Auto merge of #17356 - servo:derive-all-the-things, r=SimonSapin

Minor improvements to #[derive(ToCss)]

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components/style_derive/Cargo.toml

@@ -11,5 +11,5 @@ proc-macro = true
 
 [dependencies]
 quote = "0.3"
-syn = "0.11"
+syn = { version = "0.11", features = ["visit"] }
 synstructure = "0.5.2"

components/style_derive/to_css.rs

@@ -17,25 +17,28 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     let style = synstructure::BindStyle::Ref.into();
     let match_body = synstructure::each_variant(&input, &style, |bindings, variant| {
         let mut identifier = to_css_identifier(variant.ident.as_ref());
-        let mut expr = if bindings.is_empty() {
-            quote! {
-                ::std::fmt::Write::write_str(dest, #identifier)
+        let mut expr = if let Some((first, rest)) = bindings.split_first() {
+            if has_free_params(&first.field.ty, &input.generics.ty_params) {
+                where_clause.predicates.push(where_predicate(first.field.ty.clone()));
             }
-        } else {
-            let (first, rest) = bindings.split_first().expect("unit variants are not yet supported");
-            where_clause.predicates.push(where_predicate(first.field.ty.clone()));
             let mut expr = quote! {
                 ::style_traits::ToCss::to_css(#first, dest)
             };
             for binding in rest {
-                where_clause.predicates.push(where_predicate(binding.field.ty.clone()));
+                if has_free_params(&binding.field.ty, &input.generics.ty_params) {
+                    where_clause.predicates.push(where_predicate(binding.field.ty.clone()));
+                }
                 expr = quote! {
                     #expr?;
                     ::std::fmt::Write::write_str(dest, " ")?;
                     ::style_traits::ToCss::to_css(#binding, dest)
                 };
             }
             expr
+        } else {
+            quote! {
+                ::std::fmt::Write::write_str(dest, #identifier)
+            }
         };
         let mut css_attrs = variant.attrs.iter().filter(|attr| attr.name() == "css");
         let is_function = css_attrs.next().map_or(false, |attr| {
@@ -91,6 +94,31 @@ pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
     }
 }
 
+/// Returns whether `ty` is parameterized by any parameter from `params`.
+fn has_free_params(ty: &syn::Ty, params: &[syn::TyParam]) -> bool {
+    use syn::visit::Visitor;
+
+    struct HasFreeParams<'a> {
+        params: &'a [syn::TyParam],
+        has_free: bool,
+    }
+
+    impl<'a> Visitor for HasFreeParams<'a> {
+        fn visit_path(&mut self, path: &syn::Path) {
+            if !path.global && path.segments.len() == 1 {
+                if self.params.iter().any(|param| param.ident == path.segments[0].ident) {
+                    self.has_free = true;
+                }
+            }
+            syn::visit::walk_path(self, path);
+        }
+    }
+
+    let mut visitor = HasFreeParams { params: params, has_free: false };
+    visitor.visit_ty(ty);
+    visitor.has_free
+}
+
 /// `#ty: ::style_traits::ToCss`
 fn where_predicate(ty: syn::Ty) -> syn::WherePredicate {
     syn::WherePredicate::BoundPredicate(syn::WhereBoundPredicate {