Function is_rustdoc_only_target 

fn is_rustdoc_only_target(target: &str) -> bool

Whether roxygen2 can never resolve the link target target, so the link was written for rustdoc.

roxygen2 reads pkg::topic as a link into the R package pkg, everything before the last ::, and warns “refers to un-installed package” when no such package is installed. A target is rustdoc-only when that pkg is a Rust path root (crate, self, Self) or cannot be an R package name (a::b, my_mod; see crate::naming::is_r_package_name). Backticks around the target are ignored, as both readers do. Anything else may be an R link, so it is left alone: [topic], [`topic`], [fn()] and [pkg::topic], which roxygen2 documents, and so [`Type::method`] (Type can be a package: R6, S7, Matrix) and [super::x] (super is on CRAN).