feat: Attach actual/expected args to error instance to enable IDE diffs

src/errors.spec.ts

@@ -158,6 +158,41 @@ foobar`
       // Yeah, funky way to do a negated ansiless contains.
       expect(() => expectAnsilessContain(error.message, `bar`)).toThrow();
     });
+
+    it("should contain actual and expected values when there's a single expectation remaining", () => {
+      const matcher = It.matches(() => false, {
+        getDiff: () => ({ actual: 'actual', expected: 'expected' }),
+      });
+
+      const expectation = new StrongExpectation('foo', [matcher, matcher], {
+        value: ':irrelevant:',
+      });
+
+      const error = new UnexpectedCall(
+        'foo',
+        ['any arg', 'any arg'],
+        [expectation]
+      );
+
+      expect(error.matcherResult).toEqual({
+        actual: ['actual', 'actual'],
+        expected: ['expected', 'expected'],
+      });
+    });
+
+    it('should not contain actual and expected values when the expectation has no expected args', () => {
+      const expectation = new StrongExpectation('foo', [], {
+        value: ':irrelevant:',
+      });
+
+      const error = new UnexpectedCall(
+        'foo',
+        ['any arg', 'any arg'],
+        [expectation]
+      );
+
+      expect(error.matcherResult).toBeUndefined();
+    });
   });
 
   describe('UnexpectedCalls', () => {

src/errors.ts

@@ -1,5 +1,6 @@
 import { EXPECTED_COLOR } from 'jest-matcher-utils';
 import type { Expectation } from './expectation/expectation';
+import { getMatcherDiffs } from './expectation/matcher';
 import type { CallMap } from './expectation/repository/expectation-repository';
 import {
   printCall,
@@ -42,7 +43,16 @@ ${printRemainingExpectations(expectations)}`);
   }
 }
 
-export class UnexpectedCall extends Error {
+type MatcherResult = { expected: unknown; actual: unknown };
+
+// This is taken from jest.
+interface MatcherError {
+  matcherResult?: MatcherResult;
+}
+
+export class UnexpectedCall extends Error implements MatcherError {
+  public matcherResult?: MatcherResult;
+
   constructor(
     property: Property,
     args: unknown[],
@@ -62,8 +72,26 @@ export class UnexpectedCall extends Error {
 
 Remaining expectations:
 ${printDiffForAllExpectations(propertyExpectations, args)}`);
+
+      // If we have a single expectation we can attach the actual/expected args
+      // to the error instance, so that an IDE may show its own diff for them.
+      if (
+        propertyExpectations.length === 1 &&
+        propertyExpectations[0].args?.length
+      ) {
+        const { actual, expected } = getMatcherDiffs(
+          propertyExpectations[0].args,
+          args
+        );
+        this.matcherResult = {
+          actual,
+          expected,
+        };
+      }
     } else {
-      super(header);
+      super(`${header}
+      
+No remaining expectations.`);
     }
   }
 }

src/expectation/matcher.ts

@@ -55,3 +55,14 @@ export type TypeMatcher<T> = T & Matcher;
 export function isMatcher(f: unknown): f is Matcher {
   return !!(f && (<Matcher>f)[MATCHER_SYMBOL]);
 }
+
+export const getMatcherDiffs = (
+  matchers: Matcher[],
+  args: any[]
+): { actual?: unknown[]; expected?: unknown[] } => {
+  const matcherDiffs = matchers.map((matcher, i) => matcher.getDiff(args[i]));
+  const actual = matcherDiffs?.map((d) => d.actual);
+  const expected = matcherDiffs?.map((d) => d.expected);
+
+  return { actual, expected };
+};

src/print.ts

@@ -7,7 +7,7 @@ import {
 } from 'jest-matcher-utils';
 import type { Expectation } from './expectation/expectation';
 import { ApplyProp } from './expectation/expectation';
-import { isMatcher } from './expectation/matcher';
+import { getMatcherDiffs, isMatcher } from './expectation/matcher';
 import type { ReturnValue } from './expectation/repository/return-value';
 import type { Property } from './proxy';
 
@@ -83,13 +83,8 @@ export const printExpectationDiff = (e: Expectation, args: any[]) => {
     return '';
   }
 
-  const matcherDiffs = e.args?.map((matcher, j) => matcher.getDiff(args[j]));
-
-  const diff = printDiff(
-    matcherDiffs?.map((d) => d.expected),
-    matcherDiffs?.map((d) => d.actual),
-    { omitAnnotationLines: true }
-  );
+  const { actual, expected } = getMatcherDiffs(e.args, args);
+  const diff = printDiff(expected, actual, { omitAnnotationLines: true });
 
   if (!diff) {
     return '';