add constcast and immutable free

LANGUAGE.md

@@ -34,6 +34,7 @@ roadmap and milestone history.
 - pointer mutability via `mut`, optional pointers as nullable pointers, pointer arithmetic for many-item pointers, postfix dereference `^`, and trapping optional unwrap `?`
 - pointer-to-array `.len`, indexing, slicing, `.ptr` on slices and pointers-to-arrays, implicit address-taking for array-variable slices, and pointer/slice sentinel weakening
 - `ptrcast!(T, ptr)` as a first-pass pointer-child retype that preserves optionality, pointer kind, mutability, and sentinel shape
+- unsafe `constcast!(value)` for restoring mutability to pointers, optional pointers, and slices without changing their child type or shape
 - UTF-8 string literals as immutable pointers to static zero-terminated byte arrays, plus raw backtick multiline strings
 - narrow immutable zero-terminated byte pointer/slice conversion to `*c_char` / `?*c_char` without general `u8`/`c_char` interchange
 - optionals with `null`, `orelse`, postfix `?`, conditional unwraps, guarded unwraps, and left-to-right short-circuiting multi-unwraps

TODO.md

@@ -960,6 +960,11 @@
     value-producing control-flow forms with or without an error capture
   - void fallthrough and diverging `noreturn` fallbacks remain valid
 
+50. add `constcast!` and immutable deallocation (implemented)
+  - `constcast!` restores mutability only for pointers, optional pointers, and slices while
+    preserving child type, pointer kind, optionality, length, and sentinel shape
+  - `std/mem.free` accepts immutable slices and restores mutability only at the allocator boundary
+
 ## A word on unchecked casts
 
 For casts that bypass safety checks, Honey provides builtin functions:
@@ -968,7 +973,8 @@ For casts that bypass safety checks, Honey provides builtin functions:
 | -- | -- | -- |
 | `truncate(x, T)` | Keep low bits, discard rest | Never |
 | `bitcast(x, T)` | Reinterpret bits, no cast | Sizes don't match (compile error) |
-| `ptrcast!(p, T)` | Change pointer type | Gaining mutability (compile error) |
+| `ptrcast!(T, p)` | Change a pointer's child type while preserving its shape | Invalid child or non-pointer operand (compile error) |
+| `constcast!(p)` | Restore pointer or slice mutability | Non-pointer/slice operand (compile error) |
 
 ```honey
 # truncation
@@ -981,12 +987,10 @@ m := bitcast(n, u32)  # m == 0xFFFFFFFF (same bits)
 f: f32 = 3.14
 bits := bitcast(f, u32)   # IEEE 754 representation
 
-# pointer casts (element type, many ↔ single, pointer ↔ usize)
-buf: *u8 = get_buffer()
-ints := ptrcast!(buf, *u32)    # element type change
-single := ptrcast!(buf, @u8)   # many → single (restricting)
-addr := ptrcast!(buf, usize)   # pointer to integer
-ptr := ptrcast!(addr, @u8)     # integer to pointer
+# pointer casts
+buf *u8 = get_buffer()
+ints *u32 = ptrcast!(u32, buf)       # element type change, same pointer shape
+writable *mut u8 = constcast!(buf)   # explicit unsafe mutability restoration
 ```
 
 ## A word on multi-unwrap

compiler/checker/checker.odin

@@ -5187,6 +5187,16 @@ infer_expr :: proc(
 					_ = pop(&stack)
 					continue
 				}
+				if is_intrinsic_call(checker, expr, "constcast") {
+					if len(expr.args) == 1 {
+						operand := infer_nested_expr(checker, expr.args[0], locals, pkg, file, demanded, local_types)
+						last, _ = types.restore_mutability(&checker.module.types, operand)
+					} else {
+						last = types.INVALID
+					}
+					_ = pop(&stack)
+					continue
+				}
 				if is_intrinsic_call(checker, expr, "compile_error") {
 					last = types.VOID
 					_ = pop(&stack)
@@ -9056,6 +9066,17 @@ build_expr :: proc(
 					append(&stack, Build_Expr_Frame{expr=expr.args[1], expected=types.INVALID, template=ast.INVALID_FUNCTION})
 					continue
 				}
+				if is_intrinsic_call(checker, expr, "constcast") {
+					if len(expr.args) != 1 {
+						id := source.addf(checker.diagnostics, expr.span, "constcast! expects 1 argument, got %d", len(expr.args))
+						last = invalid_hir_expr(checker, expr.span, id)
+						_ = pop(&stack)
+						continue
+					}
+					stack[frame_index].stage = 10
+					append(&stack, Build_Expr_Frame{expr=expr.args[0], expected=types.INVALID, template=ast.INVALID_FUNCTION})
+					continue
+				}
 				if is_intrinsic_call(checker, expr, "compile_error") {
 					message := "compile_error! requires one comptime string argument"
 					if len(expr.args) == 1 {
@@ -9716,6 +9737,24 @@ build_expr :: proc(
 			}
 			_ = pop(&stack)
 		}
+		if frame.stage == 10 {
+			result, ok := types.restore_mutability(&checker.module.types, checker.module.exprs[last].type)
+			if !ok {
+				id := source.add(checker.diagnostics, expr.span, "constcast! operand must be a pointer, optional pointer, or slice")
+				last = invalid_hir_expr(checker, expr.span, id)
+			} else {
+				last = add_hir_expr(checker, hir.Expr{
+					kind=.Const_Cast,
+					span=expr.span,
+					type=result,
+					left=last,
+					target=hir.INVALID_REF,
+					right=hir.INVALID_EXPR,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+			}
+			_ = pop(&stack)
+		}
 	}
 	return last
 }

compiler/checker/comptime.odin

@@ -3243,6 +3243,9 @@ ct_eval_call_expr :: proc(state: ^Ct_State, expr: ast.Expr, expected: types.Type
 	if builtin := memory_builtin_call(checker, expr); builtin != .None {
 		return ct_eval_memory_call(state, expr, builtin, depth+1)
 	}
+	if is_intrinsic_call(checker, expr, "constcast") {
+		return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "constcast! is not available during comptime evaluation")
+	}
 	if expr.intrinsic {
 		if symbol.is_valid(expr.qualifier) {
 			return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "intrinsic calls must be unqualified")

compiler/hir/hir.odin

@@ -107,6 +107,7 @@ Expr_Kind :: enum u8 {
 	Retype,
 	Scalar_Cast,
 	Pointer_Cast,
+	Const_Cast,
 	Weaken_Pointer,
 	Weaken_Slice,
 	Decay_Array_Pointer,

compiler/ir/ir.odin

@@ -102,6 +102,7 @@ Opcode :: enum u8 {
 	Retype,
 	Scalar_Cast,
 	Pointer_Cast,
+	Const_Cast,
 	Weaken_Pointer,
 	Weaken_Slice,
 	Decay_Array_Pointer,

compiler/llvm/llvm.odin

@@ -287,7 +287,7 @@ valid_value :: proc(
 	     .Load_Global, .Function_Address, .Address_Of, .Load, .Union_Tag, .Slice, .Length, .Slice_Ptr,
 	     .Fallible_Error, .Extract, .Select, .Unwrap,
 	     .Optional_Is_Some, .Optional_Value, .Orelse,
-	     .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Pointer_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer,
+	     .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Pointer_Cast, .Const_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer,
 	     .Neg_Checked, .Add_Checked, .Sub_Checked, .Mul_Checked, .Div_Checked,
 	     .Div_Trunc_Checked, .Div_Floor_Checked, .Div_Exact_Checked, .Div_Ceil_Checked,
 	     .Rem_Checked, .Mod_Checked,
@@ -1930,6 +1930,16 @@ emit_instruction_stream :: proc(
 				continue
 			}
 			fmt.sbprintf(&emitter.builder, "  %%v%d = select i1 true, ptr %%v%d, ptr null\n", instruction_index, instruction.a)
+		case .Const_Cast:
+			if !valid_instruction(instructions, instruction.a) ||
+			   !types.same_constcast_shape(instructions[instruction.a].type, instruction.type, &emitter.module.types) {
+				emit_recovery_value(emitter, instruction_index, instruction, "invalid const cast operand")
+				continue
+			}
+			type_name := llvm_type(instruction.type, &emitter.module.types)
+			fmt.sbprintf(&emitter.builder, "  %%v%d = select i1 true, %s ", instruction_index, type_name)
+			write_operand(&emitter.builder, instructions, instruction.a, instructions[instruction.a].type, &emitter.module.types)
+			fmt.sbprintf(&emitter.builder, ", %s zeroinitializer\n", type_name)
 		case .Weaken_Slice:
 			if !valid_instruction(instructions, instruction.a) ||
 			   !types.can_weaken_slice(instructions[instruction.a].type, instruction.type, &emitter.module.types) {

compiler/lower/lower.odin

@@ -768,7 +768,7 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 					})
 				}
 				_ = pop(&stack)
-			case .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Pointer_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
+			case .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Pointer_Cast, .Const_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
 				stack[frame_index].stage = 1
 				append(&stack, Lower_Expr_Frame{expr=expr.left})
 			case .Negate, .Bit_Not:
@@ -837,6 +837,7 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 			case .Retype:              op = .Retype
 			case .Scalar_Cast:         op = .Scalar_Cast
 			case .Pointer_Cast:        op = .Pointer_Cast
+			case .Const_Cast:          op = .Const_Cast
 			case:                      op = .Widen
 			}
 			last = append_instruction(state, ir.Instruction{

compiler/types/types.odin

@@ -1340,6 +1340,45 @@ replace_pointer_child :: proc(store: ^Store, value, child: Type) -> (Type, bool)
 	return intern(store, item), true
 }
 
+restore_mutability :: proc(store: ^Store, value: Type) -> (Type, bool) {
+	item, ok := node(store, resolve_alias(value, store))
+	if !ok {
+		return INVALID, false
+	}
+	if item.kind == .Optional {
+		restored, restored_ok := restore_mutability(store, item.child)
+		if !restored_ok || !is_pointer(restored, store) {
+			return INVALID, false
+		}
+		return optional(store, restored), true
+	}
+	if item.kind != .Pointer && item.kind != .Slice {
+		return INVALID, false
+	}
+	item.mutable = true
+	return intern(store, item), true
+}
+
+same_constcast_shape :: proc(from, to: Type, store: ^Store) -> bool {
+	from_item, from_ok := node(store, resolve_alias(from, store))
+	to_item, to_ok := node(store, resolve_alias(to, store))
+	if !from_ok || !to_ok {
+		return false
+	}
+	if from_item.kind == .Optional || to_item.kind == .Optional {
+		return from_item.kind == .Optional && to_item.kind == .Optional &&
+		       is_pointer(from_item.child, store) && is_pointer(to_item.child, store) &&
+		       same_constcast_shape(from_item.child, to_item.child, store)
+	}
+	return (from_item.kind == .Pointer || from_item.kind == .Slice) &&
+	       from_item.kind == to_item.kind &&
+	       from_item.child == to_item.child &&
+	       from_item.many == to_item.many &&
+	       to_item.mutable &&
+	       from_item.has_sentinel == to_item.has_sentinel &&
+	       (!from_item.has_sentinel || from_item.sentinel == to_item.sentinel)
+}
+
 same_pointer_shape :: proc(left, right: Type, store: ^Store) -> bool {
 	left_item, left_ok := node(store, left)
 	right_item, right_ok := node(store, right)

compiler_tests.odin

@@ -2184,6 +2184,148 @@ main func() void {
 	testing.expect(t, found_target_type)
 }
 
+@(test)
+constcast_restores_pointer_and_slice_mutability :: proc(t: ^testing.T) {
+	text := `main func() i32 {
+	values [2]mut u8 = [1, 2]
+	immutable_slice []u8 = values[..]
+	mutable_slice []mut u8 = constcast!(immutable_slice)
+	mutable_slice[0] = 3
+
+	immutable_many *u8 = immutable_slice.ptr
+	mutable_many *mut u8 = constcast!(immutable_many)
+	mutable_many[1] = 4
+
+	number i32 = 5
+	immutable_single @i32 = &number
+	mutable_single @mut i32 = constcast!(immutable_single)
+	mutable_single^ = 6
+
+	maybe ?@i32 = immutable_single
+	mutable_maybe ?@mut i32 = constcast!(maybe)
+	if mutable_maybe |pointer| { pointer^ = 7 }
+
+	already_mutable []mut u8 = constcast!(mutable_slice)
+	if already_mutable[0] != 3 or already_mutable[1] != 4 or number != 7 { return 1 }
+	return 0
+}
+`
+	source_file := source.Source{path="constcast.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	ir_module := lower.lower(&hir_module)
+	defer ir.destroy_module(&ir_module)
+
+	hir_casts, ir_casts := 0, 0
+	for expr in hir_module.exprs {
+		hir_casts += 1 if expr.kind == .Const_Cast else 0
+	}
+	for function in ir_module.functions {
+		for instruction in function.instructions {
+			ir_casts += 1 if instruction.op == .Const_Cast else 0
+		}
+	}
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect_value(t, hir_casts, 5)
+	testing.expect_value(t, ir_casts, 5)
+
+	directory := "/tmp/brolang-test-constcast"
+	main_path := "/tmp/brolang-test-constcast/main.bro"
+	output := "/tmp/brolang-test-constcast-output"
+	_ = os2.remove_all(directory)
+	defer _ = os2.remove_all(directory)
+	defer _ = os.remove(output)
+	testing.expect(t, os.make_directory(directory) == nil)
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)text))
+	testing.expect_value(t, compiler_core.compile_package(directory, output), 0)
+	state := run_executable(output)
+	testing.expect_value(t, state.exit_code, 0)
+}
+
+@(test)
+invalid_constcasts_are_rejected :: proc(t: ^testing.T) {
+	text := `main func() void {
+	values [2]mut u8 = [1, 2]
+	_ = constcast!()
+	_ = constcast!(1, 2)
+	_ = constcast!(1)
+	_ = constcast!(values)
+}
+`
+	source_file := source.Source{path="invalid_constcast.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+
+	bad_arity, bad_operand := false, false
+	for diagnostic in diagnostics.items {
+		bad_arity = bad_arity || strings.contains(diagnostic.message, "constcast! expects 1 argument")
+		bad_operand = bad_operand || strings.contains(diagnostic.message, "constcast! operand must be a pointer, optional pointer, or slice")
+	}
+	testing.expect(t, bad_arity && bad_operand)
+}
+
+@(test)
+immutable_allocations_can_be_freed_and_constcast_keeps_slice_bounds_checks :: proc(t: ^testing.T) {
+	free_text := `mem :: import "@std/mem"
+main func() i32 {
+	memory []mut u8 :: mem.alloc(u8, mem.c_allocator, 4) catch |_| { return 1 }
+	memory[0] = 42
+	immutable []u8 = memory
+	mem.free(mem.c_allocator, immutable)
+	empty []u8 = mem.empty(u8)
+	mem.free(mem.c_allocator, empty)
+	return 0
+}
+`
+	directory := "/tmp/brolang-test-immutable-free"
+	main_path := "/tmp/brolang-test-immutable-free/main.bro"
+	output := "/tmp/brolang-test-immutable-free-output"
+	_ = os2.remove_all(directory)
+	defer _ = os2.remove_all(directory)
+	defer _ = os.remove(output)
+	testing.expect(t, os.make_directory(directory) == nil)
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)free_text))
+	testing.expect_value(t, compiler_core.compile_package(
+		directory, output, nil, target.DEFAULT, cimport.Options{}, ".",
+	), 0)
+	state := run_executable(output)
+	testing.expect_value(t, state.exit_code, 0)
+
+	bounds_text := `main func() void {
+	values [2]mut u8 = [1, 2]
+	immutable []u8 = values[..]
+	mutable []mut u8 = constcast!(immutable)
+	_ = mutable[mutable.len]
+}
+`
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)bounds_text))
+	testing.expect_value(t, compiler_core.compile_package(directory, output), 0)
+	bounds_state, stdout, stderr, err := os2.process_exec(
+		os2.Process_Desc{command=[]string{output}}, context.allocator,
+	)
+	defer delete(stdout)
+	defer delete(stderr)
+	testing.expect(t, err == nil)
+	testing.expect(t, !bounds_state.success)
+	testing.expect(t, strings.contains(string(stderr), "index out of bounds"))
+}
+
 @(test)
 old_intrinsic_spellings_are_not_recognized :: proc(t: ^testing.T) {
 	text := `main func() void {

std/mem/mem.bro

@@ -110,9 +110,10 @@ realloc func($T type, allocator Allocator, memory []mut T, new_count usize) []mu
 	return .out_of_memory
 }
 
-free func($T type, allocator Allocator, memory []mut T) void {
+free func($T type, allocator Allocator, memory []T) void {
 	if memory.len != 0 and sizeof!(T) != 0 {
-		raw_free(allocator, ptrcast!(u8, memory.ptr), memory.len * sizeof!(T), alignof!(T))
+		mutable_memory []mut T :: constcast!(memory)
+		raw_free(allocator, ptrcast!(u8, mutable_memory.ptr), memory.len * sizeof!(T), alignof!(T))
 	}
 }
 

tree-sitter-brolang/test/highlight/intrinsics.bro

@@ -10,4 +10,7 @@ main func() void {
 #	         ^ operator
 	_ = sizeof(i32)
 #	    ^^^^^^ function
+	_ = constcast!(memory)
+#	    ^^^^^^^^^ function.builtin
+#	            ^ operator
 }