Support nesting with unused symbols

Author: devongovett

node/index.d.ts

@@ -26,7 +26,12 @@ export interface TransformOptions {
    * Replaces user action pseudo classes with class names that can be applied from JavaScript.
    * This is useful for polyfills, for example.
    */
-  pseudoClasses?: PseudoClasses
+  pseudoClasses?: PseudoClasses,
+  /**
+   * A list of class names, ids, and custom identifiers (e.g. @keyframes) that are known
+   * to be unused. These will be removed during minification.
+   */
+  unusedSymbols?: string[]
 }
 
 export interface Drafts {

src/lib.rs

@@ -9033,5 +9033,29 @@ mod tests {
     });
     let res = stylesheet.to_css(PrinterOptions::default()).unwrap();
     assert_eq!(res.code, expected);
+
+    let source = r#"
+      .foo {
+        color: red;
+
+        &.bar {
+          color: green;
+        }
+      }
+    "#;
+
+    let expected = indoc!{r#"
+      .foo {
+        color: red;
+      }
+    "#};
+
+    let mut stylesheet = StyleSheet::parse("test.css".into(), source, ParserOptions { nesting: true, ..ParserOptions::default() }).unwrap();
+    stylesheet.minify(MinifyOptions {
+      unused_symbols: vec!["bar"].iter().map(|s| String::from(*s)).collect(),
+      ..MinifyOptions::default()
+    });
+    let res = stylesheet.to_css(PrinterOptions::default()).unwrap();
+    assert_eq!(res.code, expected);
   }
 }

src/rules/style.rs

@@ -1,12 +1,12 @@
 use cssparser::SourceLocation;
 use parcel_selectors::SelectorList;
-use crate::selector::{Selectors, is_compatible};
+use crate::selector::{Selectors, is_compatible, is_unused};
 use crate::traits::ToCss;
 use crate::printer::Printer;
 use crate::declaration::DeclarationBlock;
 use crate::vendor_prefix::VendorPrefix;
 use crate::targets::Browsers;
-use crate::rules::{CssRuleList, ToCssWithContext, StyleContext};
+use crate::rules::{CssRuleList, CssRule, ToCssWithContext, StyleContext};
 use crate::compat::Feature;
 use crate::error::PrinterError;
 use super::MinifyContext;
@@ -23,6 +23,15 @@ pub struct StyleRule {
 impl StyleRule {
   pub(crate) fn minify(&mut self, context: &mut MinifyContext) {
     self.declarations.minify(context.handler, context.important_handler, context.logical_properties);
+
+    if !context.unused_symbols.is_empty() {
+      self.rules.0.retain(|rule| {
+        match rule {
+          CssRule::Style(style) => !is_unused(&mut style.selectors.0.iter(), &context.unused_symbols),
+          _ => true
+        }
+      });
+    }
   }
 
   pub fn is_compatible(&self, targets: Option<Browsers>) -> bool {