Cargo bench is not supported

    $ cargo bench
       Compiling proc-macro2 v1.0.34 (/git/proc-macro2)
    error[E0433]: failed to resolve: use of undeclared crate or module `bench_libproc_macro`
     --> benches/bench-libproc-macro/main.rs:1:1
      |
    1 | bench_libproc_macro::bench!();
      | ^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `bench_libproc_macro`

Author: dtolnay

Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://github.com/dtolnay/proc-macro2"
 documentation = "https://docs.rs/proc-macro2"
 categories = ["development-tools::procedural-macro-helpers"]
 edition = "2018"
+autobenches = false
 rust-version = "1.31"
 description = """
 A substitute implementation of the compiler's `proc_macro` API to decouple