Inject Host Interactions at Boundaries
RUST-INJECT-HOST-INTERACTIONS-AT-BOUNDARIES
Summary
Pass filesystem, network, time, randomness, and process behavior through boundaries when tests or alternate environments need control. This keeps the core deterministic and effects explicit.
Rule
Make filesystem, network, time, randomness, process, and other host interactions injectable at a boundary when tests or alternate environments need control.
Why
Ambient host interactions make Rust code harder to test and harder to run in alternate environments. Passing a clock, random source, process runner, filesystem adapter, or network client at the boundary keeps the pure core deterministic.
Microsoft's Pragmatic Rust checklist calls out mockable I/O and system calls for resilience. The important shape is not "mock everything"; it is to keep nondeterministic host behavior at the edge so tests can exercise policy and parsing without depending on the current machine, network, clock, or process table.
Helps
- Helps tests control nondeterminism and keeps host effects at explicit boundaries.
Limits
Do not over-abstract one-off private scripts. Inject host dependencies when determinism, reuse, or alternate environments matter. Prefer concrete boundary structs or small traits over broad context objects that hide which host capabilities a function uses.
Agent Instruction
Inject Rust host dependencies at boundaries for tests or alternate environments that need
deterministic control.Mechanisms
Supported by adapter traits, concrete boundary structs, test doubles, temporary directories, fake clocks, and seeded random sources.