fix(compiler): mutable lifted object may be modified inflight (#6258)

fixes #3069
The type checker will now modify the type of preflight expressions that are being used inflight to a non-mutable version of the same type: `MutArray` -> `Array`, `MutJson` -> `Json`...

Note that since we change the type of the expression we don't get descriptive error telling us that we're trying to mutate a lifted mutable object, instead we just get the same error we'd expect if the lifted object was non-mutable to begin with. This isn't ideal but it makes the fix simpler. Let me know what you think...

## 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)
- [x] 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)*.

docs/docs/02-concepts/01-preflight-and-inflight.md

@@ -252,7 +252,7 @@ new cloud.Function(checkEndpoint);
 ```
 
 However, mutation to preflight data is not allowed.
-This mean means that variables from preflight cannot be reassigned to, and mutable collections like `MutArray` and `MutMap` cannot be modified.
+This mean means that variables from preflight cannot be reassigned to, and mutable collections like `MutArray` and `MutMap` cannot be modified (they're turned into their immutable counterparts, `Array` and `Map`, respectively when accessed inflight).
 
 ```js playground
 let var count = 3;
@@ -263,8 +263,7 @@ names.push("Jack"); // OK
 
 inflight () => {
   count = count + 1; // error: Variable cannot be reassigned from inflight
-  names.push("Jill"); // error: variable "names" cannot be mutated in inflight - error message not 
-                      // implemented yet, see https://github.com/winglang/wing/issues/3069
+  names.push("Jill"); // error: push doesn't exist in Array
 };
 ```
 

examples/tests/invalid/un_mut_lifted_objects.test.w

@@ -0,0 +1,30 @@
+let ar = MutArray<num>[];
+ar.push(1);
+
+let j = MutJson {a: 1};
+j.set("a", 2);
+
+let st = MutSet<num>[1,2];
+st.add(3);
+
+let mp = MutMap<num>{"a" => 1};
+mp.set("a", 2);
+
+let opt_ar: MutArray<num>? = MutArray<num>[];
+opt_ar?.push(1);
+
+let recursive_ar = MutArray<MutArray<num>>[];
+recursive_ar.push(MutArray<num>[1]);
+recursive_ar.at(0).push(2);
+
+inflight() => {
+  // Same code as above should be an error in inflight
+  ar.push(2); // Error: push doesn't exist in Array
+  ar[0] = 1; // Error: Cannot update elements of an immutable Array
+  j.set("a", 3); // Error: set doesn't exist in Json
+  st.add(4); // Error: add doesn't exist in Set
+  mp.set("a", 3); // Error: set doesn't exist in Map
+  opt_ar?.push(2); // Error: push doesn't exist in Array
+  recursive_ar.push(MutArray<num>[2]); // Error: push doesn't exist in Array
+  recursive_ar.at(0).push(3); // Error: push doesn't exist in Array
+};

libs/wingc/src/type_check.rs

@@ -2846,6 +2846,11 @@ impl<'a> TypeChecker<'a> {
 			}
 		}(exp, env);
 
+		// If we're inflight but the expression is a lifted (preflight) expression then make it immutable
+		if env.phase == Phase::Inflight && phase == Phase::Preflight {
+			t = self.make_immutable(t);
+		}
+
 		self.types.assign_type_to_expr(exp, t, phase);
 
 		self.curr_expr_info.pop();
@@ -4488,10 +4493,6 @@ impl<'a> TypeChecker<'a> {
 
 	fn type_check_assignment(&mut self, kind: &AssignmentKind, value: &Expr, variable: &Reference, env: &mut SymbolEnv) {
 		let (exp_type, _) = self.type_check_exp(value, env);
-
-		// TODO: we need to verify that if this variable is defined in a parent environment (i.e.
-		// being captured) it cannot be reassigned: https://github.com/winglang/wing/issues/3069
-
 		let (var, var_phase) = self.resolve_reference(&variable, env, false);
 		let var_type = match &var {
 			ResolveReferenceResult::Variable(var) => var.type_,
@@ -5691,6 +5692,29 @@ impl<'a> TypeChecker<'a> {
 			.map(|_| base_udt)
 	}
 
+	fn make_immutable(&mut self, type_: TypeRef) -> TypeRef {
+		match *type_ {
+			Type::MutArray(inner) => {
+				let inner = self.make_immutable(inner);
+				self.types.add_type(Type::Array(inner))
+			}
+			Type::MutJson => self.types.json(),
+			Type::MutMap(inner) => {
+				let inner = self.make_immutable(inner);
+				self.types.add_type(Type::Map(inner))
+			}
+			Type::MutSet(inner) => {
+				let inner = self.make_immutable(inner);
+				self.types.add_type(Type::Set(inner))
+			}
+			Type::Optional(inner) => {
+				let inner = self.make_immutable(inner);
+				self.types.add_type(Type::Optional(inner))
+			}
+			_ => type_,
+		}
+	}
+
 	fn resolve_reference(
 		&mut self,
 		reference: &Reference,

tools/hangar/__snapshots__/invalid.ts.snap

@@ -4255,6 +4255,72 @@ error: Expected type to be \\"num\\", but got \\"str\\" instead
 
 
 
+Tests 1 failed (1)
+Snapshots 1 skipped
+Test Files 1 failed (1)
+Duration <DURATION>"
+`;
+
+exports[`un_mut_lifted_objects.test.w 1`] = `
+"error: Member \\"push\\" doesn't exist in \\"Array\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:22:6
+   |
+22 |   ar.push(2); // Error: push doesn't exist in Array
+   |      ^^^^ Member \\"push\\" doesn't exist in \\"Array\\"
+
+
+error: Cannot update elements of an immutable Array
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:23:3
+   |
+23 |   ar[0] = 1; // Error: Cannot update elements of an immutable Array
+   |   ^^^^^ Cannot update elements of an immutable Array
+   |
+   = hint: Consider using MutArray instead
+
+
+error: Member \\"set\\" doesn't exist in \\"Json\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:24:5
+   |
+24 |   j.set(\\"a\\", 3); // Error: set doesn't exist in Json
+   |     ^^^ Member \\"set\\" doesn't exist in \\"Json\\"
+
+
+error: Member \\"add\\" doesn't exist in \\"Set\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:25:6
+   |
+25 |   st.add(4); // Error: add doesn't exist in Set
+   |      ^^^ Member \\"add\\" doesn't exist in \\"Set\\"
+
+
+error: Member \\"set\\" doesn't exist in \\"Map\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:26:6
+   |
+26 |   mp.set(\\"a\\", 3); // Error: set doesn't exist in Map
+   |      ^^^ Member \\"set\\" doesn't exist in \\"Map\\"
+
+
+error: Member \\"push\\" doesn't exist in \\"Array\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:27:11
+   |
+27 |   opt_ar?.push(2); // Error: push doesn't exist in Array
+   |           ^^^^ Member \\"push\\" doesn't exist in \\"Array\\"
+
+
+error: Member \\"push\\" doesn't exist in \\"Array\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:28:16
+   |
+28 |   recursive_ar.push(MutArray<num>[2]); // Error: push doesn't exist in Array
+   |                ^^^^ Member \\"push\\" doesn't exist in \\"Array\\"
+
+
+error: Member \\"push\\" doesn't exist in \\"Array\\"
+   --> ../../../examples/tests/invalid/un_mut_lifted_objects.test.w:29:22
+   |
+29 |   recursive_ar.at(0).push(3); // Error: push doesn't exist in Array
+   |                      ^^^^ Member \\"push\\" doesn't exist in \\"Array\\"
+
+
+
 Tests 1 failed (1)
 Snapshots 1 skipped
 Test Files 1 failed (1)