From f2c3011888757a0d5bd6baa68df98be6687c5b8d Mon Sep 17 00:00:00 2001
From: Joscha <joscha@plugh.de>
Date: Thu, 20 Apr 2023 20:35:11 +0200
Subject: [PATCH] Implement Document derive proc macro

---
 cove-macro/src/lib.rs | 136 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)

diff --git a/cove-macro/src/lib.rs b/cove-macro/src/lib.rs
index f561415..5e7145d 100644
--- a/cove-macro/src/lib.rs
+++ b/cove-macro/src/lib.rs
@@ -8,3 +8,139 @@
 #![warn(single_use_lifetimes)]
 // Clippy lints
 #![warn(clippy::use_self)]
+
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::punctuated::Punctuated;
+use syn::spanned::Spanned;
+use syn::{
+    parse_macro_input, Data, DeriveInput, Expr, ExprLit, Field, Lit, LitStr, MetaNameValue, Token,
+};
+
+fn strlit(expr: &Expr) -> Option<&LitStr> {
+    match expr {
+        Expr::Lit(ExprLit {
+            lit: Lit::Str(lit), ..
+        }) => Some(lit),
+        _ => None,
+    }
+}
+
+/// Given a struct field, this finds all attributes like `#[doc = "bla"]`,
+/// unindents, concatenates and returns them.
+fn docstring(field: &Field) -> syn::Result<String> {
+    let mut lines = vec![];
+
+    for attr in field
+        .attrs
+        .iter()
+        .filter(|attr| attr.path().is_ident("doc"))
+    {
+        if let Some(lit) = strlit(&attr.meta.require_name_value()?.value) {
+            let value = lit.value();
+            let value = value
+                .strip_prefix(' ')
+                .map(|value| value.to_string())
+                .unwrap_or(value);
+            lines.push(value);
+        }
+    }
+
+    Ok(lines.join("\n"))
+}
+
+/// Given a struct field, this finds all key-value pairs of the form
+/// `#[document(key = value, ...)]`.
+fn document_attributes(field: &Field) -> syn::Result<Vec<MetaNameValue>> {
+    let mut attrs = vec![];
+
+    for attr in field
+        .attrs
+        .iter()
+        .filter(|attr| attr.path().is_ident("document"))
+    {
+        let args =
+            attr.parse_args_with(Punctuated::<MetaNameValue, Token![,]>::parse_terminated)?;
+        attrs.extend(args);
+    }
+
+    Ok(attrs)
+}
+
+fn field_doc(field: &Field) -> syn::Result<Option<TokenStream>> {
+    let Some(ident) = field.ident.as_ref() else { return Ok(None); };
+    let ident = ident.to_string();
+    let ty = &field.ty;
+
+    let mut setters = vec![];
+
+    let docstring = docstring(field)?;
+    if !docstring.is_empty() {
+        setters.push(quote! {
+            doc.description = Some(#docstring.to_string());
+        });
+    }
+
+    for attr in document_attributes(field)? {
+        let value = attr.value;
+        if attr.path.is_ident("default") {
+            setters.push(quote! { doc.value_info.default = Some(#value.to_string()); });
+        } else if attr.path.is_ident("metavar") {
+            setters.push(quote! { doc.wrap_info.metavar = Some(#value.to_string()); });
+        } else {
+            return Err(syn::Error::new(attr.path.span(), "unknown argument name"));
+        }
+    }
+
+    Ok(Some(quote! {
+        fields.insert(
+            #ident.to_string(),
+            {
+                let mut doc = <#ty as Document>::doc();
+                #( #setters )*
+                Box::new(doc)
+            }
+        );
+    }))
+}
+
+fn derive_document_impl(input: DeriveInput) -> syn::Result<TokenStream> {
+    let Data::Struct(data) = input.data else {
+        return Err(syn::Error::new(input.span(), "Must be a struct"));
+    };
+
+    let mut fields = Vec::new();
+    for field in data.fields.iter() {
+        if let Some(field) = field_doc(field)? {
+            fields.push(field);
+        }
+    }
+
+    let ident = input.ident;
+    let tokens = quote!(
+        impl crate::doc::Document for #ident {
+            fn doc() -> crate::doc::Doc {
+                use ::std::{boxed::Box, collections::HashMap};
+                use crate::doc::{Doc, Document};
+
+                let mut fields = HashMap::new();
+                #( #fields )*
+
+                let mut doc = Doc::default();
+                doc.struct_info.fields = fields;
+                doc
+            }
+        }
+    );
+
+    Ok(tokens)
+}
+
+#[proc_macro_derive(Document, attributes(document))]
+pub fn derive_document(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    match derive_document_impl(input) {
+        Ok(tokens) => tokens.into(),
+        Err(err) => err.into_compile_error().into(),
+    }
+}