Auto merge of #17901 - heycam:rm-empty-variables, r=emilio

style: Remove now-unused empty Variables struct.

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The `Servo_GetEmptyVariables` FFI function no longer has any callers from Gecko, so we can remove it and the code that creates/frees that struct.

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

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/gecko/generated/bindings.rs

@@ -2866,9 +2866,6 @@ extern "C" {
                                          index: u32, name: *mut nsAString)
      -> bool;
 }
-extern "C" {
-    pub fn Servo_GetEmptyVariables() -> *const nsStyleVariables;
-}
 extern "C" {
     pub fn Gecko_CreateCSSErrorReporter(sheet: *mut ServoStyleSheet,
                                         loader: *mut Loader, uri: *mut nsIURI)

components/style/properties/gecko.mako.rs

@@ -19,13 +19,11 @@ use gecko_bindings::bindings::Gecko_Construct_Default_${style_struct.gecko_ffi_n
 use gecko_bindings::bindings::Gecko_CopyConstruct_${style_struct.gecko_ffi_name};
 use gecko_bindings::bindings::Gecko_Destroy_${style_struct.gecko_ffi_name};
 % endfor
-use gecko_bindings::bindings::Gecko_Construct_nsStyleVariables;
 use gecko_bindings::bindings::Gecko_CopyCounterStyle;
 use gecko_bindings::bindings::Gecko_CopyCursorArrayFrom;
 use gecko_bindings::bindings::Gecko_CopyFontFamilyFrom;
 use gecko_bindings::bindings::Gecko_CopyImageValueFrom;
 use gecko_bindings::bindings::Gecko_CopyListStyleImageFrom;
-use gecko_bindings::bindings::Gecko_Destroy_nsStyleVariables;
 use gecko_bindings::bindings::Gecko_EnsureImageLayersLength;
 use gecko_bindings::bindings::Gecko_FontFamilyList_AppendGeneric;
 use gecko_bindings::bindings::Gecko_FontFamilyList_AppendNamed;
@@ -43,7 +41,6 @@ use gecko_bindings::bindings::{Gecko_ResetFilters, Gecko_CopyFiltersFrom};
 use gecko_bindings::bindings::RawGeckoPresContextBorrowed;
 use gecko_bindings::structs;
 use gecko_bindings::structs::nsCSSPropertyID;
-use gecko_bindings::structs::nsStyleVariables;
 use gecko_bindings::sugar::ns_style_coord::{CoordDataValue, CoordData, CoordDataMut};
 use gecko::values::convert_nscolor_to_rgba;
 use gecko::values::convert_rgba_to_nscolor;
@@ -5030,26 +5027,3 @@ ${impl_style_struct(style_struct)}
 <%self:raw_impl_trait style_struct="${style_struct}"></%self:raw_impl_trait>
 % endif
 % endfor
-
-// This is only accessed from the Gecko main thread.
-static mut EMPTY_VARIABLES_STRUCT: Option<nsStyleVariables> = None;
-
-#[no_mangle]
-#[allow(non_snake_case)]
-pub unsafe extern "C" fn Servo_GetEmptyVariables()
-                                                 -> *const nsStyleVariables {
-    EMPTY_VARIABLES_STRUCT.as_ref().unwrap()
-}
-
-pub fn initialize() {
-    unsafe {
-        EMPTY_VARIABLES_STRUCT = Some(zeroed());
-        Gecko_Construct_nsStyleVariables(EMPTY_VARIABLES_STRUCT.as_mut().unwrap());
-    }
-}
-
-pub fn shutdown() {
-    unsafe {
-        EMPTY_VARIABLES_STRUCT.take().as_mut().map(|v| Gecko_Destroy_nsStyleVariables(v));
-    }
-}