style: Use debug_unreachable!() in matches_simple_selector

This shrinks the function by avoiding generating rather noisy panic
code.

Depends on D145486

Differential Revision: https://phabricator.services.mozilla.com/D145487

Cargo.lock

@@ -5050,6 +5050,7 @@ dependencies = [
  "derive_more",
  "fxhash",
  "log",
+ "new_debug_unreachable",
  "phf",
  "phf_codegen",
  "precomputed-hash",

components/selectors/Cargo.toml

@@ -25,6 +25,7 @@ cssparser = "0.29"
 derive_more = "0.99"
 fxhash = "0.2"
 log = "0.4"
+new_debug_unreachable = "1"
 phf = "0.10"
 precomputed-hash = "0.1"
 servo_arc = { version = "0.2", path = "../servo_arc" }

components/selectors/matching.rs

@@ -9,6 +9,7 @@ use crate::parser::{AncestorHashes, Combinator, Component, LocalName};
 use crate::parser::{NonTSPseudoClass, Selector, SelectorImpl, SelectorIter, SelectorList};
 use crate::tree::Element;
 use bitflags::bitflags;
+use debug_unreachable::debug_unreachable;
 use log::debug;
 use smallvec::SmallVec;
 use std::borrow::Borrow;
@@ -832,7 +833,9 @@ where
             }
             true
         }),
-        Component::Combinator(_) => unreachable!(),
+        Component::Combinator(_) => unsafe {
+            debug_unreachable!("Shouldn't try to selector-match combinators")
+        },
     }
 }