Constant syntax::feature_gate::EXPLAIN_CONCAT_IDENTS

pub const EXPLAIN_CONCAT_IDENTS: &'static str = "`concat_idents` is not stable enough for use and is subject to change"

🔬 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?