Introduce #[css(if_empty = "…", iterable)]

components/style/properties/helpers.mako.rs

@@ -83,10 +83,6 @@
                           need_animatable=need_animatable, **kwargs)">
         #[allow(unused_imports)]
         use smallvec::SmallVec;
-        % if allow_empty:
-        use std::fmt::{self, Write};
-        use style_traits::{CssWriter, Separator, ToCss};
-        % endif
 
         pub mod single_value {
             #[allow(unused_imports)]
@@ -120,23 +116,22 @@
             use values::computed::ComputedVecIter;
 
             /// The computed value, effectively a list of single values.
-            #[derive(Clone, Debug, MallocSizeOf, PartialEq)]
-            % if need_animatable or animation_value_type == "ComputedValue":
-            #[derive(Animate, ComputeSquaredDistance)]
-            % endif
-            % if not allow_empty:
             % if separator == "Comma":
             #[css(comma)]
             % endif
-            #[derive(ToCss)]
+            #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
+            % if need_animatable or animation_value_type == "ComputedValue":
+            #[derive(Animate, ComputeSquaredDistance)]
             % endif
             pub struct T(
-                % if allow_empty and allow_empty != "NotInitial":
-                pub Vec<single_value::T>,
-                % else:
                 % if not allow_empty:
                 #[css(iterable)]
+                % else:
+                #[css(if_empty = "none", iterable)]
                 % endif
+                % if allow_empty and allow_empty != "NotInitial":
+                pub Vec<single_value::T>,
+                % else:
                 pub SmallVec<[single_value::T; 1]>,
                 % endif
             );
@@ -165,63 +160,20 @@
             }
         }
 
-        % if allow_empty:
-        impl ToCss for computed_value::T {
-            fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
-            where
-                W: Write,
-            {
-                let mut iter = self.0.iter();
-                if let Some(val) = iter.next() {
-                    val.to_css(dest)?;
-                } else {
-                    return dest.write_str("none");
-                }
-                for i in iter {
-                    dest.write_str(::style_traits::${separator}::separator())?;
-                    i.to_css(dest)?;
-                }
-                Ok(())
-            }
-        }
-        % endif
-
         /// The specified value of ${name}.
-        #[derive(Clone, Debug, MallocSizeOf, PartialEq)]
-        % if not allow_empty:
         % if separator == "Comma":
         #[css(comma)]
         % endif
-        #[derive(ToCss)]
-        % endif
+        #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
         pub struct SpecifiedValue(
             % if not allow_empty:
             #[css(iterable)]
+            % else:
+            #[css(if_empty = "none", iterable)]
             % endif
             pub Vec<single_value::SpecifiedValue>,
         );
 
-        % if allow_empty:
-        impl ToCss for SpecifiedValue {
-            fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
-            where
-                W: Write,
-            {
-                let mut iter = self.0.iter();
-                if let Some(val) = iter.next() {
-                    val.to_css(dest)?;
-                } else {
-                    return dest.write_str("none");
-                }
-                for i in iter {
-                    dest.write_str(::style_traits::${separator}::separator())?;
-                    i.to_css(dest)?;
-                }
-                Ok(())
-            }
-        }
-        % endif
-
         pub fn get_initial_value() -> computed_value::T {
             % if allow_empty and allow_empty != "NotInitial":
                 computed_value::T(vec![])

components/style/values/specified/font.rs

@@ -736,9 +736,12 @@ pub enum VariantAlternates {
     HistoricalForms,
 }
 
-#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 /// List of Variant Alternates
-pub struct VariantAlternatesList(pub Box<[VariantAlternates]>);
+pub struct VariantAlternatesList(
+    #[css(if_empty = "normal", iterable)]
+    pub Box<[VariantAlternates]>,
+);
 
 impl VariantAlternatesList {
     /// Returns the length of all variant alternates.
@@ -759,25 +762,6 @@ impl VariantAlternatesList {
     }
 }
 
-impl ToCss for VariantAlternatesList {
-    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
-    where
-        W: Write,
-    {
-        if self.0.is_empty() {
-            return dest.write_str("normal");
-        }
-
-        let mut iter = self.0.iter();
-        iter.next().unwrap().to_css(dest)?;
-        for alternate in iter {
-            dest.write_str(" ")?;
-            alternate.to_css(dest)?;
-        }
-        Ok(())
-    }
-}
-
 #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 /// Control over the selection of these alternate glyphs
 pub enum FontVariantAlternates {

components/style_derive/to_css.rs

@@ -150,6 +150,20 @@ fn derive_single_field_expr(
     where_clause: &mut WhereClause,
 ) -> Tokens {
     if attrs.iterable {
+        if let Some(if_empty) = attrs.if_empty {
+            return quote! {
+                {
+                    let mut iter = #field.iter().peekable();
+                    if iter.peek().is_none() {
+                        writer.item(&::style_traits::values::Verbatim(#if_empty))?;
+                    } else {
+                        for item in iter {
+                            writer.item(&item)?;
+                        }
+                    }
+                }
+            };
+        }
         quote! {
             for item in #field.iter() {
                 writer.item(&item)?;
@@ -186,6 +200,7 @@ pub struct CssVariantAttrs {
 #[darling(attributes(css), default)]
 #[derive(Default, FromField)]
 struct CssFieldAttrs {
+    if_empty: Option<String>,
     ignore_bound: bool,
     iterable: bool,
     skip: bool,

components/style_traits/values.rs

@@ -27,6 +27,8 @@ use std::fmt::{self, Write};
 /// * if `#[css(iterable)]` is found on a function variant, that variant needs
 ///   to have a single member, and that member needs to be iterable. The
 ///   iterable will be serialized as the arguments for the function;
+/// * an iterable field can also be annotated with `#[css(if_empty = "foo")]`
+///   to print `"foo"` if the iterator is empty;
 /// * if `#[css(dimension)]` is found on a variant, that variant needs
 ///   to have a single member. The variant would be serialized as a CSS
 ///   dimension token, like: <member><identifier>;
@@ -210,6 +212,21 @@ where
     }
 }
 
+/// A wrapper type that implements `ToCss` by printing its inner field.
+pub struct Verbatim<'a, T>(pub &'a T)
+where
+    T: ?Sized + 'a;
+
+impl<'a, T> ToCss for Verbatim<'a, T>
+where
+    T: AsRef<str> + ?Sized + 'a,
+{
+    #[inline]
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: Write {
+        dest.write_str(self.0.as_ref())
+    }
+}
+
 /// Type used as the associated type in the `OneOrMoreSeparated` trait on a
 /// type to indicate that a serialized list of elements of this type is
 /// separated by commas.