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use proc_macro2::TokenStream;
use quote::quote;
use syn::{FnArg, Ident, ItemFn, Pat, ReturnType, Type};
use crate::registry::{self, MemoParam, MemoRegistration};
pub fn expand(item: ItemFn) -> Result<TokenStream, syn::Error> {
let fn_name = &item.sig.ident;
if item.sig.inputs.is_empty() {
return Err(syn::Error::new_spanned(
&item.sig.inputs,
format!(
"#[drv::memo] function '{}' must take at least one parameter",
fn_name,
),
));
}
// First-pass classification. `&Ident` is tentatively Lens; `&NonIdent`
// (e.g. &[u8], &dyn Trait) is ValueRef; non-references are Value.
let mut parsed: Vec<ParsedParam> = Vec::new();
for param in &item.sig.inputs {
parsed.push(classify_param(param, fn_name)?);
}
let output_ty = match &item.sig.output {
ReturnType::Type(_, ty) => (**ty).clone(),
ReturnType::Default => {
return Err(syn::Error::new_spanned(
&item.sig,
format!(
"#[drv::memo] function '{}' must have an explicit return type",
fn_name,
),
));
}
};
let fn_name_str = fn_name.to_string();
registry::with(|reg| {
// Promote tentative Lens params to ValueRef if they don't name a
// registered lens or atom. This lets users write things like `foo: &str`
// — `str` isn't a lens, so it's treated as a ToOwned'able reference.
for p in parsed.iter_mut() {
if let ParsedParam::Lens {
lens_name,
param_name,
} = p
{
let lens_name_str = lens_name.to_string();
if !reg.lens_name_exists(&lens_name_str) && !reg.atom_name_exists(&lens_name_str) {
// Not a lens or an atom — treat as a ToOwned-style reference.
*p = ParsedParam::ValueRef {
param_name: param_name.clone(),
referent: syn::Type::Path(syn::TypePath {
qself: None,
path: lens_name.clone().into(),
}),
};
}
}
}
// Require at least one lens (otherwise we have no atom for the cache).
if !parsed.iter().any(|p| matches!(p, ParsedParam::Lens { .. })) {
return Err(syn::Error::new_spanned(
fn_name,
format!(
"#[drv::memo] function '{}' must have at least one `&LensName` parameter",
fn_name,
),
));
}
if reg.memos.iter().any(|e| e.fn_name == fn_name_str) {
return Err(syn::Error::new_spanned(
fn_name,
format!(
"memo '{}' is already declared -- memo names must be unique within a crate",
fn_name
),
));
}
let body = &item.block;
let body_tokens = quote!(#body).to_string();
let vis = &item.vis;
reg.memos.push(MemoRegistration {
fn_name: fn_name_str,
vis_tokens: quote!(#vis).to_string(),
params: parsed
.iter()
.map(|p| match p {
ParsedParam::Lens {
param_name,
lens_name,
} => MemoParam::Lens {
param_name: param_name.to_string(),
lens_name: lens_name.to_string(),
},
ParsedParam::Value { param_name, ty } => MemoParam::Value {
param_name: param_name.to_string(),
ty_tokens: registry::type_to_tokens(ty),
},
ParsedParam::ValueRef {
param_name,
referent,
} => MemoParam::ValueRef {
param_name: param_name.to_string(),
referent_tokens: registry::type_to_tokens(referent),
},
})
.collect(),
output_ty_tokens: registry::type_to_tokens(&output_ty),
body_tokens,
});
Ok(())
})?;
// Emit compile-time assertions that each ValueRef referent (a) names a
// real type, and (b) implements ToOwned. Using the original syn::Type
// preserves spans, so errors point at the user's function signature.
// This catches common typos like `foo: &MyLen` (meant `&MyLens`) —
// instead of a confusing error deep in generated code, the user sees
// "cannot find type `MyLen` in this scope" pointed at their param.
let assertions: Vec<TokenStream> = parsed
.iter()
.filter_map(|p| match p {
ParsedParam::ValueRef { referent, .. } => Some(quote! {
const _: fn() = || {
fn __drv_assert_to_owned<T: ?Sized + ::std::borrow::ToOwned>() {}
__drv_assert_to_owned::<#referent>();
};
}),
_ => None,
})
.collect();
// Swallow the function body (assemble!() emits the rewritten version);
// only the compile-time assertions remain at the #[drv::memo] call site.
Ok(quote! { #(#assertions)* })
}
enum ParsedParam {
Lens {
param_name: Ident,
lens_name: Ident,
},
Value {
param_name: Ident,
ty: syn::Type,
},
ValueRef {
param_name: Ident,
referent: syn::Type,
},
}
fn classify_param(param: &FnArg, fn_name: &syn::Ident) -> Result<ParsedParam, syn::Error> {
let typed = match param {
FnArg::Typed(t) => t,
FnArg::Receiver(_) => {
return Err(syn::Error::new_spanned(
param,
format!("#[drv::memo] function '{}' cannot take self", fn_name),
));
}
};
let param_name = match typed.pat.as_ref() {
Pat::Ident(pat_ident) => pat_ident.ident.clone(),
_ => {
return Err(syn::Error::new_spanned(
&typed.pat,
format!(
"#[drv::memo] function '{}': parameter must be a simple name",
fn_name
),
));
}
};
// A shared reference is either a lens (`&MyLens`) or a ToOwned reference
// (`&str`, `&[u8]`, etc.). We classify `&Ident` tentatively as Lens; the
// caller promotes to ValueRef if the ident isn't a registered lens/atom.
if let Type::Reference(r) = typed.ty.as_ref() {
if r.mutability.is_some() {
return Err(syn::Error::new_spanned(
&typed.ty,
format!(
"#[drv::memo] function '{}': `&mut` parameters are not supported",
fn_name
),
));
}
if let Type::Path(p) = r.elem.as_ref() {
if let Some(lens_name) = p.path.get_ident() {
return Ok(ParsedParam::Lens {
param_name,
lens_name: lens_name.clone(),
});
}
}
// Non-ident referent (e.g., &[u8], &(A, B), &dyn Trait) — ToOwned ref.
return Ok(ParsedParam::ValueRef {
param_name,
referent: (*r.elem).clone(),
});
}
// Anything else: treat as an owned value parameter.
Ok(ParsedParam::Value {
param_name,
ty: (*typed.ty).clone(),
})
}