test:zig

guide/language/unreachable.zig

@@ -0,0 +1,30 @@
+const expect = @import("std").testing.expect;
+
+// `unreachable` is an assertion to the compiler that this statement will not be reached.
+// It can tell the compiler that a branch is impossible, which the optimiser can then take advantage of.
+// Reaching an `unreachable` is detectable illegal behaviour.
+// As it is of the type `noreturn`, it is compatible with all other types. Here it coerces to `u32`.
+test "unreachable" {
+    const x: i32 = 1;
+    const y: u32 = if (x == 2) 5 else unreachable;
+    _ = y;
+}
+
+// test "unreachable"...reached unreachable code
+// .\tests.zig:211:39: 0x7ff7e29b2049 in test "unreachable" (test.obj)
+//     const y: u32 = if (x == 2) 5 else unreachable;
+//                                       ^
+
+// Here is an unreachable being used in a switch.
+fn asciiToUpper(x: u8) u8 {
+    return switch (x) {
+        'a'...'z' => x + 'A' - 'a',
+        'A'...'Z' => x,
+        else => unreachable,
+    };
+}
+
+test "unreachable switch" {
+    try expect(asciiToUpper('a') == 'A');
+    try expect(asciiToUpper('A') == 'A');
+}