feat(expect): support expect.unreachable (#3556)

Co-authored-by: Vladimir <sleuths.slews0s@icloud.com>

docs/api/expect.md

@@ -42,7 +42,7 @@ type Awaitable<T> = T | PromiseLike<T>
   })
   // At the end of the test, the above errors will be output.
   ```
-  
+
   It can also be used with `expect`. if `expect` assertion fails, the test will be terminated and all errors will be displayed.
 
   ```ts
@@ -54,7 +54,7 @@ type Awaitable<T> = T | PromiseLike<T>
     expect.soft(1 + 2).toBe(4) // do not run
   })
   ```
-  
+
 ::: warning
 `expect.soft` can only be used inside the [`test`](/api/#test) function.
 :::
@@ -1136,6 +1136,49 @@ If the value in the error message is too truncated, you can increase [chaiConfig
   })
   ```
 
+## expect.unreachable
+
+- **Type:** `(message?: string) => never`
+
+  This method is used to asserting that a line should never be reached.
+
+  For example, if we want to test that `build()` throws due to receiving directories having no `src` folder, and also handle each error separately, we could do this:
+
+  ```ts
+  import { expect, test } from 'vitest'
+
+  async function build(dir) {
+    if (dir.includes('no-src'))
+      throw new Error(`${dir}/src does not exist`)
+  }
+
+  const errorDirs = [
+    'no-src-folder',
+    // ...
+  ]
+
+  test.each(errorDirs)('build fails with "%s"', async (dir) => {
+    try {
+      await build(dir)
+      expect.unreachable('Should not pass build')
+    }
+    catch (err: any) {
+      expect(err).toBeInstanceOf(Error)
+      expect(err.stack).toContain('build')
+
+      switch (dir) {
+        case 'no-src-folder':
+          expect(err.message).toBe(`${dir}/src does not exist`)
+          break
+        default:
+          // to exhaust all error tests
+          expect.unreachable('All error test must be handled')
+          break
+      }
+    }
+  })
+  ```
+
 <!-- asymmetric matchers -->
 
 ## expect.anything

packages/expect/src/types.ts

@@ -80,6 +80,7 @@ export type MatchersObject<T extends MatcherState = MatcherState> = Record<strin
 
 export interface ExpectStatic extends Chai.ExpectStatic, AsymmetricMatchersContaining {
   <T>(actual: T, message?: string): Assertion<T>
+  unreachable(message?: string): never
   soft<T>(actual: T, message?: string): Assertion<T>
   extend(expects: MatchersObject): void
   assertions(expected: number): void

packages/vitest/src/integrations/chai/index.ts

@@ -53,6 +53,10 @@ export function createExpect(test?: Test) {
     return assert
   }
 
+  expect.unreachable = (message?: string) => {
+    chai.assert.fail(`expected${message ? ` "${message}" ` : ' '}not to be reached`)
+  }
+
   function assertions(expected: number) {
     const errorGen = () => new Error(`expected number of assertions to be ${expected}, but got ${expect.getState().assertionCalls}`)
     if (Error.captureStackTrace)

test/fails/fixtures/expect-unreachable.test.ts

@@ -0,0 +1,5 @@
+import { expect, test } from 'vitest'
+
+test('', () => {
+  expect.unreachable('hi')
+})

test/fails/test/__snapshots__/runner.test.ts.snap

@@ -10,6 +10,8 @@ exports[`should fail expect.test.ts > expect.test.ts 1`] = `"AssertionError: exp
 
 exports[`should fail expect-soft.test.ts > expect-soft.test.ts 1`] = `"Error: expect.soft() can only be used inside a test"`;
 
+exports[`should fail expect-unreachable.test.ts > expect-unreachable.test.ts 1`] = `"AssertionError: expected \\"hi\\" not to be reached"`;
+
 exports[`should fail hook-timeout.test.ts > hook-timeout.test.ts 1`] = `"Error: Hook timed out in 10ms."`;
 
 exports[`should fail hooks-called.test.ts > hooks-called.test.ts 1`] = `