redundant_env_clone

Default Severity: warn

Target Resource: CPU — memory allocation, copy, and host object dispatch

What it does

Detects unnecessary .clone() calls on the Soroban Env object.

Why is this bad?

DANGER

The Soroban Env object is designed to be highly lightweight and is typically passed by value or reference. Cloning it forces MemAlloc and MemCpy operations followed by a VisitObject of the new handle — all unnecessary CPU cycles that the network charges for. See the Cost Rationale — Metered Resources for the cost types involved.

Cost impact

Cloning Env incurs MemAlloc and MemCpy operations followed by a VisitObject of the new handle — unnecessary CPU cycles charged to the transaction. Env is designed to be passed by value or reference without cloning.

Measured with Env::default() in the cost_benchmarks crate (cargo test -- --nocapture):

Pattern	Iterations	CPU instructions (delta)	Memory bytes (delta)
env.clone() (bad)	100	run cargo test -- --nocapture in cost_benchmarks/	run cargo test -- --nocapture in cost_benchmarks/
&env (good)	100	run cargo test -- --nocapture in cost_benchmarks/	run cargo test -- --nocapture in cost_benchmarks/

INFO

The cost of an individual env.clone() is small, but in hot paths (e.g., contract entry points called thousands of times across a protocol's lifetime) the cumulative overhead is real and avoidable.

How to reproduce

cd cost_benchmarks
cargo test bench_env_clone_vs_reuse -- --nocapture

Example

// ❌ Bad: Env is lightweight — no clone needed
let my_env = env.clone();

Known False Positives (Not Flagged)

The lint does not fire when the clone is genuinely required for compilation:

1. &Env receiver — Cloning through a reference produces an owned Env from a borrowed one.
2. Original binding reused after clone — If the same Env binding is used again after the .clone() call, both the original and the clone are live.
3. Non-local receiver — The receiver is not a simple local binding (e.g. a field access), so the lint cannot statically prove the clone is redundant.

// ✅ Not flagged: &Env — clone produces owned Env from a reference
fn use_ref(env: &Env) {
    let owned = env.clone();
}

// ✅ Not flagged: env is used again after the clone
fn reused(env: Env) {
    let cloned = env.clone();
    some_function(env);  // original still live
}

// ✅ Not flagged: receiver is not a simple local
fn field_access(state: &State) {
    let cloned = state.env.clone();
}

Suggested Fix

TIP

Pass env directly by value or reference without calling .clone().