by_field_name("name").unwrap())?;
 		let else_block = if let Some(else_block) = statement_node.child_by_field_name("else_block") {
@@ -577,6 +578,7 @@ impl<'s> Parser<'s> {
 		};
 		Ok(StmtKind::IfLet {
 			var_name: name,
+			reassignable,
 			value,
 			statements: if_block,
 			else_statements: else_block,

libs/wingc/src/type_check.rs

@@ -2808,6 +2808,7 @@ impl<'a> TypeChecker<'a> {
 			StmtKind::IfLet {
 				value,
 				statements,
+				reassignable,
 				var_name,
 				else_statements,
 			} => {
@@ -2838,7 +2839,7 @@ impl<'a> TypeChecker<'a> {
 				// Add the variable to if block scope
 				match stmt_env.define(
 					var_name,
-					SymbolKind::make_free_variable(var_name.clone(), var_type, false, env.phase),
+					SymbolKind::make_free_variable(var_name.clone(), var_type, *reassignable, env.phase),
 					StatementIdx::Top,
 				) {
 					Err(type_error) => {

libs/wingc/src/visit.rs

@@ -134,6 +134,7 @@ where
 		StmtKind::IfLet {
 			value,
 			statements,
+			reassignable: _,
 			var_name,
 			else_statements,
 		} => {

tools/hangar/__snapshots__/test_corpus/valid/optionals.w_compile_tf-aws.md

@@ -481,6 +481,16 @@ class $Root extends $stdlib.std.Resource {
       }
       return ({"first": (parts.at(0)),"last": (parts.at(1))});
     });
+    const a = 1;
+    {
+      const $IF_LET_VALUE = a;
+      if ($IF_LET_VALUE != undefined) {
+        let z = $IF_LET_VALUE;
+        {((cond) => {if (!cond) throw new Error("assertion failed: z == 1")})((((a,b) => { try { return require('assert').deepStrictEqual(a,b) === undefined; } catch { return false; } })(z,1)))};
+        z = 2;
+        {((cond) => {if (!cond) throw new Error("assertion failed: z == 2")})((((a,b) => { try { return require('assert').deepStrictEqual(a,b) === undefined; } catch { return false; } })(z,2)))};
+      }
+    }
     {
       const $IF_LET_VALUE = (tryParseName("Good Name"));
       if ($IF_LET_VALUE != undefined) {