Vendor dependencies

Let's see how I like this workflow.

vendor/cxx-build/src/syntax/doc.rs

@@ -0,0 +1,46 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::LitStr;
+
+pub struct Doc {
+    pub(crate) hidden: bool,
+    fragments: Vec<LitStr>,
+}
+
+impl Doc {
+    pub fn new() -> Self {
+        Doc {
+            hidden: false,
+            fragments: Vec::new(),
+        }
+    }
+
+    pub fn push(&mut self, lit: LitStr) {
+        self.fragments.push(lit);
+    }
+
+    #[allow(dead_code)] // only used by cxx-build, not cxxbridge-macro
+    pub fn is_empty(&self) -> bool {
+        self.fragments.is_empty()
+    }
+
+    #[allow(dead_code)] // only used by cxx-build, not cxxbridge-macro
+    pub fn to_string(&self) -> String {
+        let mut doc = String::new();
+        for lit in &self.fragments {
+            doc += &lit.value();
+            doc.push('\n');
+        }
+        doc
+    }
+}
+
+impl ToTokens for Doc {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let fragments = &self.fragments;
+        tokens.extend(quote! { #(#[doc = #fragments])* });
+        if self.hidden {
+            tokens.extend(quote! { #[doc(hidden)] });
+        }
+    }
+}