--- a/third_party/rust/cssparser/src/parser.rs
+++ b/third_party/rust/cssparser/src/parser.rs
@@ -473,17 +473,21 @@ impl<'i: 't, 't> Parser<'i, 't> {
     ///
     /// Successful results are accumulated in a vector.
     ///
     /// This method retuns `Err(())` the first time that a closure call does,
     /// or if a closure call leaves some input before the next comma or the end of the input.
     #[inline]
     pub fn parse_comma_separated<F, T, E>(&mut self, mut parse_one: F) -> Result<Vec<T>, ParseError<'i, E>>
     where F: for<'tt> FnMut(&mut Parser<'i, 'tt>) -> Result<T, ParseError<'i, E>> {
-        let mut values = vec![];
+        // Vec grows from 0 to 4 by default on first push().  So allocate with
+        // capacity 1, so in the somewhat common case of only one item we don't
+        // way overallocate.  Note that we always push at least one item if
+        // parsing succeeds.
+        let mut values = Vec::with_capacity(1);
         loop {
             self.skip_whitespace();  // Unnecessary for correctness, but may help try() in parse_one rewind less.
             values.push(self.parse_until_before(Delimiter::Comma, &mut parse_one)?);
             match self.next() {
                 Err(_) => return Ok(values),
                 Ok(&Token::Comma) => continue,
                 Ok(_) => unreachable!(),
             }

--- a/toolkit/library/gtest/rust/Cargo.lock
+++ b/toolkit/library/gtest/rust/Cargo.lock
@@ -292,30 +292,35 @@ dependencies = [
  "core-foundation 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "core-graphics 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.24 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "cssparser"
 version = "0.20.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cssparser-macros 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "dtoa-short 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "itoa 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "matches 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "phf 0.7.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "procedural-masquerade 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)",
  "smallvec 0.4.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
+name = "cssparser"
+version = "0.20.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+replace = "cssparser 0.20.1"
+
+[[package]]
 name = "cssparser-macros"
 version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "phf_codegen 0.7.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "procedural-masquerade 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)",
  "syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)",

--- a/toolkit/library/gtest/rust/Cargo.toml
+++ b/toolkit/library/gtest/rust/Cargo.toml
@@ -39,8 +39,11 @@ debug-assertions = true
 codegen-units = 4
 panic = "abort"
 
 [profile.release]
 opt-level = 2
 rpath = false
 debug-assertions = false
 panic = "abort"
+
+[replace]
+"cssparser:0.20.1" = { path = "/home/bzbarsky/mozilla/vanilla/mozilla/third_party/rust/cssparser" }

--- a/toolkit/library/rust/Cargo.lock
+++ b/toolkit/library/rust/Cargo.lock
@@ -290,30 +290,35 @@ dependencies = [
  "core-foundation 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "core-graphics 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.24 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "cssparser"
 version = "0.20.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cssparser-macros 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "dtoa-short 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "itoa 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "matches 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "phf 0.7.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "procedural-masquerade 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)",
  "smallvec 0.4.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
+name = "cssparser"
+version = "0.20.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+replace = "cssparser 0.20.1"
+
+[[package]]
 name = "cssparser-macros"
 version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "phf_codegen 0.7.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "procedural-masquerade 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)",
  "syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)",

--- a/toolkit/library/rust/Cargo.toml
+++ b/toolkit/library/rust/Cargo.toml
@@ -37,8 +37,11 @@ debug-assertions = true
 codegen-units = 4
 panic = "abort"
 
 [profile.release]
 opt-level = 2
 rpath = false
 debug-assertions = false
 panic = "abort"
+
+[replace]
+"cssparser:0.20.1" = { path = "/home/bzbarsky/mozilla/vanilla/mozilla/third_party/rust/cssparser" }