Add support for Option<T> attributes

Introduces the `attr=[value]` syntax that assumes `value` is an
`Option<T>`. Renders `attr="value"` for `Some(value)` and entirely
omits the attribute for `None`.

Implements and therefore closes lambda-fairy#283.

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 - Update to support axum 0.2
   [#303](https://github.com/lambda-fairy/maud/pull/303)
+- Add support for `Option<T>` attributes using the `attr=[value]` syntax.
+  [#306](https://github.com/lambda-fairy/maud/pull/306)
 
 ## [0.22.3] - 2021-09-27
 

docs/content/elements-attributes.md

@@ -101,6 +101,26 @@ html! {
 # ;
 ```
 
+## Optional attributes: `title=[Some("value")]`
+
+Add optional attributes to an element using `attr=[value]` syntax, with *square*
+brackets. These are only rendered if the value is `Some<T>`, and entirely
+omitted if the value is `None`.
+
+```rust
+# let _ = maud::
+html! {
+    p title=[Some("Good password")] { "Correct horse" }
+
+    @let value = Some(42);
+    input value=[value];
+
+    @let title: Option<&str> = None;
+    p title=[title] { "Battery staple" }
+}
+# ;
+```
+
 ## Empty attributes: `checked`
 
 Declare an empty attribute by omitting the value.

maud/tests/basic_syntax.rs

@@ -118,6 +118,40 @@ fn empty_attributes_question_mark() {
     assert_eq!(result.into_string(), "<input checked disabled>");
 }
 
+#[test]
+fn optional_attribute_some() {
+    let result = html! { input value=[Some("value")]; };
+    assert_eq!(result.into_string(), r#"<input value="value">"#);
+}
+
+#[test]
+fn optional_attribute_none() {
+    let result = html! { input value=[None as Option<&str>]; };
+    assert_eq!(result.into_string(), r#"<input>"#);
+}
+
+#[test]
+fn optional_attribute_non_string_some() {
+    let result = html! { input value=[Some(42)]; };
+    assert_eq!(result.into_string(), r#"<input value="42">"#);
+}
+
+#[test]
+fn optional_attribute_variable() {
+    let result = html! {
+        @let x = Some(42);
+        input value=[x];
+    };
+    assert_eq!(result.into_string(), r#"<input value="42">"#);
+}
+
+#[test]
+fn optional_attribute_inner_value_evaluated_only_once() {
+    let mut count = 0;
+    html! { input value=[{ count += 1; Some("picklebarrelkumquat") }]; };
+    assert_eq!(count, 1);
+}
+
 #[test]
 fn colons_in_names() {
     let result = html! { pon-pon:controls-alpha { a on:click="yay()" { "Yay!" } } };

maud_macros/src/ast.rs

@@ -170,13 +170,15 @@ impl Attribute {
 #[derive(Debug)]
 pub enum AttrType {
     Normal { value: Markup },
+    Optional { toggler: Toggler },
     Empty { toggler: Option<Toggler> },
 }
 
 impl AttrType {
     fn span(&self) -> Option<SpanRange> {
         match *self {
             AttrType::Normal { ref value } => Some(value.span()),
+            AttrType::Optional { ref toggler, .. } => Some(toggler.span()),
             AttrType::Empty { ref toggler } => toggler.as_ref().map(Toggler::span),
         }
     }

maud_macros/src/generate.rs

@@ -124,6 +124,19 @@ impl Generator {
                     self.markup(value, build);
                     build.push_str("\"");
                 }
+                AttrType::Optional { toggler } => build.push_tokens({
+                    // `inner_value` is the unpacked Some() from `toggler.cond`, see below.
+                    let markup = Markup::Splice { expr: quote!(inner_value), outer_span: toggler.cond_span };
+                    let mut build = self.builder();
+                    build.push_str(" ");
+                    self.name(name, &mut build);
+                    build.push_str("=\"");
+                    self.markup(markup, &mut build);
+                    build.push_str("\"");
+                    let body = build.finish();
+                    let cond = toggler.cond;
+                    quote!(if let Some(inner_value) = #cond { #body })
+                }),
                 AttrType::Empty { toggler: None } => {
                     build.push_str(" ");
                     self.name(name, build);

maud_macros/src/parse.rs

@@ -571,13 +571,16 @@ impl Parser {
                         // Parse a value under an attribute context
                         assert!(self.current_attr.is_none());
                         self.current_attr = Some(ast::name_to_string(name.clone()));
-                        let value = self.markup();
+                        let attr_type = match self.attr_toggler() {
+                            Some(toggler) => ast::AttrType::Optional { toggler },
+                            None => {
+                                let value = self.markup();
+                                ast::AttrType::Normal { value }
+                            }
+                        };
                         self.current_attr = None;
                         attrs.push(ast::Attr::Attribute {
-                            attribute: ast::Attribute {
-                                name,
-                                attr_type: ast::AttrType::Normal { value },
-                            },
+                            attribute: ast::Attribute { name, attr_type },
                         });
                     }
                     // Empty attribute (legacy syntax)