diff --git a/src/Bundle.js b/src/Bundle.js
index 9332b83..f2f0dd2 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
diff --git a/src/Module.js b/src/Module.js
index 97f6cca..81926c3 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -180,7 +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 );
 			}
 		}
 	}
@@ -408,17 +413,12 @@ export default class Module {
 	}
 
 	warn ( warning, pos ) {
-		if ( 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: line + 1,
-				column
-			};
-
+			warning.loc = { file: this.id, line, column };
 			warning.frame = getCodeFrame( this.code, line, column );
 		}
 
diff --git a/src/ast/nodes/ExportDefaultDeclaration.js b/src/ast/nodes/ExportDefaultDeclaration.js
index 743287e..208120f 100644
--- a/src/ast/nodes/ExportDefaultDeclaration.js
+++ b/src/ast/nodes/ExportDefaultDeclaration.js
@@ -74,8 +74,11 @@ export default class ExportDefaultDeclaration extends Node {
 					const newlineSeparated = /\n/.test( code.original.slice( start, end ) );
 
 					if ( newlineSeparated ) {
-						const { line, column } = locate( this.module.code, this.declaration.start, { offsetLine: 1 });
-						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 4da3031..d745d7d 100644
--- a/src/ast/nodes/MemberExpression.js
+++ b/src/ast/nodes/MemberExpression.js
@@ -1,5 +1,4 @@
 import isReference from 'is-reference';
-import { locate } from 'locate-character';
 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 } = locate( this.module.code, this.start, { offsetLine: 1 });
-					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 666be58..2429c65 100644
--- a/src/ast/nodes/ThisExpression.js
+++ b/src/ast/nodes/ThisExpression.js
@@ -1,8 +1,4 @@
 import Node from '../Node.js';
-import { locate } from 'locate-character';
-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 } = locate( this.module.code, this.start, { offsetLine: 1 });
-				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/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/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/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/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/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/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/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 7b499ef..a959bce 100644
--- a/test/function/warn-on-eval/_config.js
+++ b/test/function/warn-on-eval/_config.js
@@ -8,8 +8,12 @@ module.exports = {
 			loc: {
 				column: 13,
 				file: require( 'path' ).resolve( __dirname, 'main.js' ),
-				line: 2
+				line: 1
 			},
+			frame: `
+				1: var result = eval( '1 + 1' );
+				                ^
+			`,
 			url: 'https://github.com/rollup/rollup/wiki/Troubleshooting#avoiding-eval'
 		}
 	]
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 8b1d3cf..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 );
 
@@ -271,13 +292,7 @@ describe( 'rollup', function () {
 
 						if ( config.warnings ) {
 							if ( Array.isArray( config.warnings ) ) {
-								assert.deepEqual( warnings.map( warning => {
-									const clone = Object.assign( {}, warning );
-									delete clone.toString;
-									delete clone.frame;
-
-									return clone;
-								}), config.warnings );
+								compareWarnings( warnings, config.warnings );
 							} else {
 								config.warnings( warnings );
 							}
@@ -387,11 +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,
@@ -401,9 +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` );
+							}
 						});
 					});
 				});
@@ -747,11 +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` );
 			});
 		});
 	});