feat(tvix/eval): Add docstrings as documentation for builtins

Add a new `documentation: Option<&'static str>` field to Builtin, and
populate it in the `#[builtins]` macro with the docstring of the builtin
function, if any.

Change-Id: Ic68fdf9b314d15a780731974234e2ae43f6a44b0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7205
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

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

@@ -2,11 +2,12 @@ extern crate proc_macro;
 
 use proc_macro::TokenStream;
 use proc_macro2::Span;
-use quote::{quote_spanned, ToTokens};
+use quote::{quote, quote_spanned, ToTokens};
 use syn::parse::Parse;
 use syn::spanned::Spanned;
 use syn::{
-    parse_macro_input, parse_quote, FnArg, Ident, Item, ItemMod, LitStr, Pat, PatIdent, PatType,
+    parse2, parse_macro_input, parse_quote, Attribute, FnArg, Ident, Item, ItemMod, LitStr, Pat,
+    PatIdent, PatType, Token,
 };
 
 struct BuiltinArgs {
@@ -21,6 +22,35 @@ impl Parse for BuiltinArgs {
     }
 }
 
+fn extract_docstring(attrs: &[Attribute]) -> Option<LitStr> {
+    // Rust docstrings are transparently written pre-macro expansion into an attribute that looks
+    // like:
+    //
+    // #[doc = "docstring here"]
+
+    #[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"))
+        .find_map(|attr| parse2::<Docstring>(attr.tokens.clone()).ok())
+        .map(|docstring| docstring.doc)
+}
+
 /// Mark the annotated module as a module for defining Nix builtins.
 ///
 /// A function `fn builtins() -> Vec<Builtin>` will be defined within the annotated module,
@@ -144,10 +174,16 @@ pub fn builtins(_args: TokenStream, item: TokenStream) -> TokenStream {
                 let mut reversed_args = args.clone();
                 reversed_args.reverse();
 
+                let docstring = match extract_docstring(&f.attrs) {
+                    Some(docs) => quote!(Some(#docs)),
+                    None => quote!(None),
+                };
+
                 builtins.push(quote_spanned! { builtin_attr.span() => {
                     crate::internal::Builtin::new(
                         #name,
                         &[#(#builtin_arguments),*],
+                        #docstring,
                         |mut args: Vec<crate::Value>, vm: &mut crate::internal::VM| {
                             #(let #reversed_args = args.pop().unwrap();)*
                             #fn_name(vm, #(#args),*)