sentinel pointers and c strings

compiler/lower/lower.odin

@@ -227,6 +227,14 @@ lower_compound_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instructi
 				diagnostic=source.INVALID_DIAGNOSTIC,
 			})
 		}
+		_, array, array_ok := types.array_pointer(container_type, &state.hir_module.types)
+		if array_ok {
+			return append_instruction(state, ir.Instruction{
+				op=.Const, span=expr.span, type=types.USIZE, integer=i64(array.count),
+				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+				diagnostic=source.INVALID_DIAGNOSTIC,
+			})
+		}
 		value := lower_nested_expr(state, expr.left)
 		return append_instruction(state, ir.Instruction{
 			op=.Length, span=expr.span, type=types.USIZE,
@@ -324,7 +332,7 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 					})
 				}
 				_ = pop(&stack)
-			case .Widen, .C_Vararg_Promote, .Weaken_Pointer:
+			case .Widen, .C_Vararg_Promote, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
 				stack[frame_index].stage = 1
 				append(&stack, Lower_Expr_Frame{expr=expr.left})
 			case .Negate:
@@ -357,9 +365,16 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 			continue
 		}
 		if frame.stage == 1 {
+			op := ir.Opcode.Widen
+			#partial switch expr.kind {
+			case .Weaken_Pointer:      op = .Weaken_Pointer
+			case .Weaken_Slice:        op = .Weaken_Slice
+			case .Decay_Array_Pointer: op = .Decay_Array_Pointer
+			case .C_Vararg_Promote:    op = .C_Vararg_Promote
+			case:                      op = .Widen
+			}
 			last = append_instruction(state, ir.Instruction{
-				op=.Weaken_Pointer if expr.kind == .Weaken_Pointer else
-				   (.C_Vararg_Promote if expr.kind == .C_Vararg_Promote else .Widen),
+				op=op,
 				span=expr.span, type=expr.type, target=ir.INVALID_REF,
 				a=last, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
 			})