style: Convert specified value tests to compile-time tests

These were written at a time where std::mem::size_of wasn't a `const fn` in
Rust.

Now that it is, we can make these tests live in the style crate, and the build
not to compile if they fail.

Differential Revision: https://phabricator.services.mozilla.com/D146103

Cargo.lock

@@ -6042,6 +6042,12 @@ version = "1.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dba1a27d3efae4351c8051072d619e3ade2820635c3958d826bfea39d59b54c8"
 
+[[package]]
+name = "static_assertions"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
+
 [[package]]
 name = "std_test_override"
 version = "0.0.1"
@@ -6145,6 +6151,7 @@ dependencies = [
  "servo_url",
  "smallbitvec",
  "smallvec",
+ "static_assertions",
  "string_cache",
  "style_derive",
  "style_traits",

components/style/Cargo.toml

@@ -62,6 +62,7 @@ servo_config = { path = "../config", optional = true }
 servo_url = { path = "../url", optional = true }
 smallbitvec = "2.3.0"
 smallvec = "1.0"
+static_assertions = "1.1"
 string_cache = { version = "0.8", optional = true }
 style_derive = { path = "../style_derive" }
 style_traits = { path = "../style_traits" }

components/style/lib.rs

@@ -63,6 +63,8 @@ pub use servo_arc;
 #[macro_use]
 extern crate servo_atoms;
 #[macro_use]
+extern crate static_assertions;
+#[macro_use]
 extern crate style_derive;
 #[macro_use]
 extern crate to_shmem_derive;
@@ -186,7 +188,7 @@ pub mod gecko_properties {
 }
 
 macro_rules! reexport_computed_values {
-    ( $( { $name: ident, $boxed: expr } )+ ) => {
+    ( $( { $name: ident } )+ ) => {
         /// Types for [computed values][computed].
         ///
         /// [computed]: https://drafts.csswg.org/css-cascade/#computed
@@ -200,7 +202,6 @@ macro_rules! reexport_computed_values {
     }
 }
 longhand_properties_idents!(reexport_computed_values);
-
 #[cfg(feature = "gecko")]
 use crate::gecko_string_cache::WeakAtom;
 #[cfg(feature = "servo")]

components/style/properties/properties.mako.rs

@@ -4218,7 +4218,7 @@ macro_rules! css_properties_accessors {
 /// Call the given macro with tokens like this for each longhand properties:
 ///
 /// ```
-/// { snake_case_ident, true }
+/// { snake_case_ident }
 /// ```
 ///
 /// … where the boolean indicates whether the property value type
@@ -4228,12 +4228,34 @@ macro_rules! longhand_properties_idents {
     ($macro_name: ident) => {
         $macro_name! {
             % for property in data.longhands:
-                { ${property.ident}, ${"true" if property.boxed else "false"} }
+                { ${property.ident} }
             % endfor
         }
     }
 }
 
+// There are two reasons for this test to fail:
+//
+//   * Your changes made a specified value type for a given property go
+//     over the threshold. In that case, you should try to shrink it again
+//     or, if not possible, mark the property as boxed in the property
+//     definition.
+//
+//   * Your changes made a specified value type smaller, so that it no
+//     longer needs to be boxed. In this case you just need to remove
+//     boxed=True from the property definition. Nice job!
+#[cfg(target_pointer_width = "64")]
+#[allow(dead_code)] // https://github.com/rust-lang/rust/issues/96952
+const BOX_THRESHOLD: usize = 24;
+% for longhand in data.longhands:
+#[cfg(target_pointer_width = "64")]
+% if longhand.boxed:
+const_assert!(std::mem::size_of::<longhands::${longhand.ident}::SpecifiedValue>() > BOX_THRESHOLD);
+% else:
+const_assert!(std::mem::size_of::<longhands::${longhand.ident}::SpecifiedValue>() <= BOX_THRESHOLD);
+% endif
+% endfor
+
 % if engine == "servo":
 % for effect_name in ["repaint", "reflow_out_of_flow", "reflow", "rebuild_and_reflow_inline", "rebuild_and_reflow"]:
     macro_rules! restyle_damage_${effect_name} {