rustc-ap-syntax 521.0.0

Automatically published version of the package `syntax` in the rust-lang/rust repository from commit db592f4627251cfd8571a05cf8e06a56be9470c2 The publishing script for this crate lives at: https://github.com/alexcrichton/rustc-auto-publish

fn main() {
    println!("cargo:rerun-if-changed=build.rs");
    println!("cargo:rerun-if-env-changed=CFG_RELEASE_CHANNEL");
    println!("cargo:rerun-if-env-changed=CFG_DISABLE_UNSTABLE_FEATURES");
}