Isolates dependencies in Go and Rust unit tests using test-double generation - Go: `go.uber.org/mock` (gomock + mockgen codegen) and `github.com/stretchr/testify/mock` (hand-written stubs); Rust: `mockall` crate with `#[automock]` for traits and `mock!` macro for structs and external traits. Use when a unit test reaches a database, HTTP client, file system, or any interface boundary that must be replaced with a controlled fake to keep tests fast, deterministic, and isolated.