Implement procedural macro to derive {To,From}Sql traits

With this macro it is possible to #[derive(ToSql, FromSql)] for enums, that do
not contain data (C-style).

deltachat_derive/src/lib.rs

@@ -0,0 +1,45 @@
+#![recursion_limit = "128"]
+extern crate proc_macro;
+
+use crate::proc_macro::TokenStream;
+use quote::quote;
+use syn;
+
+// For now, assume (not check) that these macroses are applied to enum without
+// data.  If this assumption is violated, compiler error will point to
+// generated code, which is not very user-friendly.
+
+#[proc_macro_derive(ToSql)]
+pub fn to_sql_derive(input: TokenStream) -> TokenStream {
+    let ast: syn::DeriveInput = syn::parse(input).unwrap();
+    let name = &ast.ident;
+
+    let gen = quote! {
+        impl rusqlite::types::ToSql for #name {
+            fn to_sql(&self) -> rusqlite::Result<rusqlite::types::ToSqlOutput> {
+                let num = *self as i64;
+                let value = rusqlite::types::Value::Integer(num);
+                let output = rusqlite::types::ToSqlOutput::Owned(value);
+
+                std::result::Result::Ok(output)
+            }
+        }
+    };
+    gen.into()
+}
+
+#[proc_macro_derive(FromSql)]
+pub fn from_sql_derive(input: TokenStream) -> TokenStream {
+    let ast: syn::DeriveInput = syn::parse(input).unwrap();
+    let name = &ast.ident;
+
+    let gen = quote! {
+        impl rusqlite::types::FromSql for #name {
+            fn column_result(col: rusqlite::types::ValueRef) -> rusqlite::types::FromSqlResult<Self> {
+                let inner = rusqlite::types::FromSql::column_result(col)?;
+                num_traits::FromPrimitive::from_i64(inner).ok_or(rusqlite::types::FromSqlError::InvalidType)
+            }
+        }
+    };
+    gen.into()
+}