Add warning when unit cannot be used for fallback

lib/pixrem.js

@@ -79,7 +79,7 @@ module.exports = postcss.plugin('pixrem', function (opts) {
             value = value.replace(REGEX, function ($1) {
               // Round decimal pixels down to match webkit and opera behavior:
               // http://tylertate.com/blog/2012/01/05/subpixel-rounding.html
-              return Math.floor(parseFloat($1) * toPx(options.rootValue)) + 'px';
+              return Math.floor(parseFloat($1) * toPx(options.rootValue, result)) + 'px';
             });
 
             if (options.replace) {
@@ -122,7 +122,7 @@ function detectBrowser (browsers, browserQuery) {
 }
 
 // Return a unitless pixel value from any root font-size value.
-function toPx (value) {
+function toPx (value, result) {
   value = (typeof value === 'string' && value.indexOf('calc(') !== -1) ? calc(value) : value;
   var parts = /^(\d*\.?\d+)([a-zA-Z%]*)$/.exec(value);
   if (parts !== null) {
@@ -139,6 +139,7 @@ function toPx (value) {
       return (parseFloat(number) / 100) * BASE_FONT_SIZE;
     } else {
       // other units: vw, ex, ch, etc...
+      result.warn('Unit cannot be used for conversion, so 16px is used.');
       return BASE_FONT_SIZE;
     }
   } else {

test/test.js

@@ -57,6 +57,15 @@ describe('pixrem', function () {
     assert.equal(processed, expected);
   });
 
+  it('should warn when using browser-dependent unit', function (done) {
+    var expected = '.rule { font-size: 32px; font-size: 2rem }';
+    var processed = postcss([pixrem({rootValue: '1vw'})]).process(css).then(function (result) {
+      var warnings = result.warnings();
+      assert.deepEqual(warnings, [{type: 'warning', text: 'Unit cannot be used for conversion, so 16px is used.', plugin: 'pixrem'}]);
+      done();
+    }).catch(done);
+  });
+
   it('should generate fallbacks with a value starting with dot', function () {
     var expected = '.rule { font-size: 16px; font-size: 2rem }';
     var processed = postcss([pixrem({rootValue: '.5em'})]).process(css).css;