component: Add component and component_preview crates to power UI components (#24456)

This PR formalizes design components with the Component and
ComponentPreview traits.

You can open the preview UI with `workspace: open component preview`.

Component previews no longer need to return `Self` allowing for more
complex previews, and previews of components like `ui::Tooltip` that
supplement other components rather than are rendered by default.

`cargo-machete` incorrectly identifies `linkme` as an unused dep on
crates that have components deriving `IntoComponent`, so you may need to
add this to that crate's `Cargo.toml`:

```toml
# cargo-machete doesn't understand that linkme is used in the component macro
[package.metadata.cargo-machete]
ignored = ["linkme"]
```

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/ui_macros/Cargo.toml

@@ -13,7 +13,8 @@ path = "src/ui_macros.rs"
 proc-macro = true
 
 [dependencies]
+convert_case.workspace = true
+linkme.workspace = true
 proc-macro2.workspace = true
 quote.workspace = true
 syn.workspace = true
-convert_case.workspace = true

crates/ui_macros/src/derive_component.rs

@@ -0,0 +1,97 @@
+use convert_case::{Case, Casing};
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput, Lit, Meta, MetaList, MetaNameValue, NestedMeta};
+
+pub fn derive_into_component(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let mut scope_val = None;
+    let mut description_val = None;
+
+    for attr in &input.attrs {
+        if attr.path.is_ident("component") {
+            if let Ok(Meta::List(MetaList { nested, .. })) = attr.parse_meta() {
+                for item in nested {
+                    if let NestedMeta::Meta(Meta::NameValue(MetaNameValue {
+                        path,
+                        lit: Lit::Str(s),
+                        ..
+                    })) = item
+                    {
+                        let ident = path.get_ident().map(|i| i.to_string()).unwrap_or_default();
+                        if ident == "scope" {
+                            scope_val = Some(s.value());
+                        } else if ident == "description" {
+                            description_val = Some(s.value());
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    let name = &input.ident;
+
+    let scope_impl = if let Some(s) = scope_val {
+        quote! {
+            fn scope() -> Option<&'static str> {
+                Some(#s)
+            }
+        }
+    } else {
+        quote! {
+            fn scope() -> Option<&'static str> {
+                None
+            }
+        }
+    };
+
+    let description_impl = if let Some(desc) = description_val {
+        quote! {
+            fn description() -> Option<&'static str> {
+                Some(#desc)
+            }
+        }
+    } else {
+        quote! {}
+    };
+
+    let register_component_name = syn::Ident::new(
+        &format!(
+            "__register_component_{}",
+            Casing::to_case(&name.to_string(), Case::Snake)
+        ),
+        name.span(),
+    );
+    let register_preview_name = syn::Ident::new(
+        &format!(
+            "__register_preview_{}",
+            Casing::to_case(&name.to_string(), Case::Snake)
+        ),
+        name.span(),
+    );
+
+    let expanded = quote! {
+        impl component::Component for #name {
+            #scope_impl
+
+            fn name() -> &'static str {
+                stringify!(#name)
+            }
+
+            #description_impl
+        }
+
+        #[linkme::distributed_slice(component::__ALL_COMPONENTS)]
+        fn #register_component_name() {
+            component::register_component::<#name>();
+        }
+
+        #[linkme::distributed_slice(component::__ALL_PREVIEWS)]
+        fn #register_preview_name() {
+            component::register_preview::<#name>();
+        }
+    };
+
+    expanded.into()
+}

crates/ui_macros/src/ui_macros.rs

@@ -1,3 +1,4 @@
+mod derive_component;
 mod derive_path_str;
 mod dynamic_spacing;
 
@@ -58,3 +59,27 @@ pub fn path_str(_args: TokenStream, input: TokenStream) -> TokenStream {
 pub fn derive_dynamic_spacing(input: TokenStream) -> TokenStream {
     dynamic_spacing::derive_spacing(input)
 }
+
+/// Derives the `Component` trait for a struct.
+///
+/// This macro generates implementations for the `Component` trait and associated
+/// registration functions for the component system.
+///
+/// # Attributes
+///
+/// - `#[component(scope = "...")]`: Required. Specifies the scope of the component.
+/// - `#[component(description = "...")]`: Optional. Provides a description for the component.
+///
+/// # Example
+///
+/// ```
+/// use ui_macros::Component;
+///
+/// #[derive(Component)]
+/// #[component(scope = "toggle", description = "A element that can be toggled on and off")]
+/// struct Checkbox;
+/// ```
+#[proc_macro_derive(IntoComponent, attributes(component))]
+pub fn derive_component(input: TokenStream) -> TokenStream {
+    derive_component::derive_into_component(input)
+}