diff --git a/bin/src/runRollup.js b/bin/src/runRollup.js
index 63bbd0c..9ef3b19 100644
--- a/bin/src/runRollup.js
+++ b/bin/src/runRollup.js
@@ -1,7 +1,9 @@
 import { realpathSync } from 'fs';
 import * as rollup from 'rollup';
 import relative from 'require-relative';
+import * as chalk from 'chalk';
 import handleError from './handleError';
+import relativeId from '../../src/utils/relativeId.js';
 import SOURCEMAPPING_URL from './sourceMappingUrl.js';
 
 import { install as installSourcemapSupport } from 'source-map-support';
@@ -60,9 +62,8 @@ export default function runRollup ( command ) {
 		rollup.rollup({
 			entry: config,
 			onwarn: message => {
-				// TODO use warning codes instead of this hackery
-				if ( /treating it as an external dependency/.test( message ) ) return;
-				stderr( message );
+				if ( message.code === 'UNRESOLVED_IMPORT' ) return;
+				stderr( message.toString() );
 			}
 		}).then( bundle => {
 			const { code } = bundle.generate({
@@ -121,7 +122,7 @@ function execute ( options, command ) {
 	const optionsExternal = options.external;
 
 	if ( command.globals ) {
-		let globals = Object.create( null );
+		const globals = Object.create( null );
 
 		command.globals.split( ',' ).forEach( str => {
 			const names = str.split( ':' );
@@ -144,7 +145,32 @@ function execute ( options, command ) {
 		external = ( optionsExternal || [] ).concat( commandExternal );
 	}
 
-	options.onwarn = options.onwarn || stderr;
+	if ( !options.onwarn ) {
+		const seen = new Set();
+
+		options.onwarn = warning => {
+			const str = warning.toString();
+
+			if ( seen.has( str ) ) return;
+			seen.add( str );
+
+			stderr( `⚠️   ${chalk.bold( warning.message )}` );
+
+			if ( warning.url ) {
+				stderr( chalk.cyan( warning.url ) );
+			}
+
+			if ( warning.loc ) {
+				stderr( `${relativeId( warning.loc.file )} (${warning.loc.line}:${warning.loc.column})` );
+			}
+
+			if ( warning.frame ) {
+				stderr( chalk.dim( warning.frame ) );
+			}
+
+			stderr( '' );
+		};
+	}
 
 	options.external = external;
 
diff --git a/package.json b/package.json
index f7af247..4911fac 100644
--- a/package.json
+++ b/package.json
@@ -53,6 +53,7 @@
     "eslint-plugin-import": "^2.2.0",
     "is-reference": "^1.0.0",
     "istanbul": "^0.4.3",
+    "locate-character": "^2.0.0",
     "magic-string": "^0.15.2",
     "minimist": "^1.2.0",
     "mocha": "^3.0.0",
diff --git a/rollup.config.cli.js b/rollup.config.cli.js
index 982f6b0..5c0f673 100644
--- a/rollup.config.cli.js
+++ b/rollup.config.cli.js
@@ -15,7 +15,9 @@ export default {
 		buble(),
 		commonjs({
 			include: 'node_modules/**',
-			namedExports: { chalk: [ 'red', 'cyan', 'grey' ] }
+			namedExports: {
+				chalk: [ 'yellow', 'red', 'cyan', 'grey', 'dim', 'bold' ]
+			}
 		}),
 		nodeResolve({
 			main: true
diff --git a/src/Bundle.js b/src/Bundle.js
index 1166cb2..79cd8d0 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -188,7 +188,10 @@ export default class Bundle {
 						`'${unused[0]}' is` :
 						`${unused.slice( 0, -1 ).map( name => `'${name}'` ).join( ', ' )} and '${unused.pop()}' are`;
 
-					this.onwarn( `${names} imported from external module '${module.id}' but never used` );
+					this.warn({
+						code: 'UNUSED_EXTERNAL_IMPORT',
+						message: `${names} imported from external module '${module.id}' but never used`
+					});
 				});
 
 				this.orderedModules = this.sort();
@@ -309,7 +312,10 @@ export default class Bundle {
 
 						keys( exportAllModule.exportsAll ).forEach( name => {
 							if ( name in module.exportsAll ) {
-								this.onwarn( `Conflicting namespaces: ${module.id} re-exports '${name}' from both ${module.exportsAll[ name ]} (will be ignored) and ${exportAllModule.exportsAll[ name ]}.` );
+								this.warn({
+									code: 'NAMESPACE_CONFLICT',
+									message: `Conflicting namespaces: ${relativeId( module.id )} re-exports '${name}' from both ${relativeId( module.exportsAll[ name ] )} (will be ignored) and ${relativeId( exportAllModule.exportsAll[ name ] )}`
+								});
 							}
 							module.exportsAll[ name ] = exportAllModule.exportsAll[ name ];
 						});
@@ -333,7 +339,11 @@ export default class Bundle {
 					if ( !resolvedId && !isExternal ) {
 						if ( isRelative( source ) ) throw new Error( `Could not resolve '${source}' from ${module.id}` );
 
-						this.onwarn( `'${source}' is imported by ${relativeId( module.id )}, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency` );
+						this.warn({
+							code: 'UNRESOLVED_IMPORT',
+							message: `'${source}' is imported by ${relativeId( module.id )}, but could not be resolved – treating it as an external dependency`,
+							url: 'https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency'
+						});
 						isExternal = true;
 					}
 
@@ -380,7 +390,11 @@ export default class Bundle {
 
 	render ( options = {} ) {
 		if ( options.format === 'es6' ) {
-			this.onwarn( 'The es6 format is deprecated – use `es` instead' );
+			this.warn({
+				code: 'DEPRECATED_ES6',
+				message: 'The es6 format is deprecated – use `es` instead'
+			});
+
 			options.format = 'es';
 		}
 
@@ -404,7 +418,10 @@ export default class Bundle {
 		});
 
 		if ( !magicString.toString().trim() && this.entryModule.getExports().length === 0 ) {
-			this.onwarn( 'Generated an empty bundle' );
+			this.warn({
+				code: 'EMPTY_BUNDLE',
+				message: 'Generated an empty bundle'
+			});
 		}
 
 		timeEnd( 'render modules' );
@@ -470,7 +487,7 @@ export default class Bundle {
 				if ( typeof map.mappings === 'string' ) {
 					map.mappings = decode( map.mappings );
 				}
-				map = collapseSourcemaps( file, map, usedModules, bundleSourcemapChain, this.onwarn );
+				map = collapseSourcemaps( this, file, map, usedModules, bundleSourcemapChain );
 			} else {
 				map = magicString.generateMap({ file, includeContent: true });
 			}
@@ -535,6 +552,7 @@ export default class Bundle {
 				for ( i += 1; i < ordered.length; i += 1 ) {
 					const b = ordered[i];
 
+					// TODO reinstate this! it no longer works
 					if ( stronglyDependsOn[ a.id ][ b.id ] ) {
 						// somewhere, there is a module that imports b before a. Because
 						// b imports a, a is placed before b. We need to find the module
@@ -566,4 +584,16 @@ export default class Bundle {
 
 		return ordered;
 	}
+
+	warn ( warning ) {
+		warning.toString = () => {
+			if ( warning.loc ) {
+				return `${relativeId( warning.loc.file )} (${warning.loc.line}:${warning.loc.column}) ${warning.message}`;
+			}
+
+			return warning.message;
+		};
+
+		this.onwarn( warning );
+	}
 }
diff --git a/src/Module.js b/src/Module.js
index 71db738..81926c3 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -1,10 +1,11 @@
-import { timeStart, timeEnd } from './utils/flushTime.js';
 import { parse } from 'acorn/src/index.js';
 import MagicString from 'magic-string';
+import { locate } from 'locate-character';
+import { timeStart, timeEnd } from './utils/flushTime.js';
 import { assign, blank, deepClone, keys } from './utils/object.js';
 import { basename, extname } from './utils/path.js';
-import getLocation from './utils/getLocation.js';
 import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
+import getCodeFrame from './utils/getCodeFrame.js';
 import { SOURCEMAPPING_URL_RE } from './utils/sourceMappingURL.js';
 import error from './utils/error.js';
 import relativeId from './utils/relativeId.js';
@@ -179,7 +180,12 @@ export default class Module {
 					this.exports[ exportedName ] = { localName };
 				});
 			} else {
-				this.bundle.onwarn( `Module ${this.id} has an empty export declaration` );
+				// TODO is this really necessary? `export {}` is valid JS, and
+				// might be used as a hint that this is indeed a module
+				this.warn({
+					code: 'EMPTY_EXPORT',
+					message: `Empty export declaration`
+				}, node.start );
 			}
 		}
 	}
@@ -195,7 +201,7 @@ export default class Module {
 			if ( this.imports[ localName ] ) {
 				const err = new Error( `Duplicated import '${localName}'` );
 				err.file = this.id;
-				err.loc = getLocation( this.code, specifier.start );
+				err.loc = locate( this.code, specifier.start, { offsetLine: 1 });
 				throw err;
 			}
 
@@ -361,7 +367,7 @@ export default class Module {
 				error({
 					message: `'${importDeclaration.name}' is not exported by ${relativeId( otherModule.id )} (imported by ${relativeId( this.id )}). For help fixing this error see https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-module`,
 					file: this.id,
-					loc: getLocation( this.code, importDeclaration.specifier.start )
+					loc: locate( this.code, importDeclaration.specifier.start, { offsetLine: 1 })
 				});
 			}
 
@@ -381,7 +387,7 @@ export default class Module {
 				error({
 					message: `'${reexportDeclaration.localName}' is not exported by '${reexportDeclaration.module.id}' (imported by '${this.id}')`,
 					file: this.id,
-					loc: getLocation( this.code, reexportDeclaration.start )
+					loc: locate( this.code, reexportDeclaration.start, { offsetLine: 1 })
 				});
 			}
 
@@ -405,4 +411,17 @@ export default class Module {
 			if ( declaration ) return declaration;
 		}
 	}
+
+	warn ( warning, pos ) {
+		if ( pos !== undefined ) {
+			warning.pos = pos;
+
+			const { line, column } = locate( this.code, pos, { offsetLine: 1 }); // TODO trace sourcemaps
+
+			warning.loc = { file: this.id, line, column };
+			warning.frame = getCodeFrame( this.code, line, column );
+		}
+
+		this.bundle.warn( warning );
+	}
 }
diff --git a/src/ast/Node.js b/src/ast/Node.js
index 6a80ede..da36211 100644
--- a/src/ast/Node.js
+++ b/src/ast/Node.js
@@ -1,5 +1,5 @@
+import { locate } from 'locate-character';
 import { UNKNOWN } from './values.js';
-import getLocation from '../utils/getLocation.js';
 
 export default class Node {
 	bind ( scope ) {
@@ -74,7 +74,7 @@ export default class Node {
 
 	locate () {
 		// useful for debugging
-		const location = getLocation( this.module.code, this.start );
+		const location = locate( this.module.code, this.start, { offsetLine: 1 });
 		location.file = this.module.id;
 		location.toString = () => JSON.stringify( location );
 
diff --git a/src/ast/nodes/CallExpression.js b/src/ast/nodes/CallExpression.js
index d275100..4880cb5 100644
--- a/src/ast/nodes/CallExpression.js
+++ b/src/ast/nodes/CallExpression.js
@@ -1,4 +1,4 @@
-import getLocation from '../../utils/getLocation.js';
+import { locate } from 'locate-character';
 import error from '../../utils/error.js';
 import Node from '../Node.js';
 import isProgramLevel from '../utils/isProgramLevel.js';
@@ -14,12 +14,16 @@ export default class CallExpression extends Node {
 					message: `Cannot call a namespace ('${this.callee.name}')`,
 					file: this.module.id,
 					pos: this.start,
-					loc: getLocation( this.module.code, this.start )
+					loc: locate( this.module.code, this.start, { offsetLine: 1 })
 				});
 			}
 
 			if ( this.callee.name === 'eval' && declaration.isGlobal ) {
-				this.module.bundle.onwarn( `Use of \`eval\` (in ${this.module.id}) is strongly discouraged, as it poses security risks and may cause issues with minification. See https://github.com/rollup/rollup/wiki/Troubleshooting#avoiding-eval for more details` );
+				this.module.warn({
+					code: 'EVAL',
+					message: `Use of eval is strongly discouraged, as it poses security risks and may cause issues with minification`,
+					url: 'https://github.com/rollup/rollup/wiki/Troubleshooting#avoiding-eval'
+				}, this.start );
 			}
 		}
 
diff --git a/src/ast/nodes/ExportDefaultDeclaration.js b/src/ast/nodes/ExportDefaultDeclaration.js
index d70bd10..7027291 100644
--- a/src/ast/nodes/ExportDefaultDeclaration.js
+++ b/src/ast/nodes/ExportDefaultDeclaration.js
@@ -1,6 +1,4 @@
 import Node from '../Node.js';
-import getLocation from '../../utils/getLocation.js';
-import relativeId from '../../utils/relativeId.js';
 
 const functionOrClassDeclaration = /^(?:Function|Class)Declaration/;
 
@@ -74,8 +72,11 @@ export default class ExportDefaultDeclaration extends Node {
 					const newlineSeparated = /\n/.test( code.original.slice( start, end ) );
 
 					if ( newlineSeparated ) {
-						const { line, column } = getLocation( this.module.code, this.declaration.start );
-						this.module.bundle.onwarn( `${relativeId( this.module.id )} (${line}:${column}) Ambiguous default export (is a call expression, but looks like a function declaration). See https://github.com/rollup/rollup/wiki/Troubleshooting#ambiguous-default-export` );
+						this.module.warn({
+							code: 'AMBIGUOUS_DEFAULT_EXPORT',
+							message: `Ambiguous default export (is a call expression, but looks like a function declaration)`,
+							url: 'https://github.com/rollup/rollup/wiki/Troubleshooting#ambiguous-default-export'
+						}, this.declaration.start );
 					}
 				}
 			}
diff --git a/src/ast/nodes/MemberExpression.js b/src/ast/nodes/MemberExpression.js
index bcdcf26..d745d7d 100644
--- a/src/ast/nodes/MemberExpression.js
+++ b/src/ast/nodes/MemberExpression.js
@@ -1,5 +1,4 @@
 import isReference from 'is-reference';
-import getLocation from '../../utils/getLocation.js';
 import relativeId from '../../utils/relativeId.js';
 import Node from '../Node.js';
 import { UNKNOWN } from '../values.js';
@@ -34,8 +33,11 @@ export default class MemberExpression extends Node {
 				declaration = declaration.module.traceExport( part.name );
 
 				if ( !declaration ) {
-					const { line, column } = getLocation( this.module.code, this.start );
-					this.module.bundle.onwarn( `${relativeId( this.module.id )} (${line}:${column}) '${part.name}' is not exported by '${relativeId( exporterId )}'. See https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-module` );
+					this.module.warn({
+						code: 'MISSING_EXPORT',
+						message: `'${part.name}' is not exported by '${relativeId( exporterId )}'`,
+						url: `https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-module`
+					}, part.start );
 					this.replacement = 'undefined';
 					return;
 				}
diff --git a/src/ast/nodes/ThisExpression.js b/src/ast/nodes/ThisExpression.js
index ae35173..2429c65 100644
--- a/src/ast/nodes/ThisExpression.js
+++ b/src/ast/nodes/ThisExpression.js
@@ -1,8 +1,4 @@
 import Node from '../Node.js';
-import getLocation from '../../utils/getLocation.js';
-import relativeId from '../../utils/relativeId.js';
-
-const warning = `The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten. See https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined for more information`;
 
 export default class ThisExpression extends Node {
 	initialise ( scope ) {
@@ -11,9 +7,11 @@ export default class ThisExpression extends Node {
 		if ( lexicalBoundary.isModuleScope ) {
 			this.alias = this.module.context;
 			if ( this.alias === 'undefined' ) {
-				const { line, column } = getLocation( this.module.code, this.start );
-				const detail = `${relativeId( this.module.id )} (${line}:${column + 1})`; // use one-based column number convention
-				this.module.bundle.onwarn( `${detail} ${warning}` );
+				this.module.warn({
+					code: 'THIS_IS_UNDEFINED',
+					message: `The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten`,
+					url: `https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined`
+				}, this.start );
 			}
 		}
 	}
diff --git a/src/ast/nodes/shared/disallowIllegalReassignment.js b/src/ast/nodes/shared/disallowIllegalReassignment.js
index a40969f..5d338af 100644
--- a/src/ast/nodes/shared/disallowIllegalReassignment.js
+++ b/src/ast/nodes/shared/disallowIllegalReassignment.js
@@ -1,4 +1,4 @@
-import getLocation from '../../../utils/getLocation.js';
+import { locate } from 'locate-character';
 import error from '../../../utils/error.js';
 
 // TODO tidy this up a bit (e.g. they can both use node.module.imports)
@@ -10,7 +10,7 @@ export default function disallowIllegalReassignment ( scope, node ) {
 				message: `Illegal reassignment to import '${node.object.name}'`,
 				file: node.module.id,
 				pos: node.start,
-				loc: getLocation( node.module.code, node.start )
+				loc: locate( node.module.code, node.start, { offsetLine: 1 })
 			});
 		}
 	}
@@ -21,7 +21,7 @@ export default function disallowIllegalReassignment ( scope, node ) {
 				message: `Illegal reassignment to import '${node.name}'`,
 				file: node.module.id,
 				pos: node.start,
-				loc: getLocation( node.module.code, node.start )
+				loc: locate( node.module.code, node.start, { offsetLine: 1 })
 			});
 		}
 	}
diff --git a/src/ast/scopes/ModuleScope.js b/src/ast/scopes/ModuleScope.js
index 4c8804c..f0f5f7a 100644
--- a/src/ast/scopes/ModuleScope.js
+++ b/src/ast/scopes/ModuleScope.js
@@ -1,4 +1,5 @@
 import { forOwn } from '../../utils/object.js';
+import relativeId from '../../utils/relativeId.js';
 import Scope from './Scope.js';
 
 export default class ModuleScope extends Scope {
@@ -26,7 +27,10 @@ export default class ModuleScope extends Scope {
 			if ( specifier.name !== '*' ) {
 				const declaration = specifier.module.traceExport( specifier.name );
 				if ( !declaration ) {
-					this.module.bundle.onwarn( `Non-existent export '${specifier.name}' is imported from ${specifier.module.id} by ${this.module.id}` );
+					this.module.warn({
+						code: 'NON_EXISTENT_EXPORT',
+						message: `Non-existent export '${specifier.name}' is imported from ${relativeId( specifier.module.id )}`
+					}, specifier.specifier.start );
 					return;
 				}
 
diff --git a/src/finalisers/iife.js b/src/finalisers/iife.js
index f6d702e..2d6121e 100644
--- a/src/finalisers/iife.js
+++ b/src/finalisers/iife.js
@@ -25,7 +25,7 @@ function setupNamespace ( keypath ) {
 }
 
 export default function iife ( bundle, magicString, { exportMode, indentString, intro, outro }, options ) {
-	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle.onwarn );
+	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle );
 
 	const name = options.moduleName;
 	const isNamespaced = name && ~name.indexOf( '.' );
diff --git a/src/finalisers/shared/getGlobalNameMaker.js b/src/finalisers/shared/getGlobalNameMaker.js
index 676b416..7a1d622 100644
--- a/src/finalisers/shared/getGlobalNameMaker.js
+++ b/src/finalisers/shared/getGlobalNameMaker.js
@@ -1,11 +1,15 @@
-export default function getGlobalNameMaker ( globals, onwarn ) {
+export default function getGlobalNameMaker ( globals, bundle ) {
 	const fn = typeof globals === 'function' ? globals : id => globals[ id ];
 
 	return function ( module ) {
 		const name = fn( module.id );
 		if ( name ) return name;
 
-		onwarn( `No name was provided for external module '${module.id}' in options.globals – guessing '${module.name}'` );
+		bundle.warn({
+			code: 'MISSING_GLOBAL_NAME',
+			message: `No name was provided for external module '${module.id}' in options.globals – guessing '${module.name}'`
+		});
+
 		return module.name;
 	};
 }
diff --git a/src/finalisers/shared/warnOnBuiltins.js b/src/finalisers/shared/warnOnBuiltins.js
index e5ac1ba..c4d63f6 100644
--- a/src/finalisers/shared/warnOnBuiltins.js
+++ b/src/finalisers/shared/warnOnBuiltins.js
@@ -35,5 +35,8 @@ export default function warnOnBuiltins ( bundle ) {
 		`module ('${externalBuiltins[0]}')` :
 		`modules (${externalBuiltins.slice( 0, -1 ).map( name => `'${name}'` ).join( ', ' )} and '${externalBuiltins.pop()}')`;
 
-	bundle.onwarn( `Creating a browser bundle that depends on Node.js built-in ${detail}. You might need to include https://www.npmjs.com/package/rollup-plugin-node-builtins` );
+	bundle.warn({
+		code: 'MISSING_NODE_BUILTINS',
+		message: `Creating a browser bundle that depends on Node.js built-in ${detail}. You might need to include https://www.npmjs.com/package/rollup-plugin-node-builtins`
+	});
 }
diff --git a/src/finalisers/umd.js b/src/finalisers/umd.js
index c176242..655c77a 100644
--- a/src/finalisers/umd.js
+++ b/src/finalisers/umd.js
@@ -33,7 +33,7 @@ export default function umd ( bundle, magicString, { exportMode, indentString, i
 
 	warnOnBuiltins( bundle );
 
-	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle.onwarn );
+	const globalNameMaker = getGlobalNameMaker( options.globals || blank(), bundle );
 
 	const amdDeps = bundle.externalModules.map( quotePath );
 	const cjsDeps = bundle.externalModules.map( req );
diff --git a/src/utils/collapseSourcemaps.js b/src/utils/collapseSourcemaps.js
index 7a504ba..122a148 100644
--- a/src/utils/collapseSourcemaps.js
+++ b/src/utils/collapseSourcemaps.js
@@ -98,7 +98,7 @@ class Link {
 	}
 }
 
-export default function collapseSourcemaps ( file, map, modules, bundleSourcemapChain, onwarn ) {
+export default function collapseSourcemaps ( bundle, file, map, modules, bundleSourcemapChain ) {
 	const moduleSources = modules.filter( module => !module.excludeFromSourcemap ).map( module => {
 		let sourceMapChain = module.sourceMapChain;
 
@@ -127,7 +127,11 @@ export default function collapseSourcemaps ( file, map, modules, bundleSourcemap
 
 		sourceMapChain.forEach( map => {
 			if ( map.missing ) {
-				onwarn( `Sourcemap is likely to be incorrect: a plugin${map.plugin ? ` ('${map.plugin}')` : ``} was used to transform files, but didn't generate a sourcemap for the transformation. Consult https://github.com/rollup/rollup/wiki/Troubleshooting and the plugin documentation for more information` );
+				bundle.warn({
+					code: 'SOURCEMAP_BROKEN',
+					message: `Sourcemap is likely to be incorrect: a plugin${map.plugin ? ` ('${map.plugin}')` : ``} was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help`,
+					url: `https://github.com/rollup/rollup/wiki/Troubleshooting#sourcemap-is-likely-to-be-incorrect`
+				});
 
 				map = {
 					names: [],
diff --git a/src/utils/defaults.js b/src/utils/defaults.js
index 8db3c0b..c5dd054 100644
--- a/src/utils/defaults.js
+++ b/src/utils/defaults.js
@@ -45,9 +45,10 @@ export function resolveId ( importee, importer ) {
 export function makeOnwarn () {
 	const warned = blank();
 
-	return msg => {
-		if ( msg in warned ) return;
-		console.error( msg ); //eslint-disable-line no-console
-		warned[ msg ] = true;
+	return warning => {
+		const str = warning.toString();
+		if ( str in warned ) return;
+		console.error( str ); //eslint-disable-line no-console
+		warned[ str ] = true;
 	};
 }
diff --git a/src/utils/getCodeFrame.js b/src/utils/getCodeFrame.js
new file mode 100644
index 0000000..369829b
--- /dev/null
+++ b/src/utils/getCodeFrame.js
@@ -0,0 +1,38 @@
+function spaces ( i ) {
+	let result = '';
+	while ( i-- ) result += ' ';
+	return result;
+}
+
+
+function tabsToSpaces ( str ) {
+	return str.replace( /^\t+/, match => match.split( '\t' ).join( '  ' ) );
+}
+
+export default function getCodeFrame ( source, line, column ) {
+	let lines = source.split( '\n' );
+
+	const frameStart = Math.max( 0, line - 3 );
+	const frameEnd = Math.min( line + 2, lines.length );
+
+	const digits = String( frameEnd + 1 ).length;
+
+	lines = lines.slice( frameStart, frameEnd );
+	while ( !/\S/.test( lines[ lines.length - 1 ] ) ) lines.pop();
+
+	return lines
+		.map( ( str, i ) => {
+			const isErrorLine = frameStart + i + 1 === line;
+
+			let lineNum = String( i + frameStart + 1 );
+			while ( lineNum.length < digits ) lineNum = ` ${lineNum}`;
+
+			if ( isErrorLine ) {
+				const indicator = spaces( digits + 2 + tabsToSpaces( str.slice( 0, column ) ).length ) + '^';
+				return `${lineNum}: ${tabsToSpaces( str )}\n${indicator}`;
+			}
+
+			return `${lineNum}: ${tabsToSpaces( str )}`;
+		})
+		.join( '\n' );
+}
diff --git a/src/utils/getExportMode.js b/src/utils/getExportMode.js
index a3d00a0..d525550 100644
--- a/src/utils/getExportMode.js
+++ b/src/utils/getExportMode.js
@@ -24,7 +24,11 @@ export default function getExportMode ( bundle, {exports: exportMode, moduleName
 			exportMode = 'default';
 		} else {
 			if ( bundle.entryModule.exports.default && format !== 'es') {
-				bundle.onwarn( `Using named and default exports together. Consumers of your bundle will have to use ${moduleName || 'bundle'}['default'] to access the default export, which may not be what you want. Use \`exports: 'named'\` to disable this warning. See https://github.com/rollup/rollup/wiki/JavaScript-API#exports for more information` );
+				bundle.warn({
+					code: 'MIXED_EXPORTS',
+					message: `Using named and default exports together. Consumers of your bundle will have to use ${moduleName || 'bundle'}['default'] to access the default export, which may not be what you want. Use \`exports: 'named'\` to disable this warning`,
+					url: `https://github.com/rollup/rollup/wiki/JavaScript-API#exports`
+				});
 			}
 			exportMode = 'named';
 		}
diff --git a/src/utils/getLocation.js b/src/utils/getLocation.js
deleted file mode 100644
index 41ca492..0000000
--- a/src/utils/getLocation.js
+++ /dev/null
@@ -1,20 +0,0 @@
-export default function getLocation ( source, charIndex ) {
-	const lines = source.split( '\n' );
-	const len = lines.length;
-
-	let lineStart = 0;
-	let i;
-
-	for ( i = 0; i < len; i += 1 ) {
-		const line = lines[i];
-		const lineEnd =  lineStart + line.length + 1; // +1 for newline
-
-		if ( lineEnd > charIndex ) {
-			return { line: i + 1, column: charIndex - lineStart };
-		}
-
-		lineStart = lineEnd;
-	}
-
-	throw new Error( 'Could not determine location of character' );
-}
diff --git a/test/function/assign-namespace-to-var/_config.js b/test/function/assign-namespace-to-var/_config.js
index 2eb7e9c..a8c94ea 100644
--- a/test/function/assign-namespace-to-var/_config.js
+++ b/test/function/assign-namespace-to-var/_config.js
@@ -2,9 +2,10 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'allows a namespace to be assigned to a variable',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			'Generated an empty bundle'
-		]);
-	}
+	warnings: [
+		{
+			code: 'EMPTY_BUNDLE',
+			message: 'Generated an empty bundle'
+		}
+	]
 };
diff --git a/test/function/cannot-call-external-namespace/_config.js b/test/function/cannot-call-external-namespace/_config.js
index 263c14a..5ed02e2 100644
--- a/test/function/cannot-call-external-namespace/_config.js
+++ b/test/function/cannot-call-external-namespace/_config.js
@@ -7,6 +7,6 @@ module.exports = {
 		assert.equal( err.message, 'Cannot call a namespace (\'foo\')' );
 		assert.equal( err.file.replace( /\//g, path.sep ), path.resolve( __dirname, 'main.js' ) );
 		assert.equal( err.pos, 28 );
-		assert.deepEqual( err.loc, { line: 2, column: 0 });
+		assert.deepEqual( err.loc, { character: 28, line: 2, column: 0 });
 	}
 };
diff --git a/test/function/cannot-call-internal-namespace/_config.js b/test/function/cannot-call-internal-namespace/_config.js
index ceb42c3..d347008 100644
--- a/test/function/cannot-call-internal-namespace/_config.js
+++ b/test/function/cannot-call-internal-namespace/_config.js
@@ -7,6 +7,6 @@ module.exports = {
 		assert.equal( err.message, 'Cannot call a namespace (\'foo\')' );
 		assert.equal( err.file.replace( /\//g, path.sep ), path.resolve( __dirname, 'main.js' ) );
 		assert.equal( err.pos, 33 );
-		assert.deepEqual( err.loc, { line: 2, column: 0 });
+		assert.deepEqual( err.loc, { character: 33, line: 2, column: 0 });
 	}
 };
diff --git a/test/function/custom-path-resolver-async/_config.js b/test/function/custom-path-resolver-async/_config.js
index 83a10c1..3924db7 100644
--- a/test/function/custom-path-resolver-async/_config.js
+++ b/test/function/custom-path-resolver-async/_config.js
@@ -21,11 +21,13 @@ module.exports = {
 			}
 		}]
 	},
-	warnings: function ( warnings ) {
-		assert.deepEqual( warnings, [
-			`'path' is imported by main.js, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
-		]);
-	},
+	warnings: [
+		{
+			code: 'UNRESOLVED_IMPORT',
+			message: `'path' is imported by main.js, but could not be resolved – treating it as an external dependency`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
+		}
+	],
 	exports: function ( exports ) {
 		assert.strictEqual( exports.path, require( 'path' ) );
 	}
diff --git a/test/function/custom-path-resolver-sync/_config.js b/test/function/custom-path-resolver-sync/_config.js
index 074f671..f32cb9a 100644
--- a/test/function/custom-path-resolver-sync/_config.js
+++ b/test/function/custom-path-resolver-sync/_config.js
@@ -13,11 +13,13 @@ module.exports = {
 			}
 		}]
 	},
-	warnings: function ( warnings ) {
-		assert.deepEqual( warnings, [
-			`'path' is imported by main.js, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
-		]);
-	},
+	warnings: [
+		{
+			code: 'UNRESOLVED_IMPORT',
+			message: `'path' is imported by main.js, but could not be resolved – treating it as an external dependency`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
+		}
+	],
 	exports: function ( exports ) {
 		assert.strictEqual( exports.path, require( 'path' ) );
 	}
diff --git a/test/function/does-not-hang-on-missing-module/_config.js b/test/function/does-not-hang-on-missing-module/_config.js
index 901bfae..aeb5386 100644
--- a/test/function/does-not-hang-on-missing-module/_config.js
+++ b/test/function/does-not-hang-on-missing-module/_config.js
@@ -2,11 +2,13 @@ var assert = require( 'assert' );
 
 module.exports = {
 	description: 'does not hang on missing module (#53)',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			`'unlessYouCreatedThisFileForSomeReason' is imported by main.js, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
-		]);
-	},
+	warnings: [
+		{
+			code: 'UNRESOLVED_IMPORT',
+			message: `'unlessYouCreatedThisFileForSomeReason' is imported by main.js, but could not be resolved – treating it as an external dependency`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
+		}
+	],
 	runtimeError: function ( error ) {
 		assert.equal( "Cannot find module 'unlessYouCreatedThisFileForSomeReason'", error.message );
 	}
diff --git a/test/function/double-named-export-from/_config.js b/test/function/double-named-export-from/_config.js
deleted file mode 100644
index b3885d2..0000000
--- a/test/function/double-named-export-from/_config.js
+++ /dev/null
@@ -1,13 +0,0 @@
-const { resolve } = require('path');
-const assert = require( 'assert' );
-
-const r = path => resolve( __dirname, path );
-
-module.exports = {
-	description: 'throws on duplicate export * from',
-	warnings ( warnings ) {
-		assert.deepEqual( warnings, [
-			`Conflicting namespaces: ${r('main.js')} re-exports 'foo' from both ${r('foo.js')} (will be ignored) and ${r('deep.js')}.`
-		]);
-	}
-};
diff --git a/test/function/duplicate-import-fails/_config.js b/test/function/duplicate-import-fails/_config.js
index 4083690..492ac2f 100644
--- a/test/function/duplicate-import-fails/_config.js
+++ b/test/function/duplicate-import-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows duplicate imports',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 2, column: 9 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 36, line: 2, column: 9 });
 		assert.ok( /Duplicated import/.test( err.message ) );
 	}
 };
diff --git a/test/function/duplicate-import-specifier-fails/_config.js b/test/function/duplicate-import-specifier-fails/_config.js
index e3957b5..58446af 100644
--- a/test/function/duplicate-import-specifier-fails/_config.js
+++ b/test/function/duplicate-import-specifier-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows duplicate import specifiers',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 1, column: 12 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 12, line: 1, column: 12 });
 		assert.ok( /Duplicated import/.test( err.message ) );
 	}
 };
diff --git a/test/function/empty-exports/_config.js b/test/function/empty-exports/_config.js
index 8030197..e928a59 100644
--- a/test/function/empty-exports/_config.js
+++ b/test/function/empty-exports/_config.js
@@ -1,12 +1,23 @@
-const assert = require( 'assert' );
-const path = require( 'path' );
-
 module.exports = {
 	description: 'warns on export {}, but does not fail',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			`Module ${path.resolve( __dirname, 'main.js' )} has an empty export declaration`,
-			'Generated an empty bundle'
-		]);
-	}
+	warnings: [
+		{
+			code: 'EMPTY_EXPORT',
+			message: 'Empty export declaration',
+			pos: 0,
+			loc: {
+				file: require( 'path' ).resolve( __dirname, 'main.js' ),
+				line: 1,
+				column: 0
+			},
+			frame: `
+				1: export {};
+				   ^
+			`
+		},
+		{
+			code: 'EMPTY_BUNDLE',
+			message: 'Generated an empty bundle'
+		}
+	]
 };
diff --git a/test/function/module-tree/_config.js b/test/function/module-tree/_config.js
index 9662844..601ee7d 100644
--- a/test/function/module-tree/_config.js
+++ b/test/function/module-tree/_config.js
@@ -34,9 +34,10 @@ module.exports = {
 			}
 		]);
 	},
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			'Generated an empty bundle'
-		]);
-	}
+	warnings: [
+		{
+			code: 'EMPTY_BUNDLE',
+			message: 'Generated an empty bundle'
+		}
+	]
 };
diff --git a/test/function/namespace-missing-export/_config.js b/test/function/namespace-missing-export/_config.js
index 6ecf3b9..485a1ba 100644
--- a/test/function/namespace-missing-export/_config.js
+++ b/test/function/namespace-missing-export/_config.js
@@ -1,9 +1,21 @@
-var assert = require( 'assert' );
-
 module.exports = {
-	options: {
-		onwarn: function ( msg ) {
-			assert.equal( msg, `main.js (3:21) 'foo' is not exported by 'empty.js'. See https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-module` );
+	warnings: [
+		{
+			code: 'MISSING_EXPORT',
+			message: `'foo' is not exported by 'empty.js'`,
+			pos: 61,
+			loc: {
+				file: require( 'path' ).resolve( __dirname, 'main.js' ),
+				line: 3,
+				column: 25
+			},
+			frame: `
+				1: import * as mod from './empty.js';
+				2:
+				3: assert.equal( typeof mod.foo, 'undefined' );
+				                            ^
+			`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-module`
 		}
-	}
+	]
 };
diff --git a/test/function/namespace-reassign-import-fails/_config.js b/test/function/namespace-reassign-import-fails/_config.js
index 2d606f4..a76a36a 100644
--- a/test/function/namespace-reassign-import-fails/_config.js
+++ b/test/function/namespace-reassign-import-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows reassignments to namespace exports',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 3, column: 0 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 31, line: 3, column: 0 });
 		assert.ok( /Illegal reassignment/.test( err.message ) );
 	}
 };
diff --git a/test/function/namespace-update-import-fails/_config.js b/test/function/namespace-update-import-fails/_config.js
index 4cff5c0..dc8e0f3 100644
--- a/test/function/namespace-update-import-fails/_config.js
+++ b/test/function/namespace-update-import-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows updates to namespace exports',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 3, column: 0 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 31, line: 3, column: 0 });
 		assert.ok( /Illegal reassignment/.test( err.message ) );
 	}
 };
diff --git a/test/function/reassign-import-fails/_config.js b/test/function/reassign-import-fails/_config.js
index 22591fe..4854d02 100644
--- a/test/function/reassign-import-fails/_config.js
+++ b/test/function/reassign-import-fails/_config.js
@@ -5,8 +5,8 @@ module.exports = {
 	description: 'disallows assignments to imported bindings',
 	error: function ( err ) {
 		assert.ok( /Illegal reassignment/.test( err.message ) );
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 8, column: 0 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 113, line: 8, column: 0 });
 	}
 };
 
diff --git a/test/function/reassign-import-not-at-top-level-fails/_config.js b/test/function/reassign-import-not-at-top-level-fails/_config.js
index 6e00786..b4cba92 100644
--- a/test/function/reassign-import-not-at-top-level-fails/_config.js
+++ b/test/function/reassign-import-not-at-top-level-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows assignments to imported bindings not at the top level',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 7, column: 2 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 95, line: 7, column: 2 });
 		assert.ok( /Illegal reassignment/.test( err.message ) );
 	}
 };
diff --git a/test/function/unused-import/_config.js b/test/function/unused-import/_config.js
index b9e9b85..0f99f59 100644
--- a/test/function/unused-import/_config.js
+++ b/test/function/unused-import/_config.js
@@ -2,11 +2,19 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'warns on unused imports ([#595])',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			`'external' is imported by main.js, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`,
-			`'unused', 'notused' and 'neverused' are imported from external module 'external' but never used`,
-			`Generated an empty bundle`
-		]);
-	}
+	warnings: [
+		{
+			code: 'UNRESOLVED_IMPORT',
+			message: `'external' is imported by main.js, but could not be resolved – treating it as an external dependency`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`
+		},
+		{
+			code: 'UNUSED_EXTERNAL_IMPORT',
+			message: `'unused', 'notused' and 'neverused' are imported from external module 'external' but never used`
+		},
+		{
+			code: 'EMPTY_BUNDLE',
+			message: `Generated an empty bundle`
+		}
+	]
 };
diff --git a/test/function/update-expression-of-import-fails/_config.js b/test/function/update-expression-of-import-fails/_config.js
index 5f5c27c..cf72241 100644
--- a/test/function/update-expression-of-import-fails/_config.js
+++ b/test/function/update-expression-of-import-fails/_config.js
@@ -4,8 +4,8 @@ var assert = require( 'assert' );
 module.exports = {
 	description: 'disallows updates to imported bindings',
 	error: function ( err ) {
-		assert.equal( path.normalize(err.file), path.resolve( __dirname, 'main.js' ) );
-		assert.deepEqual( err.loc, { line: 3, column: 0 });
+		assert.equal( path.normalize( err.file ), path.resolve( __dirname, 'main.js' ) );
+		assert.deepEqual( err.loc, { character: 28, line: 3, column: 0 });
 		assert.ok( /Illegal reassignment/.test( err.message ) );
 	}
 };
diff --git a/test/function/warn-on-ambiguous-function-export/_config.js b/test/function/warn-on-ambiguous-function-export/_config.js
index 1b83422..baa08bc 100644
--- a/test/function/warn-on-ambiguous-function-export/_config.js
+++ b/test/function/warn-on-ambiguous-function-export/_config.js
@@ -2,9 +2,23 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'uses original name of default export function (#1011)',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			'foo.js (1:15) Ambiguous default export (is a call expression, but looks like a function declaration). See https://github.com/rollup/rollup/wiki/Troubleshooting#ambiguous-default-export'
-		]);
-	}
+	warnings: [
+		{
+			code: 'AMBIGUOUS_DEFAULT_EXPORT',
+			message: `Ambiguous default export (is a call expression, but looks like a function declaration)`,
+			pos: 15,
+			loc: {
+				file: require( 'path' ).resolve( __dirname, 'foo.js' ),
+				line: 1,
+				column: 15
+			},
+			frame: `
+				1: export default function foo ( a, b ) {
+				                  ^
+				2:   assert.equal( a, b );
+				3:   return 3;
+			`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#ambiguous-default-export`
+		}
+	]
 };
diff --git a/test/function/warn-on-auto-named-default-exports/_config.js b/test/function/warn-on-auto-named-default-exports/_config.js
index f093781..44e493b 100644
--- a/test/function/warn-on-auto-named-default-exports/_config.js
+++ b/test/function/warn-on-auto-named-default-exports/_config.js
@@ -1,10 +1,10 @@
-var assert = require( 'assert' );
-
 module.exports = {
 	description: 'warns if default and named exports are used in auto mode',
-	warnings: function ( warnings ) {
-		assert.deepEqual( warnings, [
-			'Using named and default exports together. Consumers of your bundle will have to use bundle[\'default\'] to access the default export, which may not be what you want. Use `exports: \'named\'` to disable this warning. See https://github.com/rollup/rollup/wiki/JavaScript-API#exports for more information'
-		]);
-	}
+	warnings: [
+		{
+			code: 'MIXED_EXPORTS',
+			message: `Using named and default exports together. Consumers of your bundle will have to use bundle['default'] to access the default export, which may not be what you want. Use \`exports: 'named'\` to disable this warning`,
+			url: `https://github.com/rollup/rollup/wiki/JavaScript-API#exports`
+		}
+	]
 };
diff --git a/test/function/warn-on-empty-bundle/_config.js b/test/function/warn-on-empty-bundle/_config.js
index 2e599ad..a6d0a23 100644
--- a/test/function/warn-on-empty-bundle/_config.js
+++ b/test/function/warn-on-empty-bundle/_config.js
@@ -1,10 +1,9 @@
-const assert = require( 'assert' );
-
 module.exports = {
 	description: 'warns if empty bundle is generated  (#444)',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			'Generated an empty bundle'
-		]);
-	}
+	warnings: [
+		{
+			code: 'EMPTY_BUNDLE',
+			message: 'Generated an empty bundle'
+		}
+	]
 };
diff --git a/test/function/warn-on-eval/_config.js b/test/function/warn-on-eval/_config.js
index d6ac89f..a959bce 100644
--- a/test/function/warn-on-eval/_config.js
+++ b/test/function/warn-on-eval/_config.js
@@ -1,16 +1,20 @@
-var assert = require( 'assert' );
-
-var warned = false;
-
 module.exports = {
 	description: 'warns about use of eval',
-	options: {
-		onwarn: function ( message ) {
-			warned = true;
-			assert.ok( /Use of `eval` \(in .+?main\.js\) is strongly discouraged, as it poses security risks and may cause issues with minification\. See https:\/\/github.com\/rollup\/rollup\/wiki\/Troubleshooting#avoiding-eval for more details/.test( message ) );
+	warnings: [
+		{
+			code: 'EVAL',
+			message: `Use of eval is strongly discouraged, as it poses security risks and may cause issues with minification`,
+			pos: 13,
+			loc: {
+				column: 13,
+				file: require( 'path' ).resolve( __dirname, 'main.js' ),
+				line: 1
+			},
+			frame: `
+				1: var result = eval( '1 + 1' );
+				                ^
+			`,
+			url: 'https://github.com/rollup/rollup/wiki/Troubleshooting#avoiding-eval'
 		}
-	},
-	exports: function () {
-		assert.ok( warned, 'did not warn' );
-	}
+	]
 };
diff --git a/test/function/warn-on-namespace-conflict/_config.js b/test/function/warn-on-namespace-conflict/_config.js
new file mode 100644
index 0000000..2054d75
--- /dev/null
+++ b/test/function/warn-on-namespace-conflict/_config.js
@@ -0,0 +1,9 @@
+module.exports = {
+	description: 'warns on duplicate export * from',
+	warnings: [
+		{
+			code: 'NAMESPACE_CONFLICT',
+			message: `Conflicting namespaces: main.js re-exports 'foo' from both foo.js (will be ignored) and deep.js`
+		}
+	]
+};
diff --git a/test/function/double-named-export-from/bar.js b/test/function/warn-on-namespace-conflict/bar.js
similarity index 100%
rename from test/function/double-named-export-from/bar.js
rename to test/function/warn-on-namespace-conflict/bar.js
diff --git a/test/function/double-named-export-from/deep.js b/test/function/warn-on-namespace-conflict/deep.js
similarity index 100%
rename from test/function/double-named-export-from/deep.js
rename to test/function/warn-on-namespace-conflict/deep.js
diff --git a/test/function/double-named-export-from/foo.js b/test/function/warn-on-namespace-conflict/foo.js
similarity index 100%
rename from test/function/double-named-export-from/foo.js
rename to test/function/warn-on-namespace-conflict/foo.js
diff --git a/test/function/double-named-export-from/main.js b/test/function/warn-on-namespace-conflict/main.js
similarity index 100%
rename from test/function/double-named-export-from/main.js
rename to test/function/warn-on-namespace-conflict/main.js
diff --git a/test/function/warn-on-top-level-this/_config.js b/test/function/warn-on-top-level-this/_config.js
index 99c3f2c..4d9033e 100644
--- a/test/function/warn-on-top-level-this/_config.js
+++ b/test/function/warn-on-top-level-this/_config.js
@@ -2,11 +2,25 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'warns on top-level this (#770)',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			`main.js (3:1) The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten. See https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined for more information`
-		]);
-	},
+	warnings: [
+		{
+			code: 'THIS_IS_UNDEFINED',
+			message: `The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten`,
+			pos: 81,
+			loc: {
+				file: require( 'path' ).resolve( __dirname, 'main.js' ),
+				line: 3,
+				column: 0
+			},
+			frame: `
+				1: const someVariableJustToCheckOnCorrectLineNumber = true; // eslint-disable-line
+				2:
+				3: this.foo = 'bar';
+				   ^
+			`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#this-is-undefined`
+		}
+	],
 	runtimeError: err => {
 		assert.equal( err.message, `Cannot set property 'foo' of undefined` );
 	}
diff --git a/test/function/warn-on-unused-missing-imports/_config.js b/test/function/warn-on-unused-missing-imports/_config.js
index 1fca512..08cc6ba 100644
--- a/test/function/warn-on-unused-missing-imports/_config.js
+++ b/test/function/warn-on-unused-missing-imports/_config.js
@@ -3,9 +3,22 @@ const assert = require( 'assert' );
 
 module.exports = {
 	description: 'warns on missing (but unused) imports',
-	warnings: warnings => {
-		assert.deepEqual( warnings, [
-			`Non-existent export 'b' is imported from ${path.resolve(__dirname, 'foo.js')} by ${path.resolve(__dirname, 'main.js')}`
-		]);
-	}
+	warnings: [
+		{
+			code: 'NON_EXISTENT_EXPORT',
+			message: `Non-existent export 'b' is imported from foo.js`,
+			pos: 12,
+			loc: {
+				file: path.resolve( __dirname, 'main.js' ),
+				line: 1,
+				column: 12
+			},
+			frame: `
+				1: import { a, b } from './foo.js';
+				               ^
+				2:
+				3: assert.equal( a, 42 );
+			`
+		}
+	]
 };
diff --git a/test/sourcemaps/transform-without-sourcemap/_config.js b/test/sourcemaps/transform-without-sourcemap/_config.js
index ad33354..0ce79c0 100644
--- a/test/sourcemaps/transform-without-sourcemap/_config.js
+++ b/test/sourcemaps/transform-without-sourcemap/_config.js
@@ -1,10 +1,5 @@
-const assert = require( 'assert' );
-
-let warnings = [];
-
 module.exports = {
 	description: 'preserves sourcemap chains when transforming',
-	before: () => warnings = [], // reset
 	options: {
 		plugins: [
 			{
@@ -13,14 +8,13 @@ module.exports = {
 					return code;
 				}
 			}
-		],
-		onwarn ( msg ) {
-			warnings.push( msg );
-		}
+		]
 	},
-	test: () => {
-		assert.deepEqual( warnings, [
-			`Sourcemap is likely to be incorrect: a plugin ('fake plugin') was used to transform files, but didn't generate a sourcemap for the transformation. Consult https://github.com/rollup/rollup/wiki/Troubleshooting and the plugin documentation for more information`
-		]);
-	}
+	warnings: [
+		{
+			code: `SOURCEMAP_BROKEN`,
+			message: `Sourcemap is likely to be incorrect: a plugin ('fake plugin') was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help`,
+			url: `https://github.com/rollup/rollup/wiki/Troubleshooting#sourcemap-is-likely-to-be-incorrect`
+		}
+	]
 };
diff --git a/test/test.js b/test/test.js
index 50f63e8..789ac44 100644
--- a/test/test.js
+++ b/test/test.js
@@ -58,6 +58,27 @@ function loader ( modules ) {
 	};
 }
 
+function compareWarnings ( actual, expected ) {
+	assert.deepEqual(
+		actual.map( warning => {
+			const clone = Object.assign( {}, warning );
+			delete clone.toString;
+
+			if ( clone.frame ) {
+				clone.frame = clone.frame.replace( /\s+$/gm, '' );
+			}
+
+			return clone;
+		}),
+		expected.map( warning => {
+			if ( warning.frame ) {
+				warning.frame = warning.frame.slice( 1 ).replace( /^\t+/gm, '' ).replace( /\s+$/gm, '' ).trim();
+			}
+			return warning;
+		})
+	);
+}
+
 describe( 'rollup', function () {
 	this.timeout( 10000 );
 
@@ -270,7 +291,11 @@ describe( 'rollup', function () {
 						}
 
 						if ( config.warnings ) {
-							config.warnings( warnings );
+							if ( Array.isArray( config.warnings ) ) {
+								compareWarnings( warnings, config.warnings );
+							} else {
+								config.warnings( warnings );
+							}
 						} else if ( warnings.length ) {
 							throw new Error( `Got unexpected warnings:\n${warnings.join('\n')}` );
 						}
@@ -377,11 +402,18 @@ describe( 'rollup', function () {
 				const entry = path.resolve( SOURCEMAPS, dir, 'main.js' );
 				const dest = path.resolve( SOURCEMAPS, dir, '_actual/bundle' );
 
-				const options = extend( {}, config.options, { entry });
+				let warnings;
+
+				const options = extend( {}, config.options, {
+					entry,
+					onwarn: warning => warnings.push( warning )
+				});
 
 				PROFILES.forEach( profile => {
 					( config.skip ? it.skip : config.solo ? it.only : it )( 'generates ' + profile.format, () => {
 						process.chdir( SOURCEMAPS + '/' + dir );
+						warnings = [];
+
 						return rollup.rollup( options ).then( bundle => {
 							const options = extend( {}, {
 								format: profile.format,
@@ -391,9 +423,16 @@ describe( 'rollup', function () {
 
 							bundle.write( options );
 
-							if ( config.before ) config.before();
-							const result = bundle.generate( options );
-							config.test( result.code, result.map );
+							if ( config.test ) {
+								const { code, map } = bundle.generate( options );
+								config.test( code, map );
+							}
+
+							if ( config.warnings ) {
+								compareWarnings( warnings, config.warnings );
+							} else if ( warnings.length ) {
+								throw new Error( `Unexpected warnings` );
+							}
 						});
 					});
 				});
@@ -737,11 +776,9 @@ describe( 'rollup', function () {
 					moduleName: 'myBundle'
 				});
 
-				assert.deepEqual( warnings, [
-					`'util' is imported by entry, but could not be resolved – treating it as an external dependency. For help see https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency`,
-					`Creating a browser bundle that depends on Node.js built-in module ('util'). You might need to include https://www.npmjs.com/package/rollup-plugin-node-builtins`,
-					`No name was provided for external module 'util' in options.globals – guessing 'util'`
-				]);
+				const relevantWarnings = warnings.filter( warning => warning.code === 'MISSING_NODE_BUILTINS' );
+				assert.equal( relevantWarnings.length, 1 );
+				assert.equal( relevantWarnings[0].message, `Creating a browser bundle that depends on Node.js built-in module ('util'). You might need to include https://www.npmjs.com/package/rollup-plugin-node-builtins` );
 			});
 		});
 	});