Print diagnostics when a protoplugin fails to emit files

packages/protoplugin-test/src/transpile.test.ts.ts → packages/protoplugin-test/src/transpile.test.ts

@@ -97,4 +97,66 @@ describe("built-in transpile", () => {
       ]);
     });
   });
+
+  describe("failing to emit", () => {
+    test("raises error with helpful message", async () => {
+      await expect(async () =>
+        testTranspileToDts([
+          `export interface Foo {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+        ]),
+      ).rejects.toThrow(
+        /^A problem occurred during transpilation and files were not generated\. {2}Contact the plugin author for support\.\n/,
+      );
+    });
+    test("raises error with diagnostics", async () => {
+      await expect(async () =>
+        testTranspileToDts([
+          `export interface Foo {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+        ]),
+      ).rejects.toThrow(
+        /test\.ts\(3,17\): error TS4033: Property 'p' of exported interface has or is using private name 'P'\.$/,
+      );
+    });
+    test("raises error with 3 diagnostics, and elides the rest", async () => {
+      await expect(async () =>
+        testTranspileToDts([
+          `export interface Foo1 {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+          `export interface Foo2 {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+          `export interface Foo3 {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+          `export interface Foo4 {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+          `export interface Foo5 {`,
+          `  p: {`,
+          `    [K in keyof P]: string;`,
+          `  },`,
+          `}`,
+        ]),
+      ).rejects.toThrow(
+        /(?:test\.ts\(\d+,\d+\): .+\n){3}2 more diagnostics elided/,
+      );
+    });
+  });
 });

packages/protoplugin/src/transpile.ts

@@ -158,9 +158,35 @@ export function transpile(
     throw err;
   }
   if (result.emitSkipped) {
+    // When compilation fails, this error message is printed to stderr.
+    const diagnostics = formatDiagnostics(result.diagnostics);
     throw Error(
-      "A problem occurred during transpilation and files were not generated.  Contact the plugin author for support.",
+      `A problem occurred during transpilation and files were not generated.  Contact the plugin author for support.\n\n${diagnostics}`,
     );
   }
   return results;
 }
+
+function formatDiagnostics(diagnostics: readonly ts.Diagnostic[]): string {
+  const sorted = ts.sortAndDeduplicateDiagnostics(diagnostics);
+  if (sorted.length == 0) {
+    return "";
+  }
+  const first = sorted.slice(0, 3);
+  const formatHost: ts.FormatDiagnosticsHost = {
+    getCanonicalFileName(fileName: string): string {
+      return fileName;
+    },
+    getCurrentDirectory(): string {
+      return ".";
+    },
+    getNewLine(): string {
+      return "\n";
+    },
+  };
+  let out = ts.formatDiagnostics(first, formatHost).trim();
+  if (first.length < sorted.length) {
+    out += `\n${sorted.length - first.length} more diagnostics elided`;
+  }
+  return out;
+}