Auto merge of #16347 - tictakk:ticbranch, r=emilio

Removing recursion from ComplexSelector

<!-- Please describe your changes on the following line: -->
Using a smallvec instead of recursively calling to_css for ComplexSelector to handle running out of stack.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16019  (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Cargo.lock

@@ -2395,6 +2395,7 @@ dependencies = [
  "fnv 1.0.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "matches 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "precomputed-hash 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "smallvec 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]

components/selectors/Cargo.toml

@@ -21,3 +21,4 @@ matches = "0.1"
 cssparser = "0.12.1"
 fnv = "1.0"
 precomputed-hash = "0.1"
+smallvec = "0.3"

components/selectors/lib.rs

@@ -7,6 +7,7 @@
 #[macro_use] extern crate matches;
 extern crate fnv;
 extern crate precomputed_hash;
+extern crate smallvec;
 
 pub mod bloom;
 pub mod matching;

components/selectors/parser.rs

@@ -363,12 +363,29 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
 
 impl<Impl: SelectorImpl> ToCss for ComplexSelector<Impl> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        if let Some((ref next, ref combinator)) = self.next {
-            next.to_css(dest)?;
-            combinator.to_css(dest)?;
-        }
-        for simple in &self.compound_selector {
-            simple.to_css(dest)?;
+       use smallvec::SmallVec;
+       let mut current = self;
+       let mut nodes = SmallVec::<[&Self;8]>::new();
+       nodes.push(current);
+
+       loop {
+           match current.next {
+               None => break,
+               Some((ref next, _)) => {
+                   current = &**next;
+                   nodes.push(next);
+               }
+           }
+       }
+
+       for selector in nodes.iter().rev() {
+           if let Some((_, ref combinator)) = selector.next {
+               combinator.to_css(dest)?;
+           }
+
+           for simple in &selector.compound_selector {
+               simple.to_css(dest)?;
+           }
         }
         Ok(())
     }