When I working on Cargo.toml file, this cargo-jump-xref.el help me to execute xref-find-definitions on a cargo dependency, jump to the dependency's source code.
You need to add cargo-jump-xref-backend to xref-backend-functions, then you can open a Cargo.toml file, move cursor on a cargo dependency, execute xref-find-definitions will jump to the cargo dependency's source file.
(use-package cargo-jump-xref
:ensure
(:type git :host github :repo "eval-exec/cargo-jump-xref.el")
:config
(add-to-list 'xref-backend-functions #'cargo-jump-xref-backend))