feat: #[component] now handles impl Trait by converting to generi…

…c type params, fix #2274

Book needs to be updated to remove this line:
https://github.com/leptos-rs/book/blob/35c380ffc8f7d2e50ef21db97943138b3dba2728/src/view/03_components.md?plain=1#L233

leptos_macro/src/component.rs

@@ -8,10 +8,11 @@ use leptos_hot_reload::parsing::value_to_string;
 use proc_macro2::{Ident, Span, TokenStream};
 use quote::{format_ident, quote, quote_spanned, ToTokens, TokenStreamExt};
 use syn::{
-    parse::Parse, parse_quote, spanned::Spanned,
-    AngleBracketedGenericArguments, Attribute, FnArg, GenericArgument, Item,
-    ItemFn, LitStr, Meta, Pat, PatIdent, Path, PathArguments, ReturnType,
-    Signature, Stmt, Type, TypePath, Visibility,
+    parse::Parse, parse_quote, spanned::Spanned, token::Colon,
+    visit_mut::VisitMut, AngleBracketedGenericArguments, Attribute, FnArg,
+    GenericArgument, GenericParam, Item, ItemFn, LitStr, Meta, Pat, PatIdent,
+    Path, PathArguments, ReturnType, Signature, Stmt, Type, TypeImplTrait,
+    TypeParam, TypePath, Visibility,
 };
 
 pub struct Model {
@@ -28,6 +29,7 @@ pub struct Model {
 impl Parse for Model {
     fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
         let mut item = ItemFn::parse(input)?;
+        convert_impl_trait_to_generic(&mut item.sig);
 
         let docs = Docs::new(&item.attrs);
 
@@ -1229,3 +1231,57 @@ fn is_valid_into_view_return_type(ty: &ReturnType) -> bool {
 pub fn unmodified_fn_name_from_fn_name(ident: &Ident) -> Ident {
     Ident::new(&format!("__{ident}"), ident.span())
 }
+
+/// Converts all `impl Trait`s in a function signature to use generic params instead.
+pub fn convert_impl_trait_to_generic(sig: &mut Signature) {
+    fn new_generic_ident(i: usize, span: Span) -> Ident {
+        Ident::new(&format!("__ImplTrait{}", i), span)
+    }
+
+    // First: visit all `impl Trait`s and replace them with new generic params.
+    #[derive(Default)]
+    struct RemoveImplTrait(Vec<TypeImplTrait>);
+    impl VisitMut for RemoveImplTrait {
+        fn visit_type_mut(&mut self, ty: &mut Type) {
+            syn::visit_mut::visit_type_mut(self, ty);
+            if matches!(ty, Type::ImplTrait(_)) {
+                let ident = new_generic_ident(self.0.len(), ty.span());
+                let generic_type = Type::Path(TypePath {
+                    qself: None,
+                    path: Path::from(ident),
+                });
+                let Type::ImplTrait(impl_trait) =
+                    std::mem::replace(ty, generic_type)
+                else {
+                    unreachable!();
+                };
+                self.0.push(impl_trait);
+            }
+        }
+
+        // Early exits.
+        fn visit_attribute_mut(&mut self, _: &mut Attribute) {}
+        fn visit_pat_mut(&mut self, _: &mut Pat) {}
+    }
+    let mut visitor = RemoveImplTrait::default();
+    for fn_arg in sig.inputs.iter_mut() {
+        visitor.visit_fn_arg_mut(fn_arg);
+    }
+    let RemoveImplTrait(impl_traits) = visitor;
+
+    // Second: Add the new generic params into the signature.
+    for (i, impl_trait) in impl_traits.into_iter().enumerate() {
+        let span = impl_trait.span();
+        let ident = new_generic_ident(i, span);
+        // We can simply append to the end (only lifetime params must be first).
+        // Note currently default generics are not allowed in `fn`, so not a concern.
+        sig.generics.params.push(GenericParam::Type(TypeParam {
+            attrs: vec![],
+            ident,
+            colon_token: Some(Colon { spans: [span] }),
+            bounds: impl_trait.bounds,
+            eq_token: None,
+            default: None,
+        }));
+    }
+}

leptos_macro/src/lib.rs

@@ -37,7 +37,9 @@ impl Default for Mode {
 
 mod params;
 mod view;
-use crate::component::unmodified_fn_name_from_fn_name;
+use component::{
+    convert_impl_trait_to_generic, unmodified_fn_name_from_fn_name,
+};
 use view::{client_template::render_template, render_view};
 mod component;
 mod slice;
@@ -608,6 +610,8 @@ pub fn component(args: proc_macro::TokenStream, s: TokenStream) -> TokenStream {
     let Ok(mut dummy) = syn::parse::<DummyModel>(s.clone()) else {
         return s;
     };
+    convert_impl_trait_to_generic(&mut dummy.sig);
+
     let parse_result = syn::parse::<component::Model>(s);
 
     if let (ref mut unexpanded, Ok(model)) = (&mut dummy, parse_result) {

leptos_macro/tests/component.rs

@@ -8,20 +8,27 @@ fn Component(
     #[prop(strip_option)] strip_option: Option<u8>,
     #[prop(default = NonZeroUsize::new(10).unwrap())] default: NonZeroUsize,
     #[prop(into)] into: String,
+    impl_trait: impl Fn() -> i32 + 'static,
 ) -> impl IntoView {
     _ = optional;
     _ = optional_no_strip;
     _ = strip_option;
     _ = default;
     _ = into;
+    _ = impl_trait;
 }
 
 #[test]
 fn component() {
-    let cp = ComponentProps::builder().into("").strip_option(9).build();
+    let cp = ComponentProps::builder()
+        .into("")
+        .strip_option(9)
+        .impl_trait(|| 42)
+        .build();
     assert!(!cp.optional);
     assert_eq!(cp.optional_no_strip, None);
     assert_eq!(cp.strip_option, Some(9));
     assert_eq!(cp.default, NonZeroUsize::new(10).unwrap());
     assert_eq!(cp.into, "");
+    assert_eq!((cp.impl_trait)(), 42);
 }