[Content collections] Better error formatting (#6508)

* feat: new error map with unions, literal, and fbs

* fix: bad type casting on type error

* refactor: add comments, clean up edge cases

* refactor: JSON.stringify, add colon for follow ups

* refactor: bold file name in msg

* fix: prefix union errors

* test: new error mapping

* refactor: clean up Required handling

* test: required, fallthrough

* chore: changeset

* fix: out-of-date error msg

* chore: stray console log

* docs: revert to old error msg

7 files changed, 220 insertions, 16 deletions

diff --git a/.changeset/quick-turtles-decide.md b/.changeset/quick-turtles-decide.md
new file mode 100644
index 000000000..e3ce95beb
--- /dev/null
+++ b/.changeset/quick-turtles-decide.md
@@ -0,0 +1,8 @@
+---
+'astro': patch
+---
+
+Improve content collection error formatting:
+- Bold the collection and entry that failed
+- Consistently list the frontmatter key at the start of every error
+- Rich errors for union types
diff --git a/packages/astro/src/content/error-map.ts b/packages/astro/src/content/error-map.ts
new file mode 100644
index 000000000..51c8cdde1
--- /dev/null
+++ b/packages/astro/src/content/error-map.ts
@@ -0,0 +1,99 @@
+import type { ZodErrorMap } from 'zod';
+
+type TypeOrLiteralErrByPathEntry = {
+	code: 'invalid_type' | 'invalid_literal';
+	received: unknown;
+	expected: unknown[];
+};
+
+export const errorMap: ZodErrorMap = (baseError, ctx) => {
+	const baseErrorPath = flattenErrorPath(baseError.path);
+	if (baseError.code === 'invalid_union') {
+		// Optimization: Combine type and literal errors for keys that are common across ALL union types
+		// Ex. a union between `{ key: z.literal('tutorial') }` and `{ key: z.literal('blog') }` will
+		// raise a single error when `key` does not match:
+		// > Did not match union.
+		// > key: Expected `'tutorial' | 'blog'`, received 'foo'
+		let typeOrLiteralErrByPath: Map<string, TypeOrLiteralErrByPathEntry> = new Map();
+		for (const unionError of baseError.unionErrors.map((e) => e.errors).flat()) {
+			if (unionError.code === 'invalid_type' || unionError.code === 'invalid_literal') {
+				const flattenedErrorPath = flattenErrorPath(unionError.path);
+				if (typeOrLiteralErrByPath.has(flattenedErrorPath)) {
+					typeOrLiteralErrByPath.get(flattenedErrorPath)!.expected.push(unionError.expected);
+				} else {
+					typeOrLiteralErrByPath.set(flattenedErrorPath, {
+						code: unionError.code,
+						received: unionError.received,
+						expected: [unionError.expected],
+					});
+				}
+			}
+		}
+		let messages: string[] = [
+			prefix(
+				baseErrorPath,
+				typeOrLiteralErrByPath.size ? 'Did not match union:' : 'Did not match union.'
+			),
+		];
+		return {
+			message: messages
+				.concat(
+					[...typeOrLiteralErrByPath.entries()]
+						// If type or literal error isn't common to ALL union types,
+						// filter it out. Can lead to confusing noise.
+						.filter(([, error]) => error.expected.length === baseError.unionErrors.length)
+						.map(([key, error]) =>
+							key === baseErrorPath
+								? // Avoid printing the key again if it's a base error
+								  `> ${getTypeOrLiteralMsg(error)}`
+								: `> ${prefix(key, getTypeOrLiteralMsg(error))}`
+						)
+				)
+				.join('\n'),
+		};
+	}
+	if (baseError.code === 'invalid_literal' || baseError.code === 'invalid_type') {
+		return {
+			message: prefix(
+				baseErrorPath,
+				getTypeOrLiteralMsg({
+					code: baseError.code,
+					received: baseError.received,
+					expected: [baseError.expected],
+				})
+			),
+		};
+	} else if (baseError.message) {
+		return { message: prefix(baseErrorPath, baseError.message) };
+	} else {
+		return { message: prefix(baseErrorPath, ctx.defaultError) };
+	}
+};
+
+const getTypeOrLiteralMsg = (error: TypeOrLiteralErrByPathEntry): string => {
+	if (error.received === 'undefined') return 'Required';
+	const expectedDeduped = new Set(error.expected);
+	switch (error.code) {
+		case 'invalid_type':
+			return `Expected type \`${unionExpectedVals(expectedDeduped)}\`, received ${JSON.stringify(
+				error.received
+			)}`;
+		case 'invalid_literal':
+			return `Expected \`${unionExpectedVals(expectedDeduped)}\`, received ${JSON.stringify(
+				error.received
+			)}`;
+	}
+};
+
+const prefix = (key: string, msg: string) => (key.length ? `**${key}**: ${msg}` : msg);
+
+const unionExpectedVals = (expectedVals: Set<unknown>) =>
+	[...expectedVals]
+		.map((expectedVal, idx) => {
+			if (idx === 0) return JSON.stringify(expectedVal);
+			const sep = ' | ';
+			return `${sep}${JSON.stringify(expectedVal)}`;
+		})
+		.join('');
+
+const flattenErrorPath = (errorPath: (string | number)[]) => errorPath.join('.');
diff --git a/packages/astro/src/content/index.ts b/packages/astro/src/content/index.ts
index f12106b0a..7504389e9 100644
--- a/packages/astro/src/content/index.ts
+++ b/packages/astro/src/content/index.ts
@@ -4,3 +4,4 @@ export { contentObservable, getContentPaths, getDotAstroTypeReference } from './
 export { astroContentAssetPropagationPlugin } from './vite-plugin-content-assets.js';
 export { astroContentImportPlugin } from './vite-plugin-content-imports.js';
 export { astroContentVirtualModPlugin } from './vite-plugin-content-virtual-mod.js';
+export { errorMap } from './error-map.js';
diff --git a/packages/astro/src/content/utils.ts b/packages/astro/src/content/utils.ts
index 00d0f61e4..2db053dd3 100644
--- a/packages/astro/src/content/utils.ts
+++ b/packages/astro/src/content/utils.ts
@@ -10,6 +10,7 @@ import type { AstroConfig, AstroSettings } from '../@types/astro.js';
 import { emitESMImage } from '../assets/utils/emitAsset.js';
 import { AstroError, AstroErrorData } from '../core/errors/index.js';
 import { CONTENT_TYPES_FILE } from './consts.js';
+import { errorMap } from './error-map.js';
 
 export const collectionConfigParser = z.object({
 	schema: z.any().optional(),
@@ -221,20 +222,6 @@ function hasUnderscoreBelowContentDirectoryPath(
 	return false;
 }
 
-const flattenErrorPath = (errorPath: (string | number)[]) => errorPath.join('.');
-
-const errorMap: z.ZodErrorMap = (error, ctx) => {
-	if (error.code === 'invalid_type') {
-		const badKeyPath = JSON.stringify(flattenErrorPath(error.path));
-		if (error.received === 'undefined') {
-			return { message: `${badKeyPath} is required.` };
-		} else {
-			return { message: `${badKeyPath} should be ${error.expected}, not ${error.received}.` };
-		}
-	}
-	return { message: ctx.defaultError };
-};
-
 function getFrontmatterErrorLine(rawFrontmatter: string, frontmatterKey: string) {
 	const indexOfFrontmatterKey = rawFrontmatter.indexOf(`\n${frontmatterKey}`);
 	if (indexOfFrontmatterKey === -1) return 0;
diff --git a/packages/astro/src/core/errors/errors-data.ts b/packages/astro/src/core/errors/errors-data.ts
index a4db5ab3e..44e210ad3 100644
--- a/packages/astro/src/core/errors/errors-data.ts
+++ b/packages/astro/src/core/errors/errors-data.ts
@@ -771,7 +771,9 @@ See https://docs.astro.build/en/guides/server-side-rendering/ for more informati
 		code: 9001,
 		message: (collection: string, entryId: string, error: ZodError) => {
 			return [
-				`${String(collection)} → ${String(entryId)} frontmatter does not match collection schema.`,
+				`**${String(collection)} → ${String(
+					entryId
+				)}** frontmatter does not match collection schema.`,
 				...error.errors.map((zodError) => zodError.message),
 			].join('\n');
 		},
diff --git a/packages/astro/test/content-collections.test.js b/packages/astro/test/content-collections.test.js
index 67ef08235..dd86288d1 100644
--- a/packages/astro/test/content-collections.test.js
+++ b/packages/astro/test/content-collections.test.js
@@ -210,7 +210,7 @@ describe('Content Collections', () => {
 			} catch (e) {
 				error = e.message;
 			}
-			expect(error).to.include('"title" should be string, not number.');
+			expect(error).to.include('**title**: Expected type `"string"`, received "number"');
 		});
 	});
 
diff --git a/packages/astro/test/units/content-collections/error-map.test.js b/packages/astro/test/units/content-collections/error-map.test.js
new file mode 100644
index 000000000..4284ad721
--- /dev/null
+++ b/packages/astro/test/units/content-collections/error-map.test.js
@@ -0,0 +1,107 @@
+import { z } from '../../../zod.mjs';
+import { errorMap } from '../../../dist/content/index.js';
+import { fixLineEndings } from '../../test-utils.js';
+import { expect } from 'chai';
+
+describe('Content Collections - error map', () => {
+	it('Prefixes messages with object key', () => {
+		const error = getParseError(
+			z.object({
+				base: z.string(),
+				nested: z.object({
+					key: z.string(),
+				}),
+				union: z.union([z.string(), z.number()]),
+			}),
+			{ base: 1, nested: { key: 2 }, union: true }
+		);
+		const msgs = messages(error).sort();
+		expect(msgs).to.have.length(3);
+		// expect "**" for bolding
+		expect(msgs[0].startsWith('**base**')).to.equal(true);
+		expect(msgs[1].startsWith('**nested.key**')).to.equal(true);
+		expect(msgs[2].startsWith('**union**')).to.equal(true);
+	});
+	it('Returns formatted error for type mismatch', () => {
+		const error = getParseError(
+			z.object({
+				foo: z.string(),
+			}),
+			{ foo: 1 }
+		);
+		expect(messages(error)).to.deep.equal(['**foo**: Expected type `"string"`, received "number"']);
+	});
+	it('Returns formatted error for literal mismatch', () => {
+		const error = getParseError(
+			z.object({
+				lang: z.literal('en'),
+			}),
+			{ lang: 'es' }
+		);
+		expect(messages(error)).to.deep.equal(['**lang**: Expected `"en"`, received "es"']);
+	});
+	it('Replaces undefined errors with "Required"', () => {
+		const error = getParseError(
+			z.object({
+				foo: z.string(),
+				bar: z.string(),
+			}),
+			{ foo: 'foo' }
+		);
+		expect(messages(error)).to.deep.equal(['**bar**: Required']);
+	});
+	it('Returns formatted error for basic union mismatch', () => {
+		const error = getParseError(
+			z.union([z.boolean(), z.number()]),
+			'not a boolean or a number, oops!'
+		);
+		expect(messages(error)).to.deep.equal([
+			fixLineEndings(
+				'Did not match union:\n> Expected type `"boolean" | "number"`, received "string"'
+			),
+		]);
+	});
+	it('Returns formatted error for union mismatch on nested object properties', () => {
+		const error = getParseError(
+			z.union([
+				z.object({
+					type: z.literal('tutorial'),
+				}),
+				z.object({
+					type: z.literal('article'),
+				}),
+			]),
+			{ type: 'integration-guide' }
+		);
+		expect(messages(error)).to.deep.equal([
+			fixLineEndings(
+				'Did not match union:\n> **type**: Expected `"tutorial" | "article"`, received "integration-guide"'
+			),
+		]);
+	});
+	it('Lets unhandled errors fall through', () => {
+		const error = getParseError(
+			z.object({
+				lang: z.enum(['en', 'fr']),
+			}),
+			{ lang: 'jp' }
+		);
+		expect(messages(error)).to.deep.equal([
+			"**lang**: Invalid enum value. Expected 'en' | 'fr', received 'jp'",
+		]);
+	});
+});
+
+/**
+ * @param {z.ZodError} error
+ * @returns string[]
+ */
+function messages(error) {
+	return error.errors.map((e) => e.message);
+}
+
+function getParseError(schema, entry, parseOpts = { errorMap }) {
+	const res = schema.safeParse(entry, parseOpts);
+	expect(res.success).to.equal(false, 'Schema should raise error');
+	return res.error;
+}