style: Remove ThinBoxedSlice

The only remaining consumers are ::-moz-tree pseudo-elements (we used to
use ThinBoxedSlice for other data structures in the past).

Those are not particularly performance sensitive so I think just
double-boxing is fine. In the future, if we wanted to avoid the double
indirection, we could probably use the "thin" crate
(https://docs.rs/thin) or similar, which stores the length of the slice
along with the allocation, making the pointer thin in all
configurations, much like "ThinArc" does:

  https://searchfox.org/mozilla-central/rev/1ce2eea39442190a71a1f8f650d098f286bf4a01/servo/components/servo_arc/lib.rs#891

In practice though, I don't think it's particularly worth it for this
specific case.

Differential Revision: https://phabricator.services.mozilla.com/D134672

components/style/Cargo.toml

@@ -17,8 +17,7 @@ path = "lib.rs"
 doctest = false
 
 [features]
-gecko = ["style_traits/gecko", "bindgen", "regex", "toml",
-         "num_cpus", "thin-slice"]
+gecko = ["style_traits/gecko", "bindgen", "regex", "toml", "num_cpus"]
 servo = ["serde", "style_traits/servo", "servo_atoms", "servo_config", "html5ever",
          "cssparser/serde", "encoding_rs", "malloc_size_of/servo", "servo_url",
          "string_cache", "to_shmem/servo",
@@ -68,7 +67,6 @@ smallvec = "1.0"
 string_cache = { version = "0.8", optional = true }
 style_derive = { path = "../style_derive" }
 style_traits = { path = "../style_traits" }
-thin-slice = { version = "0.1.0", optional = true }
 time = "0.1"
 to_shmem = { path = "../to_shmem" }
 to_shmem_derive = { path = "../to_shmem_derive" }

components/style/gecko/pseudo_element.rs

@@ -17,7 +17,6 @@ use crate::string_cache::Atom;
 use crate::values::serialize_atom_identifier;
 use cssparser::ToCss;
 use std::fmt;
-use thin_slice::ThinBoxedSlice;
 
 include!(concat!(
     env!("OUT_DIR"),

components/style/gecko/pseudo_element_definition.mako.rs

@@ -3,12 +3,15 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 /// Gecko's pseudo-element definition.
+///
+/// We intentionally double-box legacy ::-moz-tree pseudo-elements to keep the
+/// size of PseudoElement (and thus selector components) small.
 #[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq, ToShmem)]
 pub enum PseudoElement {
     % for pseudo in PSEUDOS:
         /// ${pseudo.value}
         % if pseudo.is_tree_pseudo_element():
-        ${pseudo.capitalized_pseudo()}(ThinBoxedSlice<Atom>),
+        ${pseudo.capitalized_pseudo()}(Box<Box<[Atom]>>),
         % else:
         ${pseudo.capitalized_pseudo()},
         % endif