Work around Rust error message bug

Summary: Without this line, Rust error messages involving types from Serde get associated with a somewhat surprising/misleading trait path. This is a compiler bug (rust-lang/rust#46991) but it's easy to work around here.

Reviewed By: yfeldblum

Differential Revision: D23326186

fbshipit-source-id: 0bb2c3e905a6545dd8abc8f0746c7bce838272c1

thrift/compiler/generate/templates/rust/lib.rs.mustache

@@ -19,6 +19,10 @@
 
 #![allow(non_camel_case_types, non_snake_case, non_upper_case_globals, unused_crate_dependencies)]
 
+{{#program:serde?}}
+{{! work around https://github.com/rust-lang/rust/issues/46991 }}
+extern crate serde;
+{{/program:serde?}}
 {{#program:constants?}}
 pub use self::consts::*;
 {{/program:constants?}}