feat(tvix/eval): Add a proc-macro for defining builtins

Add a single new proc macro to a new proc-macro crate,
`tvix-eval-proc-macros` for defining an inline module containing nix
builtins, and automatically generating a function within that module
which returns a list of those builtins as `tvix_eval::value::Builtin`.

Change-Id: Ie4afae438914d2af93d15637151a49b4c68aa352
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7198
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/builtin-macros/src/lib.rs

@@ -0,0 +1,143 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::Span;
+use quote::{quote_spanned, ToTokens};
+use syn::parse::Parse;
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, parse_quote, FnArg, Ident, Item, ItemMod, LitStr, PatType};
+
+struct BuiltinArgs {
+    name: LitStr,
+}
+
+impl Parse for BuiltinArgs {
+    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
+        Ok(BuiltinArgs {
+            name: input.parse()?,
+        })
+    }
+}
+
+/// Mark the annotated module as a module for defining Nix builtins.
+///
+/// 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.
+///
+/// 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);
+
+    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();
+                }
+
+                let strictness = f
+                    .sig
+                    .inputs
+                    .iter_mut()
+                    .skip(1)
+                    .map(|input| {
+                        let mut lazy = false;
+                        if let FnArg::Typed(PatType { attrs, .. }) = input {
+                            attrs.retain(|attr| {
+                                attr.path.get_ident().into_iter().any(|id| {
+                                    if id == "lazy" {
+                                        lazy = true;
+                                        false
+                                    } else {
+                                        true
+                                    }
+                                })
+                            });
+                        }
+                        !lazy
+                    })
+                    .collect::<Vec<_>>();
+
+                let fn_name = f.sig.ident.clone();
+                let args = (0..(f.sig.inputs.len() - 1))
+                    .map(|n| Ident::new(&format!("arg_{n}"), Span::call_site()))
+                    .collect::<Vec<_>>();
+                let mut reversed_args = args.clone();
+                reversed_args.reverse();
+
+                builtins.push(quote_spanned! { builtin_attr.span() => {
+                    crate::value::Builtin::new(
+                        #name,
+                        &[#(#strictness),*],
+                        |mut args: Vec<Value>, vm: &mut VM| {
+                            #(let #reversed_args = args.pop().unwrap();)*
+                            #fn_name(vm, #(#args),*)
+                        }
+                    )
+                }});
+            }
+        }
+    }
+
+    items.push(parse_quote! {
+        pub fn builtins() -> Vec<crate::value::Builtin> {
+            vec![#(#builtins),*]
+        }
+    });
+
+    module.into_token_stream().into()
+}