Ignore specific warnings and make the rest into errors (#3695)

hardhat/ignore-unreachable-warnings.js

@@ -0,0 +1,47 @@
+// Warnings about unreachable code are emitted with a source location that corresponds to the unreachable code.
+// We have some testing contracts that purposely cause unreachable code, but said code is in the library contracts, and
+// with hardhat-ignore-warnings we are not able to selectively ignore them without potentially ignoring relevant
+// warnings that we don't want to miss.
+// Thus, we need to handle these warnings separately. We force Hardhat to compile them in a separate compilation job and
+// then ignore the warnings about unreachable code that come from that compilation job.
+
+const { task } = require('hardhat/config');
+const {
+  TASK_COMPILE_SOLIDITY_GET_COMPILATION_JOB_FOR_FILE,
+  TASK_COMPILE_SOLIDITY_COMPILE,
+} = require('hardhat/builtin-tasks/task-names');
+
+const marker = Symbol('unreachable');
+const markedCache = new WeakMap();
+
+task(TASK_COMPILE_SOLIDITY_GET_COMPILATION_JOB_FOR_FILE, async (params, _, runSuper) => {
+  const job = await runSuper(params);
+  // If the file is in the unreachable directory, we make a copy of the config and mark it, which will cause it to get
+  // compiled separately (along with the other marked files).
+  if (params.file.sourceName.startsWith('contracts/mocks/') && /\bunreachable\b/.test(params.file.sourceName)) {
+    const originalConfig = job.solidityConfig;
+    let markedConfig = markedCache.get(originalConfig);
+    if (markedConfig === undefined) {
+      markedConfig = { ...originalConfig, [marker]: true };
+      markedCache.set(originalConfig, markedConfig);
+    }
+    job.solidityConfig = markedConfig;
+  }
+  return job;
+});
+
+const W_UNREACHABLE_CODE = '5740';
+
+task(TASK_COMPILE_SOLIDITY_COMPILE, async (params, _, runSuper) => {
+  const marked = params.compilationJob.solidityConfig[marker];
+  const result = await runSuper(params);
+  if (marked) {
+    result.output = {
+      ...result.output,
+      errors: result.output.errors?.filter(
+        e => e.severity !== 'warning' || e.errorCode !== W_UNREACHABLE_CODE,
+      ),
+    };
+  }
+  return result;
+});