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

(cherry picked from commit 046121e080)

hardhat.config.js

@@ -50,6 +50,7 @@ const argv = require('yargs/yargs')()
   .argv;
 
 require('@nomiclabs/hardhat-truffle5');
+require('hardhat-ignore-warnings');
 
 if (argv.gas) {
   require('hardhat-gas-reporter');
@@ -75,6 +76,13 @@ module.exports = {
       viaIR: withOptimizations && argv.ir,
     },
   },
+  warnings: {
+    '*': {
+      'code-size': withOptimizations,
+      'unused-param': !argv.coverage, // coverage causes unused-param warnings
+      default: 'error',
+    },
+  },
   networks: {
     hardhat: {
       blockGasLimit: 10000000,