feat: adds --output to flatten

packages/hardhat-core/src/builtin-tasks/flatten.ts

@@ -1,4 +1,5 @@
 import picocolors from "picocolors";
+import { writeFileSync } from "node:fs";
 import { subtask, task, types } from "../internal/core/config/config-env";
 import { HardhatError } from "../internal/core/errors";
 import { ERRORS } from "../internal/core/errors-list";
@@ -303,46 +304,63 @@ task(
     undefined,
     types.inputFile
   )
-  .setAction(async ({ files }: { files: string[] | undefined }, { run }) => {
-    const [flattenedFile, metadata]: [string, FlattenMetadata | null] =
-      await run(TASK_FLATTEN_GET_FLATTENED_SOURCE_AND_METADATA, { files });
-
-    console.log(flattenedFile);
-
-    if (metadata === null) return;
-
-    if (metadata.filesWithoutLicenses.length > 0) {
-      console.warn(
-        picocolors.yellow(
-          `\nThe following file(s) do NOT specify SPDX licenses: ${metadata.filesWithoutLicenses.join(
-            ", "
-          )}`
-        )
-      );
-    }
+  .addOptionalParam(
+    "output",
+    "The output file containing the flattened contracts",
+    undefined,
+    types.string
+  )
+  .setAction(
+    async (
+      {
+        files,
+        output,
+      }: { files: string[] | undefined; output: string | undefined },
+      { run }
+    ) => {
+      const [flattenedFile, metadata]: [string, FlattenMetadata | null] =
+        await run(TASK_FLATTEN_GET_FLATTENED_SOURCE_AND_METADATA, { files });
+
+      if (output !== undefined) {
+        writeFileSync(output, flattenedFile, { encoding: "utf-8" });
+      } else {
+        console.log(flattenedFile);
+      }
+      if (metadata === null) return;
+
+      if (metadata.filesWithoutLicenses.length > 0) {
+        console.warn(
+          picocolors.yellow(
+            `\nThe following file(s) do NOT specify SPDX licenses: ${metadata.filesWithoutLicenses.join(
+              ", "
+            )}`
+          )
+        );
+      }
 
-    if (
-      metadata.pragmaDirective !== "" &&
-      metadata.filesWithoutPragmaDirectives.length > 0
-    ) {
-      console.warn(
-        picocolors.yellow(
-          `\nPragma abicoder directives are defined in some files, but they are not defined in the following ones: ${metadata.filesWithoutPragmaDirectives.join(
-            ", "
-          )}`
-        )
-      );
-    }
+      if (
+        metadata.pragmaDirective !== "" &&
+        metadata.filesWithoutPragmaDirectives.length > 0
+      ) {
+        console.warn(
+          picocolors.yellow(
+            `\nPragma abicoder directives are defined in some files, but they are not defined in the following ones: ${metadata.filesWithoutPragmaDirectives.join(
+              ", "
+            )}`
+          )
+        );
+      }
 
-    if (metadata.filesWithDifferentPragmaDirectives.length > 0) {
-      console.warn(
-        picocolors.yellow(
-          `\nThe flattened file is using the pragma abicoder directive '${
-            metadata.pragmaDirective
-          }' but these files have a different pragma abicoder directive: ${metadata.filesWithDifferentPragmaDirectives.join(
-            ", "
-          )}`
-        )
-      );
+      if (metadata.filesWithDifferentPragmaDirectives.length > 0) {
+        console.warn(
+          picocolors.yellow(
+            `\nThe flattened file is using the pragma abicoder directive '${
+              metadata.pragmaDirective
+            }' but these files have a different pragma abicoder directive: ${metadata.filesWithDifferentPragmaDirectives.join(
+              ", "
+            )}`
+          )
+        );
+      }
     }
-  });
+  );

packages/hardhat-core/test/builtin-tasks/flatten.ts

@@ -1,8 +1,11 @@
 import { assert } from "chai";
-import fs from "fs";
+import fs, { readFileSync } from "fs";
 
 import sinon, { SinonSpy } from "sinon";
 import picocolors from "picocolors";
+import { removeSync } from "fs-extra";
+import { readSync } from "node:fs";
+import { readFile } from "node:fs/promises";
 import {
   TASK_FLATTEN,
   TASK_FLATTEN_GET_FLATTENED_SOURCE,
@@ -412,6 +415,18 @@ describe("Flatten task", () => {
       assert(!spyFunctionConsoleWarn.called);
     });
 
+    it("should write to an output file when the parameter output is specified", async function () {
+      const outputFile = "flatten.sol";
+      await this.env.run(TASK_FLATTEN, {
+        files: ["contracts/A.sol", "contracts/D.sol"],
+        output: outputFile,
+      });
+      const expected = await getExpectedSol();
+      const actual = readFileSync(outputFile, "utf8");
+      assert.equal(actual, expected);
+      removeSync(outputFile);
+    });
+
     describe("No contracts to flatten", () => {
       useFixtureProject("flatten-task/no-contracts");