Update Cargo.toml of rustc-workspace-hack

Author: flip1995

src/tools/rustc-workspace-hack/Cargo.toml

@@ -60,7 +60,7 @@ parking_lot = { version = "0.6", features = ['nightly'] }
 rand = { version = "0.5.5", features = ["i128_support"] }
 serde = { version = "1.0.82", features = ['derive'] }
 serde_json = { version = "1.0.31", features = ["raw_value"] }
-smallvec = { version = "0.6", features = ['union'] }
+smallvec = { version = "0.6", features = ['union', 'may_dangle'] }
 scopeguard = { version = "0.3.3", features = ["use_std", "default"]}
 byteorder = { version = "1.2.7", features = ["i128"]}