Function is_rust_std_crate 

pub fn is_rust_std_crate(crate_name: &str) -> bool

Standard distribution crates documented on doc.rust-lang.org.

The std, core, alloc, proc_macro, and test crates are not published to docs.rs; their rustdoc lives on doc.rust-lang.org using the same item-page layout but without a version path segment. Item and index (all.html) URLs must target that host or every lookup 404s and silently falls back to the crate overview.