next

    fn next(it: *StackIterator, io: Io) Result {
        switch (it.*) {
            .ctx_first => |context_ptr| {
                // After the first frame, start actually unwinding.
                if (SelfInfo != void and SelfInfo.can_unwind and fp_usability != .ideal) {
                    it.* = .{ .di = .init(context_ptr) };
                } else {
                    const fp = applyOffset(context_ptr.getFp(), stack_bias) orelse return .end;
                    it.* = .{ .fp = fp };
                }

                // The caller expects *return* addresses, where they will subtract 1 to find the address of the call.
                // However, we have the actual current PC, which should not be adjusted. Compensate by adding 1.
                return .{ .frame = context_ptr.getPc() +| 1 };
            },
            .di => |*unwind_context| {
                const di = getSelfDebugInfo() catch unreachable;
                const ret_addr = di.unwindFrame(io, unwind_context) catch |err| {
                    const pc = unwind_context.pc;
                    const fp = applyOffset(unwind_context.getFp(), stack_bias) orelse return .end;
                    unwind_context.deinit();
                    it.* = .{ .fp = fp };
                    return .{ .switch_to_fp = .{
                        .address = pc,
                        .err = err,
                    } };
                };
                if (ret_addr <= 1) return .end;
                return .{ .frame = ret_addr };
            },
            .fp => |fp| {
                if (fp == 0) return .end; // we reached the "sentinel" base pointer

                const bp_addr = applyOffset(fp, fp_to_bp_offset) orelse return .end;
                const ra_addr = applyOffset(fp, fp_to_ra_offset) orelse return .end;

                if (bp_addr == 0 or !mem.isAligned(bp_addr, @alignOf(usize)) or
                    ra_addr == 0 or !mem.isAligned(ra_addr, @alignOf(usize)))
                {
                    // This isn't valid, but it most likely indicates end of stack.
                    return .end;
                }

                const bp_ptr: *const usize = @ptrFromInt(bp_addr);
                const ra_ptr: *const usize = @ptrFromInt(ra_addr);
                const bp = applyOffset(bp_ptr.*, stack_bias) orelse return .end;

                // If the stack grows downwards, `bp > fp` should always hold; conversely, if it
                // grows upwards, `bp < fp` should always hold. If that is not the case, this
                // frame is invalid, so we'll treat it as though we reached end of stack. The
                // exception is address 0, which is a graceful end-of-stack signal, in which case
                // *this* return address is valid and the *next* iteration will be the last.
                if (bp != 0 and switch (comptime builtin.target.stackGrowth()) {
                    .down => bp <= fp,
                    .up => bp >= fp,
                }) return .end;

                it.fp = bp;
                const ra = stripInstructionPtrAuthCode(ra_ptr.*);
                if (ra <= 1) return .end;
                return .{ .frame = ra };
            },
        }
    }