feat: add a slice!() macro (leptos-rs#1867)

leptos_macro/src/lib.rs

@@ -34,6 +34,7 @@ mod view;
 use view::{client_template::render_template, render_view};
 mod component;
 mod server;
+mod slice;
 mod slot;
 
 /// The `view` macro uses RSX (like JSX, but Rust!) It follows most of the
@@ -958,7 +959,7 @@ pub fn params_derive(
     input: proc_macro::TokenStream,
 ) -> proc_macro::TokenStream {
     match syn::parse(input) {
-        Ok(ast) => params::impl_params(&ast),
+        Ok(ast) => params::params_impl(&ast),
         Err(err) => err.to_compile_error().into(),
     }
 }
@@ -969,3 +970,36 @@ pub(crate) fn attribute_value(attr: &KeyedAttribute) -> &syn::Expr {
         None => abort!(attr.key, "attribute should have value"),
     }
 }
+
+/// Generates a `lens` into struct with a default getter and setter
+///
+/// Can be used to access deeply nested fields within a global state object
+///
+/// ```rust
+/// # use leptos::{create_runtime, create_rw_signal};
+/// # use leptos_macro::slice;
+/// # let runtime = create_runtime();
+///
+/// #[derive(Default)]
+/// pub struct Outer {
+///     count: i32,
+///     inner: Inner,
+/// }
+///
+/// #[derive(Default)]
+/// pub struct Inner {
+///     inner_count: i32,
+///     inner_name: String,
+/// }
+///
+/// let outer_signal = create_rw_signal(Outer::default());
+///
+/// let (count, set_count) = slice!(outer_signal.count);
+///
+/// let (inner_count, set_inner_count) = slice!(outer_signal.inner.inner_count);
+/// let (inner_name, set_inner_name) = slice!(outer_signal.inner.inner_name);
+/// ```
+#[proc_macro]
+pub fn slice(input: TokenStream) -> TokenStream {
+    slice::slice_impl(input)
+}

leptos_macro/src/params.rs

@@ -1,7 +1,7 @@
 use quote::{quote, quote_spanned};
 use syn::spanned::Spanned;
 
-pub fn impl_params(ast: &syn::DeriveInput) -> proc_macro::TokenStream {
+pub fn params_impl(ast: &syn::DeriveInput) -> proc_macro::TokenStream {
     let name = &ast.ident;
 
     let fields = if let syn::Data::Struct(syn::DataStruct {

leptos_macro/src/slice.rs

@@ -0,0 +1,59 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::Ident;
+use quote::quote;
+use syn::{
+    parse::{Parse, ParseStream},
+    parse_macro_input,
+    punctuated::Punctuated,
+    token::Dot,
+    Token, Type,
+};
+
+struct SliceMacroInput {
+    pub root: Ident,
+    pub path: Punctuated<Type, Dot>,
+}
+
+impl Parse for SliceMacroInput {
+    fn parse(input: ParseStream) -> Result<Self, syn::Error> {
+        let root = syn::Ident::parse(input)?;
+        let _dot = <Token![.]>::parse(input)?;
+        let path = input.parse_terminated(Type::parse, Token![.])?;
+
+        if path.is_empty() {
+            return Err(syn::Error::new(input.span(), "Expected identifier"));
+        }
+
+        if path.trailing_punct() {
+            return Err(syn::Error::new(
+                input.span(),
+                "Unexpected trailing `.`",
+            ));
+        }
+
+        Ok(SliceMacroInput { root, path })
+    }
+}
+
+impl From<SliceMacroInput> for TokenStream {
+    fn from(val: SliceMacroInput) -> Self {
+        let root = val.root;
+        let path = val.path;
+
+        quote! {
+            ::leptos::create_slice(
+                #root,
+                |st: &_| st.#path.clone(),
+                |st: &mut _, n| st.#path = n
+            )
+        }
+        .into()
+    }
+}
+
+pub fn slice_impl(tokens: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(tokens as SliceMacroInput);
+    input.into()
+}

leptos_macro/tests/slice.rs

@@ -0,0 +1,32 @@
+use leptos::{create_runtime, create_rw_signal};
+use leptos_macro::slice;
+
+#[derive(Default)]
+pub struct OuterState {
+    count: i32,
+    inner: InnerState,
+}
+
+#[derive(Clone, PartialEq, Default)]
+pub struct InnerState {
+    inner_count: i32,
+    inner_name: String,
+}
+
+#[test]
+fn green() {
+    let _ = create_runtime();
+
+    let outer_signal = create_rw_signal(OuterState::default());
+
+    let (_, _) = slice!(outer_signal.count);
+
+    let (_, _) = slice!(outer_signal.inner.inner_count);
+    let (_, _) = slice!(outer_signal.inner.inner_name);
+}
+
+#[test]
+fn red() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/slice/red.rs")
+}

leptos_macro/tests/slice/red.rs

@@ -0,0 +1,28 @@
+use leptos::{create_runtime, create_rw_signal};
+use leptos_macro::slice;
+
+#[derive(Default, PartialEq)]
+pub struct OuterState {
+    count: i32,
+    inner: InnerState,
+}
+
+#[derive(Clone, PartialEq, Default)]
+pub struct InnerState {
+    inner_count: i32,
+    inner_name: String,
+}
+
+fn main() {
+    let _ = create_runtime();
+
+    let outer_signal = create_rw_signal(OuterState::default());
+
+    let (_, _) = slice!();
+
+    let (_, _) = slice!(outer_signal);
+
+    let (_, _) = slice!(outer_signal.);
+
+    let (_, _) = slice!(outer_signal.inner.);
+}

leptos_macro/tests/slice/red.stderr

@@ -0,0 +1,31 @@
+error: unexpected end of input, expected identifier
+  --> tests/slice/red.rs:21:18
+   |
+21 |     let (_, _) = slice!();
+   |                  ^^^^^^^^
+   |
+   = note: this error originates in the macro `slice` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: expected `.`
+  --> tests/slice/red.rs:23:18
+   |
+23 |     let (_, _) = slice!(outer_signal);
+   |                  ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `slice` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: Expected identifier
+  --> tests/slice/red.rs:25:18
+   |
+25 |     let (_, _) = slice!(outer_signal.);
+   |                  ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `slice` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: Unexpected trailing `.`
+  --> tests/slice/red.rs:27:18
+   |
+27 |     let (_, _) = slice!(outer_signal.inner.);
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `slice` (in Nightly builds, run with -Z macro-backtrace for more info)