fix(cargo): don't inject a redundant summary line when 0 crates compiled

[pszymkowiak]

Summary

cargo_build_success_line() always prepended a "cargo build (N crates compiled)" line — even when N == 0 and cargo's own Finished line already said so. On a no-op build (nothing to recompile), this turns a single-line raw output into two lines: a net token increase, not a reduction.

Caught live via scripts/benchmark.sh:

🔴 cargo build   │ cargo build 2>&1 || true │ rtk cargo build 2>&1 │  18 →  25 (-38%)

Also covers the sibling case where no Finished line is captured at all (still 0 crates compiled) — distinguishing a genuine success from a silent failure.

Fix

fn cargo_build_success_line(compiled: usize, finished: Option<&str>, label: &str) -> String {
    match finished {
        Some(f) if compiled == 0 => format!("{}\n", f),
        Some(f) => format!("cargo {} ({} crates compiled)\n{}\n", label, compiled, f),
        None if compiled == 0 => format!("cargo {}: ok (up to date, nothing recompiled)\n", label),
        None => format!("cargo {} ({} crates compiled)\n", label, compiled),
    }
}

Context

• Fixes cargo output on success is too terse, making it ambiguous #759 (cargo output on success is too terse / ambiguous)
• Supersedes fix(cargo): disambiguate success output when 0 crates compiled #1140, which targeted the same root cause but was closed for mechanical reasons (auto-generated PR branched off master instead of develop, producing a diverging/conflicting diff) — not rejected on merit. This PR covers both the case fix(cargo): disambiguate success output when 0 crates compiled #1140 addressed (finished_line == None) and the case it left unchanged (finished_line == Some(..), which is the one actually caught by the benchmark).

Test plan

• 2 new unit tests: test_filter_cargo_build_up_to_date_with_finished_line, test_filter_cargo_build_up_to_date_no_finished_line
• Existing test_filter_cargo_build_success (compiled > 0 path) unaffected
• cargo fmt --all && cargo clippy --all-targets && cargo test --all — 2494 passed, 0 failed
• Verified against the real release binary:

  raw: "    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s"
  rtk: "Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s"
  

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com