Constant rustc::hir::def_id::CRATE_DEF_INDEX

pub const CRATE_DEF_INDEX: DefIndex = DefIndex(0)

🔬 This is a nightly-only experimental API. (rustc_private)

this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via Cargo.toml instead?

The crate root is always assigned index 0 by the AST Map code, thanks to NodeCollector::new.