style: Allow to derive Parse/ToCss/SpecifiedValueInfo on bitflags

We keep getting this pattern of properties that have a set of joint and
disjoint flags, and copy-pasting or writing the same parsing and
serialization code in slightly different ways.

container-type is one such type, and I think we should have a single way
of dealing with this, thus implement deriving for various traits for
bitflags, with an attribute that says which flags are single vs mixed.

See docs and properties I ported. The remaining ones I left TODOs with,
they are a bit trickier but can be ported with some care.

Differential Revision: https://phabricator.services.mozilla.com/D142418

components/style_derive/parse.rs

@@ -2,14 +2,14 @@
  * 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 crate::to_css::CssVariantAttrs;
+use crate::to_css::{CssBitflagAttrs, CssVariantAttrs};
 use darling::FromField;
 use darling::FromVariant;
 use derive_common::cg;
-use proc_macro2::TokenStream;
-use quote::quote;
+use proc_macro2::{TokenStream, Span};
+use quote::{quote, TokenStreamExt};
 use syn::parse_quote;
-use syn::{self, DeriveInput, Path};
+use syn::{self, Ident, DeriveInput, Path};
 use synstructure::{Structure, VariantInfo};
 
 #[derive(Default, FromVariant)]
@@ -25,6 +25,70 @@ pub struct ParseFieldAttrs {
     field_bound: bool,
 }
 
+fn parse_bitflags(bitflags: &CssBitflagAttrs) -> TokenStream {
+    let mut match_arms = TokenStream::new();
+    for (rust_name, css_name) in bitflags.single_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        match_arms.append_all(quote! {
+            #css_name if result.is_empty() => {
+                single_flag = true;
+                Self::#rust_ident
+            },
+        });
+    }
+
+    for (rust_name, css_name) in bitflags.mixed_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        match_arms.append_all(quote! {
+            #css_name => Self::#rust_ident,
+        });
+    }
+
+    let mut validate_condition = quote! { !result.is_empty() };
+    if let Some(ref function) = bitflags.validate_mixed {
+        validate_condition.append_all(quote! {
+            && #function(result)
+        });
+    }
+
+    // NOTE(emilio): this loop has this weird structure because we run this code
+    // to parse stuff like text-decoration-line in the text-decoration
+    // shorthand, so we need to be a bit careful that we don't error if we don't
+    // consume the whole thing because we find an invalid identifier or other
+    // kind of token. Instead, we should leave it unconsumed.
+    quote! {
+        let mut result = Self::empty();
+        loop {
+            let mut single_flag = false;
+            let flag: Result<_, style_traits::ParseError<'i>> = input.try_parse(|input| {
+                Ok(try_match_ident_ignore_ascii_case! { input,
+                    #match_arms
+                })
+            });
+
+            let flag = match flag {
+                Ok(flag) => flag,
+                Err(..) => break,
+            };
+
+            if single_flag {
+                return Ok(flag);
+            }
+
+            if result.intersects(flag) {
+                return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+            }
+
+            result.insert(flag);
+        }
+        if #validate_condition {
+            Ok(result)
+        } else {
+            Err(input.new_custom_error(style_traits::StyleParseErrorKind::UnspecifiedError))
+        }
+    }
+}
+
 fn parse_non_keyword_variant(
     where_clause: &mut Option<syn::WhereClause>,
     name: &syn::Ident,
@@ -46,6 +110,13 @@ fn parse_non_keyword_variant(
     let binding_ast = &bindings[0].ast();
     let ty = &binding_ast.ty;
 
+    if let Some(ref bitflags) = variant_attrs.bitflags {
+        assert!(skip_try, "Should be the only variant");
+        assert!(parse_attrs.condition.is_none(), "Should be the only variant");
+        assert!(where_clause.is_none(), "Generic bitflags?");
+        return parse_bitflags(bitflags)
+    }
+
     let field_attrs = cg::parse_field_attrs::<ParseFieldAttrs>(binding_ast);
     if field_attrs.field_bound {
         cg::add_predicate(where_clause, parse_quote!(#ty: crate::parser::Parse));

components/style_derive/specified_value_info.rs

@@ -71,7 +71,14 @@ pub fn derive(mut input: DeriveInput) -> TokenStream {
                 }
             },
             Data::Struct(ref s) => {
-                if !derive_struct_fields(&s.fields, &mut types, &mut values) {
+                if let Some(ref bitflags) = css_attrs.bitflags {
+                    for (_rust_name, css_name) in bitflags.single_flags() {
+                        values.push(css_name)
+                    }
+                    for (_rust_name, css_name) in bitflags.mixed_flags() {
+                        values.push(css_name)
+                    }
+                } else if !derive_struct_fields(&s.fields, &mut types, &mut values) {
                     values.push(input_name());
                 }
             },

components/style_derive/to_css.rs

@@ -7,13 +7,64 @@ use darling::FromDeriveInput;
 use darling::FromField;
 use darling::FromVariant;
 use derive_common::cg;
-use proc_macro2::TokenStream;
+use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use quote::{ToTokens, TokenStreamExt};
 use syn::parse_quote;
-use syn::{self, Data, Path, WhereClause};
+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;
+    });
+
+    for (rust_name, css_name) in bitflags.mixed_flags() {
+        let rust_ident = Ident::new(&rust_name, Span::call_site());
+        body.append_all(quote! {
+            if self.intersects(Self::#rust_ident) {
+                if has_any {
+                    dest.write_char(' ')?;
+                }
+                has_any = true;
+                dest.write_str(#css_name)?;
+            }
+        });
+    }
+
+    body.append_all(quote! {
+        debug_assert!(has_any, "Shouldn't have parsed empty");
+        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() {
@@ -21,12 +72,21 @@ pub fn derive(mut input: syn::DeriveInput) -> TokenStream {
     }
 
     let input_attrs = cg::parse_input_attrs::<CssInputAttrs>(&input);
-    if let Data::Enum(_) = input.data {
+    if matches!(input.data, Data::Enum(..)) || input_attrs.bitflags.is_some() {
         assert!(
             input_attrs.function.is_none(),
-            "#[css(function)] is not allowed on enums"
+            "#[css(function)] is not allowed on enums or bitflags"
         );
-        assert!(!input_attrs.comma, "#[css(comma)] is not allowed on enums");
+        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 = {
@@ -249,6 +309,32 @@ fn derive_single_field_expr(
     expr
 }
 
+#[derive(Default, FromMeta)]
+pub struct CssBitflagAttrs {
+    /// Flags that can only go on their own, comma-separated.
+    pub single: String,
+    /// Flags that can go mixed with each other, comma-separated.
+    pub mixed: String,
+    /// Extra validation of the resulting mixed flags.
+    #[darling(default)]
+    pub validate_mixed: Option<Path>,
+}
+
+impl CssBitflagAttrs {
+    /// Returns a vector of (rust_name, css_name) of a given flag list.
+    fn names(s: &str) -> Vec<(String, String)> {
+        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 {
@@ -257,6 +343,7 @@ pub struct CssInputAttrs {
     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)]
@@ -266,6 +353,7 @@ pub struct CssVariantAttrs {
     // 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,