Work around overflow evaluating the requirement in Nightly rustdoc

See rust-lang/rust#62132
and rust-lang/rust#62059

The fix in rustdoc is at the “investigating the feasibility” stage,
and this is preventing Servo from upgrading its Rust toolchain.

Author: SimonSapin

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "synstructure"
-version = "0.10.2"
+version = "0.10.3"
 authors = ["Nika Layzell <nika@thelayzells.com>"]
 
 description = "Helper methods and macros for custom derives"

src/lib.rs

@@ -159,6 +159,8 @@
 //! For more example usage, consider investigating the `abomonation_derive` crate,
 //! which makes use of this crate, and is fairly simple.
 
+#![recursion_limit="128"]
+
 extern crate proc_macro;
 extern crate proc_macro2;
 #[macro_use]

test_traits/Cargo.toml

@@ -8,3 +8,7 @@ authors = ["Nika Layzell <nika@thelayzells.com>"]
 
 [lib]
 path = "lib.rs"
+
+[dependencies]
+# To trigger https://github.com/rust-lang/rust/issues/62132
+syn = { version = "0.15", features = ["full"] }