unreachable

In Debug and ReleaseSafe mode unreachable emits a call to panic with the message reached unreachable code.

In ReleaseFast and ReleaseSmall mode, the optimizer uses the assumption that unreachable code will never be hit to perform optimizations.

Basics

test_unreachable.zig

  1. // unreachable is used to assert that control flow will never reach a
  2. // particular location:
  3. test "basic math" {
  4.     const x = 1;
  5.     const y = 2;
  6.     if (x + y != 3) {
  7.         unreachable;
  8.     }
  9. }

Shell

  1. $ zig test test_unreachable.zig
  2. 1/1 test.basic math... OK
  3. All 1 tests passed.

In fact, this is how std.debug.assert is implemented:

test_assertion_failure.zig

  1. // This is how std.debug.assert is implemented
  2. fn assert(ok: bool) void {
  3.     if (!ok) unreachable; // assertion failure
  4. }
  6. // This test will fail because we hit unreachable.
  7. test "this will fail" {
  8.     assert(false);
  9. }

Shell

  1. $ zig test test_assertion_failure.zig
  2. 1/1 test.this will fail... thread 2453337 panic: reached unreachable code
  3. /home/ci/actions-runner/_work/zig-bootstrap/zig/docgen_tmp/test_assertion_failure.zig:3:14: 0x2241e2 in assert (test)
  4.     if (!ok) unreachable; // assertion failure
  5.              ^
  6. /home/ci/actions-runner/_work/zig-bootstrap/zig/docgen_tmp/test_assertion_failure.zig:8:11: 0x22419a in test.this will fail (test)
  7.     assert(false);
  8.           ^
  9. /home/ci/actions-runner/_work/zig-bootstrap/out/host/lib/zig/test_runner.zig:176:28: 0x22ceb9 in mainTerminal (test)
  10.         } else test_fn.func();
  11.                            ^
  12. /home/ci/actions-runner/_work/zig-bootstrap/out/host/lib/zig/test_runner.zig:36:28: 0x22503a in main (test)
  13.         return mainTerminal();
  14.                            ^
  15. /home/ci/actions-runner/_work/zig-bootstrap/out/host/lib/zig/std/start.zig:564:22: 0x2246b2 in posixCallMainAndExit (test)
  16.             root.main();
  17.                      ^
  18. /home/ci/actions-runner/_work/zig-bootstrap/out/host/lib/zig/std/start.zig:243:5: 0x224201 in _start (test)
  19.     asm volatile (switch (native_arch) {
  20.     ^
  21. ???:?:?: 0x0 in ??? (???)
  22. error: the following test command crashed:
  23. /home/ci/actions-runner/_work/zig-bootstrap/out/zig-local-cache/o/3f6575b81fc83074e649e9b167fa28e1/test

At Compile-Time

test_comptime_unreachable.zig

  1. const assert = @import("std").debug.assert;
  3. test "type of unreachable" {
  4.     comptime {
  5.         // The type of unreachable is noreturn.
  7.         // However this assertion will still fail to compile because
  8.         // unreachable expressions are compile errors.
  10.         assert(@TypeOf(unreachable) == noreturn);
  11.     }
  12. }

Shell

  1. $ zig test test_comptime_unreachable.zig
  2. docgen_tmp/test_comptime_unreachable.zig:10:16: error: unreachable code
  3.         assert(@TypeOf(unreachable) == noreturn);
  4.                ^~~~~~~~~~~~~~~~~~~~
  5. docgen_tmp/test_comptime_unreachable.zig:10:24: note: control flow is diverted here
  6.         assert(@TypeOf(unreachable) == noreturn);
  7.                        ^~~~~~~~~~~

See also: