To add to the list, my favorites are:
* cargo-expand [0]: dumps the expansion of troublesome macros
* cargo-bloat [1]: explains why a package is so huge
* ripgrep [2]: RIP grep
* llvm-tools-preview (via [3]): the LLVM tools with which Rust was compiled (because LLVM versions sometimes don't interoperate)
[0] https://crates.io/crates/cargo-expand [1] https://crates.io/crates/cargo-bloat [2] https://crates.io/crates/ripgrep [3] https://crates.io/crates/cargo-binutils
I'll add to this cargo-edit [0] which lets you add, remove and update dependencies from terminal. Small additions to cargo but huge timesavers.