extern crate proc_macro;
use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::{quote, quote_spanned, ToTokens};
use syn::parse::Parse;
use syn::spanned::Spanned;
use syn::{
parse2, parse_macro_input, parse_quote, Attribute, FnArg, Ident, Item, ItemMod, LitStr, Meta,
Pat, PatIdent, PatType, Token, Type,
};
struct BuiltinArgs {
name: LitStr,
}
impl Parse for BuiltinArgs {
fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
Ok(BuiltinArgs {
name: input.parse()?,
})
}
}
fn extract_docstring(attrs: &[Attribute]) -> Option<String> {
// Rust docstrings are transparently written pre-macro expansion into an attribute that looks
// like:
//
// #[doc = "docstring here"]
//
// Multi-line docstrings yield multiple attributes in order, which we assemble into a single
// string below.
#[allow(dead_code)]
#[derive(Debug)]
struct Docstring {
eq: Token![=],
doc: LitStr,
}
impl Parse for Docstring {
fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
Ok(Self {
eq: input.parse()?,
doc: input.parse()?,
})
}
}
attrs
.iter()
.filter(|attr| attr.path.get_ident().into_iter().any(|id| id == "doc"))
.filter_map(|attr| parse2::<Docstring>(attr.tokens.clone()).ok())
.map(|docstring| docstring.doc.value())
.reduce(|mut fst, snd| {
if snd.is_empty() {
// An empty string represents a spacing newline that was added in the
// original doc comment.
fst.push_str("\n\n");
} else {
fst.push_str(&snd);
}
fst
})
}
/// Parse arguments to the `builtins` macro itself, such as `#[builtins(state = Rc<State>)]`.
fn parse_module_args(args: TokenStream) -> Option<Type> {
if args.is_empty() {
return None;
}
let meta: Meta = syn::parse(args).expect("could not parse arguments to `builtins`-attribute");
let name_value = match meta {
Meta::NameValue(nv) => nv,
_ => panic!("arguments to `builtins`-attribute must be of the form `name = value`"),
};
if name_value.path.get_ident().unwrap().to_string() != "state" {
return None;
}
if let syn::Lit::Str(type_name) = name_value.lit {
let state_type: Type =
syn::parse_str(&type_name.value()).expect("failed to parse builtins state type");
return Some(state_type);
}
panic!("state attribute must be a quoted Rust type");
}
/// Mark the annotated module as a module for defining Nix builtins.
///
/// An optional type definition may be specified as an argument (e.g. `#[builtins(Rc<State>)]`),
/// which will add a parameter to the `builtins` function of that type which is passed to each
/// builtin upon instantiation. Using this, builtins that close over some external state can be
/// written.
///
/// A function `fn builtins() -> Vec<Builtin>` will be defined within the annotated module,
/// returning a list of [`tvix_eval::Builtin`] for each function annotated with the `#[builtin]`
/// attribute within the module. If a `state` type is specified, the `builtins` function will take a
/// value of that type.
///
/// Each invocation of the `#[builtin]` annotation within the module should be passed a string
/// literal for the name of the builtin.
///
/// # Examples
/// ```ignore
/// # use tvix_eval_builtin_macros::builtins;
/// # mod value {
/// # pub use tvix_eval::Builtin;
/// # }
///
/// #[builtins]
/// mod builtins {
/// use tvix_eval::{ErrorKind, Value, VM};
///
/// #[builtin("identity")]
/// pub fn builtin_identity(_vm: &mut VM, x: Value) -> Result<Value, ErrorKind> {
/// Ok(x)
/// }
///
/// // Builtins can request their argument not be forced before being called by annotating the
/// // argument with the `#[lazy]` attribute
///
/// #[builtin("tryEval")]
/// pub fn builtin_try_eval(vm: &mut VM, #[lazy] x: Value) -> Result<Value, ErrorKind> {
/// todo!()
/// }
/// }
/// ```
#[proc_macro_attribute]
pub fn builtins(args: TokenStream, item: TokenStream) -> TokenStream {
let mut module = parse_macro_input!(item as ItemMod);
// parse the optional state type, which users might want to pass to builtins
let state_type = parse_module_args(args);
let (_, items) = match &mut module.content {
Some(content) => content,
None => {
return (quote_spanned!(module.span() =>
compile_error!("Builtin modules must be defined in-line")
))
.into();
}
};
let mut builtins = vec![];
for item in items.iter_mut() {
if let Item::Fn(f) = item {
if let Some(builtin_attr_pos) = f
.attrs
.iter()
.position(|attr| attr.path.get_ident().iter().any(|id| *id == "builtin"))
{
let builtin_attr = f.attrs.remove(builtin_attr_pos);
let BuiltinArgs { name } = match builtin_attr.parse_args() {
Ok(args) => args,
Err(err) => return err.into_compile_error().into(),
};
if f.sig.inputs.len() <= 1 {
return (quote_spanned!(
f.sig.inputs.span() =>
compile_error!("Builtin functions must take at least two arguments")
))
.into();
}
// Determine if this function is taking the state parameter.
let mut args_iter = f.sig.inputs.iter_mut().peekable();
let mut captures_state = false;
if let Some(FnArg::Typed(PatType { pat, .. })) = args_iter.peek() {
if let Pat::Ident(PatIdent { ident, .. }) = pat.as_ref() {
if ident.to_string() == "state" {
if state_type.is_none() {
panic!("builtin captures a `state` argument, but no state type was defined");
}
captures_state = true;
}
}
}
// skip state and/or VM args ..
let skip_num = if captures_state { 2 } else { 1 };
let builtin_arguments = args_iter
.skip(skip_num)
.map(|arg| {
let mut strict = true;
let name = match arg {
FnArg::Receiver(_) => {
return Err(quote_spanned!(arg.span() => {
compile_error!("Unexpected receiver argument in builtin")
}))
}
FnArg::Typed(PatType { attrs, pat, .. }) => {
attrs.retain(|attr| {
attr.path.get_ident().into_iter().any(|id| {
if id == "lazy" {
strict = false;
false
} else {
true
}
})
});
match pat.as_ref() {
Pat::Ident(PatIdent { ident, .. }) => ident.to_string(),
_ => "unknown".to_string(),
}
}
};
Ok(quote_spanned!(arg.span() => {
crate::BuiltinArgument {
strict: #strict,
name: #name,
}
}))
})
.collect::<Result<Vec<_>, _>>();
let builtin_arguments = match builtin_arguments {
Ok(args) => args,
Err(err) => return err.into(),
};
let fn_name = f.sig.ident.clone();
let num_args = f.sig.inputs.len() - skip_num;
let args = (0..num_args)
.map(|n| Ident::new(&format!("arg_{n}"), Span::call_site()))
.collect::<Vec<_>>();
let mut reversed_args = args.clone();
reversed_args.reverse();
let docstring = match extract_docstring(&f.attrs) {
Some(docs) => quote!(Some(#docs)),
None => quote!(None),
};
if captures_state {
builtins.push(quote_spanned! { builtin_attr.span() => {
let inner_state = state.clone();
crate::Builtin::new(
#name,
&[#(#builtin_arguments),*],
#docstring,
move |mut args: Vec<crate::Value>, vm: &mut crate::VM| {
#(let #reversed_args = args.pop().unwrap();)*
#fn_name(inner_state.clone(), vm, #(#args),*)
}
)
}});
} else {
builtins.push(quote_spanned! { builtin_attr.span() => {
crate::Builtin::new(
#name,
&[#(#builtin_arguments),*],
#docstring,
|mut args: Vec<crate::Value>, vm: &mut crate::VM| {
#(let #reversed_args = args.pop().unwrap();)*
#fn_name(vm, #(#args),*)
}
)
}});
}
}
}
}
if let Some(state_type) = state_type {
items.push(parse_quote! {
pub fn builtins(state: #state_type) -> Vec<(&'static str, Value)> {
vec![#(#builtins),*].into_iter().map(|b| (b.name(), Value::Builtin(b))).collect()
}
});
} else {
items.push(parse_quote! {
pub fn builtins() -> Vec<(&'static str, Value)> {
vec![#(#builtins),*].into_iter().map(|b| (b.name(), Value::Builtin(b))).collect()
}
});
}
module.into_token_stream().into()
}
