SOLVED!

Hey everyone, I managed to solve the issue and wanted to share the solution.

The Main Reason: The whole problem was happening because I was trying to use rustup (even though installed via Nix) alongside the Rust tools (rustc, cargo, rust-analyzer, rustfmt) also installed via Nix. The conclusion is: on NixOS, you generally shouldn't install rustup if you're managing your Rust toolchain directly with Nix.

Explanation:

Nix/NixOS already does the "job" that rustup would do on a traditional system, but in a declarative way that's integrated with the Nix package system. rustup is designed to manage different Rust versions and components in its own directory (~/.rustup), acting as a proxy for commands like rustc, cargo, etc.

When I installed rustup via Nix, it was placed in the system path, but it still tried to act like rustup, looking for toolchains managed by itself and expecting commands like rustup default stable or rustup component add.