Add resolver for custom snapshots paths

packages/jest-snapshot/src/__tests__/__snapshots__/snapshot_resolver.test.js.snap

@@ -1,5 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`malformed custom resolver in project config inconsistent functions throws  1`] = `"<bold>Custom snapshot resolver functions must transform paths consistently, i.e. expects resolveTestPath(resolveSnapshotPath('some-path/__tests__/snapshot_resolver.test.js')) === some-path/__SPECS__/snapshot_resolver.test.js</>"`;
+
 exports[`malformed custom resolver in project config missing resolveSnapshotPath throws  1`] = `
 "<bold>Custom snapshot resolver must implement a \`resolveSnapshotPath\` function.</>
 Documentation: https://facebook.github.io/jest/docs/en/configuration.html#snapshotResolver"

packages/jest-snapshot/src/__tests__/fixtures/customSnapshotResolver-inconsistent-fns.js

@@ -0,0 +1,9 @@
+module.exports = {
+  resolveSnapshotPath: (testPath, snapshotExtension) =>
+    testPath.replace('__tests__', '__snapshots__') + snapshotExtension,
+
+  resolveTestPath: (snapshotFilePath, snapshotExtension) =>
+    snapshotFilePath
+      .replace('__snapshots__', '__SPECS__')
+      .slice(0, -snapshotExtension.length),
+};

packages/jest-snapshot/src/__tests__/snapshot_resolver.test.js

@@ -96,4 +96,13 @@ describe('malformed custom resolver in project config', () => {
       buildSnapshotResolver(projectConfig);
     }).toThrowErrorMatchingSnapshot();
   });
+
+  it('inconsistent functions throws ', () => {
+    const projectConfig = newProjectConfig(
+      'customSnapshotResolver-inconsistent-fns.js',
+    );
+    expect(() => {
+      buildSnapshotResolver(projectConfig);
+    }).toThrowErrorMatchingSnapshot();
+  });
 });

packages/jest-snapshot/src/snapshot_resolver.js

@@ -51,12 +51,16 @@ function createCustomSnapshotResolver(
     throw new TypeError(mustImplement('resolveTestPath'));
   }
 
-  return {
+  const customResolver = {
     resolveSnapshotPath: testPath =>
       custom.resolveSnapshotPath(testPath, DOT_EXTENSION),
     resolveTestPath: snapshotPath =>
       custom.resolveTestPath(snapshotPath, DOT_EXTENSION),
   };
+
+  verifyConsistentTransformations(customResolver);
+
+  return customResolver;
 }
 
 function mustImplement(functionName: string) {
@@ -67,3 +71,21 @@ function mustImplement(functionName: string) {
     '\nDocumentation: https://facebook.github.io/jest/docs/en/configuration.html#snapshotResolver'
   );
 }
+
+function verifyConsistentTransformations(custom: SnapshotResolver) {
+  const fakeTestPath = path.join(
+    'some-path',
+    '__tests__',
+    'snapshot_resolver.test.js',
+  );
+  const transformedPath = custom.resolveTestPath(
+    custom.resolveSnapshotPath(fakeTestPath),
+  );
+  if (transformedPath !== fakeTestPath) {
+    throw new Error(
+      chalk.bold(
+        `Custom snapshot resolver functions must transform paths consistently, i.e. expects resolveTestPath(resolveSnapshotPath('${fakeTestPath}')) === ${transformedPath}`,
+      ),
+    );
+  }
+}