Add support for optional attributes on HTML elements/tags

docs/concepts/html/elements.md

@@ -89,6 +89,20 @@ html! {
 ```
 <!--END_DOCUSAURUS_CODE_TABS-->
 
+## Optional attributes for HTML elements
+
+Most HTML attributes can be marked as optional by placing a `?` in front of
+the `=` sign. This makes them accept the same type of value as otherwise, but
+wrapped in an `Option<T>`:
+```rust
+let maybe_id = Some("foobar");
+
+html! {
+    <div id?=maybe_id></div>
+}
+```
+If the attribute is set to `None`, it will behave as though it wasn't set.
+
 ## Classes
 
 There are a number of convenient ways to specify classes for an element:

yew-macro/src/html_tree/html_component.rs

@@ -117,7 +117,7 @@ impl ToTokens for HtmlComponent {
 
         let init_props = match &props.prop_type {
             PropType::List(list_props) => {
-                let set_props = list_props.iter().map(|HtmlProp { label, value }| {
+                let set_props = list_props.iter().map(|HtmlProp { label, value, .. }| {
                     quote_spanned! { value.span()=> .#label(
                         <::yew::virtual_dom::VComp as ::yew::virtual_dom::Transformer<_, _>>::transform(
                             #value
@@ -405,6 +405,13 @@ impl Parse for Props {
                 return Err(syn::Error::new_spanned(&prop.label, "expected identifier"));
             }
 
+            if prop.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &prop.label,
+                    "optional attributes are only supported on HTML tags. Yew components can use `Option<T>` properties to accomplish the same thing.",
+                ));
+            }
+
             match props.prop_type {
                 ref mut prop_type @ PropType::None => {
                     *prop_type = PropType::List(vec![prop]);

yew-macro/src/html_tree/html_dashed_name.rs

@@ -81,6 +81,6 @@ impl ToTokens for HtmlDashedName {
         let dashes = extended.iter().map(|(dash, _)| quote! {#dash});
         let idents = extended.iter().map(|(_, ident)| quote! {#ident});
         let extended = quote! { #(#dashes#idents)* };
-        tokens.extend(quote! {#name#extended});
+        tokens.extend(quote! { #name#extended });
     }
 }

yew-macro/src/html_tree/html_list.rs

@@ -92,7 +92,7 @@ impl PeekValue<()> for HtmlListOpen {
         // make sure it's either a property (key=value) or it's immediately closed
         if let Some((_, cursor)) = HtmlDashedName::peek(cursor) {
             let (punct, _) = cursor.punct()?;
-            (punct.as_char() == '=').as_option()
+            (punct.as_char() == '=' || punct.as_char() == '?').as_option()
         } else {
             let (punct, _) = cursor.punct()?;
             (punct.as_char() == '>').as_option()
@@ -136,6 +136,13 @@ impl Parse for HtmlListProps {
                 ));
             }
 
+            if prop.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    prop.label,
+                    "the 'key' attribute does not support being used as an optional attribute",
+                ));
+            }
+
             Some(prop.value)
         };
 

yew-macro/src/html_tree/html_prop.rs

@@ -7,6 +7,7 @@ use syn::{Expr, Token};
 
 pub struct HtmlProp {
     pub label: HtmlPropLabel,
+    pub question_mark: Option<Token![?]>,
     pub value: Expr,
 }
 
@@ -19,6 +20,11 @@ impl PeekValue<()> for HtmlProp {
 impl Parse for HtmlProp {
     fn parse(input: ParseStream) -> ParseResult<Self> {
         let label = input.parse::<HtmlPropLabel>()?;
+        let question_mark = if input.peek(Token![?]) {
+            Some(input.parse()?)
+        } else {
+            None
+        };
         let equals = input
             .parse::<Token![=]>()
             .map_err(|_| syn::Error::new_spanned(&label, "this prop doesn't have a value"))?;
@@ -31,7 +37,11 @@ impl Parse for HtmlProp {
         let value = input.parse::<Expr>()?;
         // backwards compat
         let _ = input.parse::<Token![,]>();
-        Ok(HtmlProp { label, value })
+        Ok(Self {
+            label,
+            question_mark,
+            value,
+        })
     }
 }
 

yew-macro/src/html_tree/html_tag/mod.rs

@@ -136,32 +136,84 @@ impl ToTokens for HtmlTag {
         } = &attributes;
 
         let vtag = Ident::new("__yew_vtag", tag_name.span());
-        let attr_pairs = attributes.iter().map(|TagAttribute { label, value }| {
+        let attr_pairs = attributes.iter().map(|TagAttribute { label, question_mark, value }| {
             let label_str = label.to_string();
-            quote_spanned! {value.span()=> (#label_str.to_owned(), (#value).to_string()) }
+            if question_mark.is_some() {
+                quote_spanned! {value.span()=>
+                    {
+                        let __yew_value = ::std::option::Option::as_ref(&(#value)).map(::std::string::ToString::to_string);
+                        (::std::string::String::from(#label_str), __yew_value)
+                    }
+                }
+            } else {
+                quote_spanned! {value.span()=>
+                    {
+                        (::std::string::String::from(#label_str), ::std::option::Option::Some(::std::string::ToString::to_string(&#value)))
+                    }
+                }
+            }
         });
-        let set_booleans = booleans.iter().map(|TagAttribute { label, value }| {
+        let set_booleans = booleans.iter().map(|TagAttribute { label, value, .. }| {
             let label_str = label.to_string();
             quote_spanned! {value.span()=>
-                if #value {
-                    #vtag.add_attribute(&#label_str, &#label_str);
+                {
+                    if #value {
+                        #vtag.add_attribute(#label_str, &#label_str);
+                    }
                 }
             }
         });
         let set_kind = kind.iter().map(|kind| {
-            quote_spanned! {kind.span()=> #vtag.set_kind(&(#kind)); }
+            let value = &kind.value;
+            if kind.question_mark.is_some() {
+                quote_spanned! {value.span()=>
+                    {
+                        if let ::std::option::Option::Some(__yew_kind) = ::std::option::Option::as_ref(&(#value)) {
+                            #vtag.set_kind(__yew_kind)
+                        }
+                    }
+                }
+            } else {
+                quote_spanned! {value.span()=> #vtag.set_kind(&(#value)); }
+            }
         });
         let set_value = value.iter().map(|value| {
-            quote_spanned! {value.span()=> #vtag.set_value(&(#value)); }
+            let value_value = &value.value;
+            if value.question_mark.is_some() {
+                quote_spanned! {value_value.span()=>
+                    {
+                        if let ::std::option::Option::Some(__yew_value) = ::std::option::Option::as_ref(&(#value_value)) {
+                            #vtag.set_value(__yew_value);
+                        }
+                    }
+                }
+            } else {
+                quote_spanned! {value_value.span()=> #vtag.set_value(&(#value_value)); }
+            }
         });
         let add_href = href.iter().map(|href| {
-            quote_spanned! {href.span()=>
-                let __yew_href: ::yew::html::Href = (#href).into();
-                #vtag.add_attribute("href", &__yew_href);
+            let value = &href.value;
+            if href.question_mark.is_some() {
+                quote_spanned! {value.span()=>
+                    {
+                        let __yew_href = ::std::option::Option::map(#value, ::yew::html::Href::from);
+                        if let ::std::option::Option::Some(__yew_href) = __yew_href {
+                            #vtag.add_attribute("href", &__yew_href);
+                        }
+                    }
+                }
+            } else {
+                quote_spanned! {value.span()=>
+                    {
+                        let __yew_href = ::yew::html::Href::from(#value);
+                        #vtag.add_attribute("href", &__yew_href);
+                    }
+                }
             }
         });
         let set_checked = checked.iter().map(|checked| {
-            quote_spanned! {checked.span()=> #vtag.set_checked(#checked); }
+            let value = &checked.value;
+            quote_spanned! {value.span()=> #vtag.set_checked(#value); }
         });
         let set_classes = classes.iter().map(|classes_form| match classes_form {
             ClassesForm::Tuple(classes) => quote! {
@@ -171,7 +223,7 @@ impl ToTokens for HtmlTag {
                 }
             },
             ClassesForm::Single(classes) => quote! {
-                let __yew_classes = ::std::convert::Into::<::yew::virtual_dom::Classes>::into(#classes);
+                let __yew_classes = ::yew::virtual_dom::Classes::from(#classes);
                 if !__yew_classes.is_empty() {
                     #vtag.add_attribute("class", &__yew_classes);
                 }
@@ -191,13 +243,27 @@ impl ToTokens for HtmlTag {
             let name = &listener.label.name;
             let callback = &listener.value;
 
-            quote_spanned! {name.span()=> {
-                ::yew::html::#name::Wrapper::new(
-                    <::yew::virtual_dom::VTag as ::yew::virtual_dom::Transformer<_, _>>::transform(
-                        #callback
-                    )
-                )
-            }}
+            if listener.question_mark.is_some() {
+                quote_spanned! {name.span()=>
+                    {
+                        ::std::option::Option::map(#callback, |__yew_callback| {
+                            ::yew::html::#name::Wrapper::new(
+                                <::yew::virtual_dom::VTag as ::yew::virtual_dom::Transformer<_, _>>::transform(
+                                    __yew_callback,
+                                ),
+                            )
+                        })
+                    }
+                }
+            } else {
+                quote_spanned! {name.span()=> {
+                    ::std::option::Option::Some(::yew::html::#name::Wrapper::new(
+                        <::yew::virtual_dom::VTag as ::yew::virtual_dom::Transformer<_, _>>::transform(
+                            #callback
+                        )
+                    ))
+                }}
+            }
         });
 
         // These are the runtime-checks exclusive to dynamic tags.
@@ -241,8 +307,20 @@ impl ToTokens for HtmlTag {
             #(#set_classes)*
             #(#set_node_ref)*
             #(#set_key)*
-            #vtag.add_attributes(vec![#(#attr_pairs),*]);
-            #vtag.add_listeners(vec![#(::std::rc::Rc::new(#listeners)),*]);
+            #vtag.add_attributes({
+                let mut attributes = vec![];
+                #(if let (l, Some(v)) = #attr_pairs {
+                    attributes.push((l, v));
+                })*
+                attributes
+            });
+            #vtag.add_listeners({
+                let mut listeners: ::std::vec::Vec<::std::rc::Rc<dyn ::yew::virtual_dom::Listener>> = vec![];
+                #(if let Some(l) = #listeners {
+                    listeners.push(::std::rc::Rc::new(l));
+                })*
+                listeners
+            });
             #vtag.add_children(#children);
             #dyn_tag_runtime_checks
             ::yew::virtual_dom::VNode::from(#vtag)
@@ -383,10 +461,11 @@ impl Parse for HtmlTagOpen {
                 match name.to_ascii_lowercase_string().as_str() {
                     "input" | "textarea" => {}
                     _ => {
-                        if let Some(value) = attributes.value.take() {
+                        if let Some(attribute) = attributes.value.take() {
                             attributes.attributes.push(TagAttribute {
                                 label: HtmlDashedName::new(Ident::new("value", Span::call_site())),
-                                value,
+                                question_mark: attribute.question_mark,
+                                value: attribute.value,
                             });
                         }
                     }

yew-macro/src/html_tree/html_tag/tag_attributes.rs

@@ -11,12 +11,12 @@ pub struct TagAttributes {
     pub listeners: Vec<TagAttribute>,
     pub classes: Option<ClassesForm>,
     pub booleans: Vec<TagAttribute>,
-    pub value: Option<Expr>,
-    pub kind: Option<Expr>,
-    pub checked: Option<Expr>,
+    pub value: Option<TagAttribute>,
+    pub kind: Option<TagAttribute>,
+    pub checked: Option<TagAttribute>,
     pub node_ref: Option<Expr>,
     pub key: Option<Expr>,
-    pub href: Option<Expr>,
+    pub href: Option<TagAttribute>,
 }
 
 pub enum ClassesForm {
@@ -242,11 +242,11 @@ impl TagAttributes {
         drained
     }
 
-    fn remove_attr(attrs: &mut Vec<TagAttribute>, name: &str) -> Option<Expr> {
+    fn remove_attr(attrs: &mut Vec<TagAttribute>, name: &str) -> Option<TagAttribute> {
         let mut i = 0;
         while i < attrs.len() {
             if attrs[i].label.to_string() == name {
-                return Some(attrs.remove(i).value);
+                return Some(attrs.remove(i));
             } else {
                 i += 1;
             }
@@ -307,14 +307,56 @@ impl Parse for TagAttributes {
             i += 1;
         }
         let booleans = TagAttributes::drain_boolean(&mut attributes);
+        for boolean in &booleans {
+            if boolean.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &boolean.label,
+                        "boolean attributes don't support being used as an option attribute (hint: a value of false results in the attribute not being set)"
+                ));
+            }
+        }
 
-        let classes =
-            TagAttributes::remove_attr(&mut attributes, "class").map(TagAttributes::map_classes);
+        let classes = TagAttributes::remove_attr(&mut attributes, "class");
+        if let Some(classes) = &classes {
+            if classes.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &classes.label,
+                    "the 'class' attribute does not support being used as an optional attribute",
+                ));
+            }
+        }
+        let classes = classes.map(|a| TagAttributes::map_classes(a.value));
         let value = TagAttributes::remove_attr(&mut attributes, "value");
         let kind = TagAttributes::remove_attr(&mut attributes, "type");
         let checked = TagAttributes::remove_attr(&mut attributes, "checked");
+        if let Some(checked) = &checked {
+            if checked.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &checked.label,
+                    "boolean attributes don't support being used as an option attribute (hint: a value of false results in the attribute not being set)",
+                ));
+            }
+        }
         let node_ref = TagAttributes::remove_attr(&mut attributes, "ref");
+        if let Some(node_ref) = &node_ref {
+            if node_ref.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &node_ref.label,
+                    "the 'ref' attribute does not support being used as an optional attribute",
+                ));
+            }
+        }
+        let node_ref = node_ref.map(|n| n.value);
         let key = TagAttributes::remove_attr(&mut attributes, "key");
+        if let Some(key) = &key {
+            if key.question_mark.is_some() {
+                return Err(syn::Error::new_spanned(
+                    &key.label,
+                    "the 'key' attribute does not support being used as an optional attribute",
+                ));
+            }
+        }
+        let key = key.map(|k| k.value);
 
         let href = TagAttributes::remove_attr(&mut attributes, "href");
 

yew-macro/tests/macro/html-component-fail.rs

@@ -121,6 +121,8 @@ fn compile_fail() {
         <span>{ 1 }</span>
         <span>{ 2 }</span>
     };
+
+    html! { <TestComponent value?="not_supported" /> };
 }
 
 fn main() {}

yew-macro/tests/macro/html-component-fail.stderr

@@ -198,6 +198,14 @@ error: only one root html element is allowed (hint: you can wrap multiple html e
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error: optional attributes are only supported on HTML tags. Yew components can use `Option<T>` properties to accomplish the same thing.
+   --> $DIR/html-component-fail.rs:125:28
+    |
+125 |     html! { <TestComponent value?="not_supported" /> };
+    |                            ^^^^^
+    |
+    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error[E0425]: cannot find value `blah` in this scope
   --> $DIR/html-component-fail.rs:91:25
    |