Simplify :is with a single selector when possible

Author: devongovett

src/lib.rs

@@ -5614,6 +5614,24 @@ mod tests {
     minify_test(":foo(bar) { color: yellow }", ":foo(bar){color:#ff0}");
     minify_test("::foo(bar) { color: yellow }", "::foo(bar){color:#ff0}");
     minify_test("::foo(*) { color: yellow }", "::foo(*){color:#ff0}");
+
+    minify_test(":is(.foo) { color: yellow }", ".foo{color:#ff0}");
+    minify_test(":is(#foo) { color: yellow }", "#foo{color:#ff0}");
+    minify_test("a:is(.foo) { color: yellow }", "a.foo{color:#ff0}");
+    minify_test("a:is([data-test]) { color: yellow }", "a[data-test]{color:#ff0}");
+    minify_test(".foo:is(a) { color: yellow }", ".foo:is(a){color:#ff0}");
+    minify_test(".foo:is(*|a) { color: yellow }", ".foo:is(a){color:#ff0}");
+    minify_test(".foo:is(*) { color: yellow }", ".foo:is(*){color:#ff0}");
+    minify_test(
+      "@namespace svg url(http://www.w3.org/2000/svg); .foo:is(svg|a) { color: yellow }",
+      "@namespace svg \"http://www.w3.org/2000/svg\";.foo:is(svg|a){color:#ff0}",
+    );
+    minify_test("a:is(.foo .bar) { color: yellow }", "a:is(.foo .bar){color:#ff0}");
+    minify_test(":is(.foo, .bar) { color: yellow }", ":is(.foo,.bar){color:#ff0}");
+    minify_test("a:is(:not(.foo)) { color: yellow }", "a:not(.foo){color:#ff0}");
+    minify_test("a:is(:first-child) { color: yellow }", "a:first-child{color:#ff0}");
+    minify_test("a:is(:has(.foo)) { color: yellow }", "a:has(.foo){color:#ff0}");
+    minify_test("a:is(:is(.foo)) { color: yellow }", "a.foo{color:#ff0}");
   }
 
   #[test]

src/selector.rs

@@ -1164,7 +1164,16 @@ impl<'a, 'i> ToCssWithContext<'a, 'i> for Component<'i, Selectors> {
       Is(ref list) | Where(ref list) | Negation(ref list) | Any(_, ref list) => {
         match *self {
           Where(..) => dest.write_str(":where(")?,
-          Is(..) => {
+          Is(ref selectors) => {
+            // If there's only one simple selector, serialize it directly.
+            if selectors.len() == 1 {
+              let first = selectors.first().unwrap();
+              if !has_type_selector(first) && is_simple(first) {
+                serialize_selector(first, dest, context, false)?;
+                return Ok(());
+              }
+            }
+
             let vp = dest.vendor_prefix;
             if vp.intersects(VendorPrefix::WebKit | VendorPrefix::Moz) {
               dest.write_char(':')?;