Improve properties.mako.rs file structure, take 2

This is a new attempt of #10586, after Simon Sapin's great cleanups in #10749 has landed. I have adjusted the changes to the new structure that was introduced, and also only done a few of the longhand ones. Will certainly continue on this as soon as we have a basic agreement that this style is reasonable.

components/style/properties/longhand/box.mako.rs

@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+ <%page args="data, helpers" />
+
+<% data.new_style_struct("Box", inherited=False, gecko_ffi_name="nsStyleDisplay",
+                   additional_methods=[data.new_method("clone_display",
+                                                       "longhands::display::computed_value::T"),
+                                       data.new_method("clone_position",
+                                                       "longhands::position::computed_value::T"),
+                                       data.new_method("is_floated", "bool"),
+                                       data.new_method("overflow_x_is_visible", "bool"),
+                                       data.new_method("overflow_y_is_visible", "bool"),
+                                       data.new_method("transition_count", "usize")]) %>
+
+// TODO(SimonSapin): don't parse `inline-table`, since we don't support it
+<%helpers:longhand name="display" custom_cascade="${product == 'servo'}">
+    <%
+        values = """inline block inline-block
+            table inline-table table-row-group table-header-group table-footer-group
+            table-row table-column-group table-column table-cell table-caption
+            list-item flex
+            none
+        """.split()
+        experimental_values = set("flex".split())
+    %>
+    pub use self::computed_value::T as SpecifiedValue;
+    use values::computed::{Context, ComputedValueAsSpecified};
+
+    pub mod computed_value {
+        #[allow(non_camel_case_types)]
+        #[derive(Clone, Eq, PartialEq, Copy, Hash, RustcEncodable, Debug, HeapSizeOf)]
+        #[derive(Deserialize, Serialize)]
+        pub enum T {
+            % for value in values:
+                ${data.to_rust_ident(value)},
+            % endfor
+        }
+
+        impl ::cssparser::ToCss for T {
+            fn to_css<W>(&self, dest: &mut W) -> ::std::fmt::Result
+            where W: ::std::fmt::Write {
+                match *self {
+                    % for value in values:
+                        T::${data.to_rust_ident(value)} => dest.write_str("${value}"),
+                    % endfor
+                }
+            }
+        }
+    }
+    #[inline] pub fn get_initial_value() -> computed_value::T {
+        computed_value::T::${data.to_rust_ident(values[0])}
+    }
+    pub fn parse(_context: &ParserContext, input: &mut Parser)
+                 -> Result<SpecifiedValue, ()> {
+        match_ignore_ascii_case! { try!(input.expect_ident()),
+            % for value in values:
+                "${value}" => {
+                    % if value in experimental_values:
+                        if !::util::prefs::get_pref("layout.${value}.enabled")
+                            .as_boolean().unwrap_or(false) {
+                            return Err(())
+                        }
+                    % endif
+                    Ok(computed_value::T::${data.to_rust_ident(value)})
+                },
+            % endfor
+            _ => Err(())
+        }
+    }
+
+    impl ComputedValueAsSpecified for SpecifiedValue {}
+
+    % if product == "servo":
+        fn cascade_property_custom<C: ComputedValues>(
+                                   _declaration: &PropertyDeclaration,
+                                   _inherited_style: &C,
+                                   context: &mut computed::Context<C>,
+                                   _seen: &mut PropertyBitField,
+                                   _cacheable: &mut bool,
+                                   _error_reporter: &mut StdBox<ParseErrorReporter + Send>) {
+            longhands::_servo_display_for_hypothetical_box::derive_from_display(context);
+            longhands::_servo_text_decorations_in_effect::derive_from_display(context);
+        }
+    % endif
+
+</%helpers:longhand>
+
+${helpers.single_keyword("position", "static absolute relative fixed", extra_gecko_values="sticky")}
+
+<%helpers:single_keyword_computed name="float" values="none left right" gecko_ffi_name="mFloats">
+    impl ToComputedValue for SpecifiedValue {
+        type ComputedValue = computed_value::T;
+
+        #[inline]
+        fn to_computed_value<Cx: TContext>(&self, context: &Cx) -> computed_value::T {
+            let positioned = matches!(context.style().get_box().clone_position(),
+                longhands::position::SpecifiedValue::absolute |
+                longhands::position::SpecifiedValue::fixed);
+            if positioned {
+                SpecifiedValue::none
+            } else {
+                *self
+            }
+        }
+    }
+
+</%helpers:single_keyword_computed>
+
+${helpers.single_keyword("clear", "none left right both", gecko_ffi_name="mBreakType")}
+
+<%helpers:longhand name="-servo-display-for-hypothetical-box" derived_from="display" products="servo">
+    pub use super::display::{SpecifiedValue, get_initial_value};
+    pub use super::display::{parse};
+
+    pub mod computed_value {
+        pub type T = super::SpecifiedValue;
+    }
+
+    #[inline]
+    pub fn derive_from_display<Cx: TContext>(context: &mut Cx) {
+        let d = context.style().get_box().clone_display();
+        context.mutate_style().mutate_box().set__servo_display_for_hypothetical_box(d);
+    }
+
+</%helpers:longhand>