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/ctx.rs

@@ -0,0 +1,50 @@
+use std::cell::RefCell;
+use std::fmt;
+use std::thread::panicking;
+
+use quote::ToTokens;
+
+pub struct Context {
+    errors: RefCell<Option<Vec<syn::Error>>>,
+}
+
+impl Context {
+    pub fn new() -> Context {
+        Context {
+            errors: RefCell::new(Some(Vec::new())),
+        }
+    }
+
+    pub fn syn_error(&self, error: syn::Error) {
+        self.errors
+            .borrow_mut()
+            .as_mut()
+            .take()
+            .unwrap()
+            .push(error);
+    }
+
+    pub fn error_spanned<T: ToTokens, D: fmt::Display>(&self, tokens: T, message: D) {
+        self.syn_error(syn::Error::new_spanned(tokens, message));
+    }
+
+    pub fn check(&self) -> syn::Result<()> {
+        let mut iter = self.errors.borrow_mut().take().unwrap().into_iter();
+        let mut err = match iter.next() {
+            None => return Ok(()),
+            Some(err) => err,
+        };
+        for next_err in iter {
+            err.combine(next_err);
+        }
+        Err(err)
+    }
+}
+
+impl Drop for Context {
+    fn drop(&mut self) {
+        if self.errors.borrow().is_some() && !panicking() {
+            panic!("Context dropped without checking errors");
+        }
+    }
+}