1 files changed, 396 insertions, 0 deletions
diff --git a/servo/components/style_derive/to_css.rs b/servo/components/style_derive/to_css.rs
new file mode 100644
index 0000000000..aa33536648
--- /dev/null
+++ b/servo/components/style_derive/to_css.rs
@@ -0,0 +1,396 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use darling::util::Override;
+use derive_common::cg;
+use proc_macro2::{Span, TokenStream};
+use quote::{ToTokens, TokenStreamExt};
+use syn::{self, Data, Ident, Path, WhereClause};
+use synstructure::{BindingInfo, Structure, VariantInfo};
+
+fn derive_bitflags(input: &syn::DeriveInput, bitflags: &CssBitflagAttrs) -> TokenStream {
+    let name = &input.ident;
+    let mut body = TokenStream::new();
+    for (rust_name, css_name) in bitflags.single_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        body.append_all(quote! {
+            if *self == Self::#rust_ident {
+                return dest.write_str(#css_name);
+            }
+        });
+    }
+
+    body.append_all(quote! {
+        let mut has_any = false;
+    });
+
+    if bitflags.overlapping_bits {
+        body.append_all(quote! {
+            let mut serialized = Self::empty();
+        });
+    }
+
+    for (rust_name, css_name) in bitflags.mixed_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        let serialize = quote! {
+            if has_any {
+                dest.write_char(' ')?;
+            }
+            has_any = true;
+            dest.write_str(#css_name)?;
+        };
+        if bitflags.overlapping_bits {
+            body.append_all(quote! {
+                if self.contains(Self::#rust_ident) && !serialized.intersects(Self::#rust_ident) {
+                    #serialize
+                    serialized.insert(Self::#rust_ident);
+                }
+            });
+        } else {
+            body.append_all(quote! {
+                if self.intersects(Self::#rust_ident) {
+                    #serialize
+                }
+            });
+        }
+    }
+
+    body.append_all(quote! {
+        Ok(())
+    });
+
+    quote! {
+        impl style_traits::ToCss for #name {
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_css<W>(
+                &self,
+                dest: &mut style_traits::CssWriter<W>,
+            ) -> std::fmt::Result
+            where
+                W: std::fmt::Write,
+            {
+                #body
+            }
+        }
+    }
+}
+
+pub fn derive(mut input: syn::DeriveInput) -> TokenStream {
+    let mut where_clause = input.generics.where_clause.take();
+    for param in input.generics.type_params() {
+        cg::add_predicate(&mut where_clause, parse_quote!(#param: style_traits::ToCss));
+    }
+
+    let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
+    if matches!(input.data, Data::Enum(..)) || input_attrs.bitflags.is_some() {
+        assert!(
+            input_attrs.function.is_none(),
+            "#[css(function)] is not allowed on enums or bitflags"
+        );
+        assert!(
+            !input_attrs.comma,
+            "#[css(comma)] is not allowed on enums or bitflags"
+        );
+    }
+
+    if let Some(ref bitflags) = input_attrs.bitflags {
+        assert!(
+            !input_attrs.derive_debug,
+            "Bitflags can derive debug on their own"
+        );
+        assert!(where_clause.is_none(), "Generic bitflags?");
+        return derive_bitflags(&input, bitflags);
+    }
+
+    let match_body = {
+        let s = Structure::new(&input);
+        s.each_variant(|variant| derive_variant_arm(variant, &mut where_clause))
+    };
+    input.generics.where_clause = where_clause;
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let mut impls = quote! {
+        impl #impl_generics style_traits::ToCss for #name #ty_generics #where_clause {
+            #[allow(unused_variables)]
+            #[inline]
+            fn to_css<W>(
+                &self,
+                dest: &mut style_traits::CssWriter<W>,
+            ) -> std::fmt::Result
+            where
+                W: std::fmt::Write,
+            {
+                match *self {
+                    #match_body
+                }
+            }
+        }
+    };
+
+    if input_attrs.derive_debug {
+        impls.append_all(quote! {
+            impl #impl_generics std::fmt::Debug for #name #ty_generics #where_clause {
+                fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+                    style_traits::ToCss::to_css(
+                        self,
+                        &mut style_traits::CssWriter::new(f),
+                    )
+                }
+            }
+        });
+    }
+
+    impls
+}
+
+fn derive_variant_arm(variant: &VariantInfo, generics: &mut Option<WhereClause>) -> TokenStream {
+    let bindings = variant.bindings();
+    let identifier = cg::to_css_identifier(&variant.ast().ident.to_string());
+    let ast = variant.ast();
+    let variant_attrs = cg::parse_variant_attrs_from_ast::<CssVariantAttrs>(&ast);
+    let separator = if variant_attrs.comma { ", " } else { " " };
+
+    if variant_attrs.skip {
+        return quote!(Ok(()));
+    }
+    if variant_attrs.dimension {
+        assert_eq!(bindings.len(), 1);
+        assert!(
+            variant_attrs.function.is_none() && variant_attrs.keyword.is_none(),
+            "That makes no sense"
+        );
+    }
+
+    let mut expr = if let Some(keyword) = variant_attrs.keyword {
+        assert!(bindings.is_empty());
+        quote! {
+            std::fmt::Write::write_str(dest, #keyword)
+        }
+    } else if !bindings.is_empty() {
+        derive_variant_fields_expr(bindings, generics, separator)
+    } else {
+        quote! {
+            std::fmt::Write::write_str(dest, #identifier)
+        }
+    };
+
+    if variant_attrs.dimension {
+        expr = quote! {
+            #expr?;
+            std::fmt::Write::write_str(dest, #identifier)
+        }
+    } else if let Some(function) = variant_attrs.function {
+        let mut identifier = function.explicit().map_or(identifier, |name| name);
+        identifier.push('(');
+        expr = quote! {
+            std::fmt::Write::write_str(dest, #identifier)?;
+            #expr?;
+            std::fmt::Write::write_str(dest, ")")
+        }
+    }
+    expr
+}
+
+fn derive_variant_fields_expr(
+    bindings: &[BindingInfo],
+    where_clause: &mut Option<WhereClause>,
+    separator: &str,
+) -> TokenStream {
+    let mut iter = bindings
+        .iter()
+        .filter_map(|binding| {
+            let attrs = cg::parse_field_attrs::<CssFieldAttrs>(&binding.ast());
+            if attrs.skip {
+                return None;
+            }
+            Some((binding, attrs))
+        })
+        .peekable();
+
+    let (first, attrs) = match iter.next() {
+        Some(pair) => pair,
+        None => return quote! { Ok(()) },
+    };
+    if attrs.field_bound {
+        let ty = &first.ast().ty;
+        // TODO(emilio): IntoIterator might not be enough for every type of
+        // iterable thing (like ArcSlice<> or what not). We might want to expose
+        // an `item = "T"` attribute to handle that in the future.
+        let predicate = if attrs.iterable {
+            parse_quote!(<#ty as IntoIterator>::Item: style_traits::ToCss)
+        } else {
+            parse_quote!(#ty: style_traits::ToCss)
+        };
+        cg::add_predicate(where_clause, predicate);
+    }
+    if !attrs.iterable && iter.peek().is_none() {
+        let mut expr = quote! { style_traits::ToCss::to_css(#first, dest) };
+        if let Some(condition) = attrs.skip_if {
+            expr = quote! {
+                if !#condition(#first) {
+                    #expr
+                }
+            }
+        }
+
+        if let Some(condition) = attrs.contextual_skip_if {
+            expr = quote! {
+                if !#condition(#(#bindings), *) {
+                    #expr
+                }
+            }
+        }
+        return expr;
+    }
+
+    let mut expr = derive_single_field_expr(first, attrs, where_clause, bindings);
+    for (binding, attrs) in iter {
+        derive_single_field_expr(binding, attrs, where_clause, bindings).to_tokens(&mut expr)
+    }
+
+    quote! {{
+        let mut writer = style_traits::values::SequenceWriter::new(dest, #separator);
+        #expr
+        Ok(())
+    }}
+}
+
+fn derive_single_field_expr(
+    field: &BindingInfo,
+    attrs: CssFieldAttrs,
+    where_clause: &mut Option<WhereClause>,
+    bindings: &[BindingInfo],
+) -> TokenStream {
+    let mut expr = 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.raw_item(#if_empty)?;
+                    } else {
+                        for item in iter {
+                            writer.item(&item)?;
+                        }
+                    }
+                }
+            };
+        }
+        quote! {
+            for item in #field.iter() {
+                writer.item(&item)?;
+            }
+        }
+    } else if attrs.represents_keyword {
+        let ident = field
+            .ast()
+            .ident
+            .as_ref()
+            .expect("Unnamed field with represents_keyword?");
+        let ident = cg::to_css_identifier(&ident.to_string()).replace("_", "-");
+        quote! {
+            if *#field {
+                writer.raw_item(#ident)?;
+            }
+        }
+    } else {
+        if attrs.field_bound {
+            let ty = &field.ast().ty;
+            cg::add_predicate(where_clause, parse_quote!(#ty: style_traits::ToCss));
+        }
+        quote! { writer.item(#field)?; }
+    };
+
+    if let Some(condition) = attrs.skip_if {
+        expr = quote! {
+            if !#condition(#field) {
+                #expr
+            }
+        }
+    }
+
+    if let Some(condition) = attrs.contextual_skip_if {
+        expr = quote! {
+            if !#condition(#(#bindings), *) {
+                #expr
+            }
+        }
+    }
+
+    expr
+}
+
+#[derive(Default, FromMeta)]
+#[darling(default)]
+pub struct CssBitflagAttrs {
+    /// Flags that can only go on their own, comma-separated.
+    pub single: Option<String>,
+    /// Flags that can go mixed with each other, comma-separated.
+    pub mixed: Option<String>,
+    /// Extra validation of the resulting mixed flags.
+    pub validate_mixed: Option<Path>,
+    /// Whether there are overlapping bits we need to take care of when
+    /// serializing.
+    pub overlapping_bits: bool,
+}
+
+impl CssBitflagAttrs {
+    /// Returns a vector of (rust_name, css_name) of a given flag list.
+    fn names(s: &Option<String>) -> Vec<(String, String)> {
+        let s = match s {
+            Some(s) => s,
+            None => return vec![],
+        };
+        s.split(',')
+            .map(|css_name| (cg::to_scream_case(css_name), css_name.to_owned()))
+            .collect()
+    }
+
+    pub fn single_flags(&self) -> Vec<(String, String)> {
+        Self::names(&self.single)
+    }
+
+    pub fn mixed_flags(&self) -> Vec<(String, String)> {
+        Self::names(&self.mixed)
+    }
+}
+
+#[derive(Default, FromDeriveInput)]
+#[darling(attributes(css), default)]
+pub struct CssInputAttrs {
+    pub derive_debug: bool,
+    // Here because structs variants are also their whole type definition.
+    pub function: Option<Override<String>>,
+    // Here because structs variants are also their whole type definition.
+    pub comma: bool,
+    pub bitflags: Option<CssBitflagAttrs>,
+}
+
+#[derive(Default, FromVariant)]
+#[darling(attributes(css), default)]
+pub struct CssVariantAttrs {
+    pub function: Option<Override<String>>,
+    // Here because structs variants are also their whole type definition.
+    pub derive_debug: bool,
+    pub comma: bool,
+    pub bitflags: Option<CssBitflagAttrs>,
+    pub dimension: bool,
+    pub keyword: Option<String>,
+    pub skip: bool,
+}
+
+#[derive(Default, FromField)]
+#[darling(attributes(css), default)]
+pub struct CssFieldAttrs {
+    pub if_empty: Option<String>,
+    pub field_bound: bool,
+    pub iterable: bool,
+    pub skip: bool,
+    pub represents_keyword: bool,
+    pub contextual_skip_if: Option<Path>,
+    pub skip_if: Option<Path>,
+}