feat(tvix/nix-compat-derive): Add deriver for NixDeserialize

This adds a nix-compat-derive derive crate that implements a deriver
for NixDeserialize implementations. This is to reduce the amount of
code needed to implement deserialization for all the types used by
the Nix daemon protocol.

Change-Id: I484724b550e8a1d5e9adad9555d9dc1374ae95c2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12022
Autosubmit: Brian Olsen <me@griff.name>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat-derive/src/internal/inputs.rs

@@ -0,0 +1,110 @@
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RemoteInput {
+    pub attrs: Vec<syn::Attribute>,
+    pub ident: syn::Type,
+}
+
+impl syn::parse::Parse for RemoteInput {
+    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
+        let attrs = input.call(syn::Attribute::parse_outer)?;
+
+        let ident = input.parse::<syn::Type>()?;
+        Ok(RemoteInput { attrs, ident })
+    }
+}
+
+impl quote::ToTokens for RemoteInput {
+    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
+        fn is_outer(attr: &&syn::Attribute) -> bool {
+            match attr.style {
+                syn::AttrStyle::Outer => true,
+                syn::AttrStyle::Inner(_) => false,
+            }
+        }
+        for attr in self.attrs.iter().filter(is_outer) {
+            attr.to_tokens(tokens);
+        }
+        self.ident.to_tokens(tokens);
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use syn::parse_quote;
+    //use syn::parse::Parse;
+
+    use super::*;
+
+    #[test]
+    fn test_input() {
+        let p: RemoteInput = parse_quote!(u64);
+        assert_eq!(
+            p,
+            RemoteInput {
+                attrs: vec![],
+                ident: parse_quote!(u64),
+            }
+        );
+    }
+
+    #[test]
+    fn test_input_attr() {
+        let p: RemoteInput = parse_quote!(
+            #[nix]
+            u64
+        );
+        assert_eq!(
+            p,
+            RemoteInput {
+                attrs: vec![parse_quote!(#[nix])],
+                ident: parse_quote!(u64),
+            }
+        );
+    }
+
+    #[test]
+    fn test_input_attr_multiple() {
+        let p: RemoteInput = parse_quote!(
+            #[nix]
+            #[hello]
+            u64
+        );
+        assert_eq!(
+            p,
+            RemoteInput {
+                attrs: vec![parse_quote!(#[nix]), parse_quote!(#[hello])],
+                ident: parse_quote!(u64),
+            }
+        );
+    }
+
+    #[test]
+    fn test_input_attr_full() {
+        let p: RemoteInput = parse_quote!(
+            #[nix(try_from = "u64")]
+            usize
+        );
+        assert_eq!(
+            p,
+            RemoteInput {
+                attrs: vec![parse_quote!(#[nix(try_from="u64")])],
+                ident: parse_quote!(usize),
+            }
+        );
+    }
+
+    #[test]
+    fn test_input_attr_other() {
+        let p: RemoteInput = parse_quote!(
+            #[muh]
+            u64
+        );
+        assert_eq!(
+            p,
+            RemoteInput {
+                attrs: vec![parse_quote!(#[muh])],
+                ident: parse_quote!(u64),
+            }
+        );
+    }
+}