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

Removing recursion from ComplexSelector

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Using a smallvec instead of recursively calling to_css for ComplexSelector to handle running out of stack.

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

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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)",
 ]
 
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