nh darwin: fix activation without darwin-rebuild

As part of supporting the new nix-darwin activation style in https://github.com/nix-community/nh/pull/238, there was a breaking change made to use darwin-rebuild instead of calling the activation scripts directly. Unfortunately this change meant it was no longer meant to use nh as a darwin-rebuild replacement, as it would depend on darwin-rebuild itself. So if darwin-rebuild was disabled (e.g. by setting system.tools.enable = false), nh would fail to activate the system with an error like the following:

> Comparing changes
<<< /run/current-system
>>> /var/folders/h7/34fnxyx94lv_hkg1fmg658fr0000gn/T/nh-osusP10v/result
No version or selection state changes.
Closure size: 2055 -> 2055 (0 paths added, 0 paths removed, delta +0, disk usage +0B).
> Activating configuration
env: ‘/var/folders/h7/34fnxyx94lv_hkg1fmg658fr0000gn/T/nh-osusP10v/result/sw/bin/darwin-rebuild’: No such file or directory

This PR simply adds a check whether darwin-rebuild exists, and if not, falls back to the old behavior.

(If it's undesirable for this configuration to be supported then perhaps a warning should be added when activate is used, but I think compatibility should be kept at least for now, because it was done in a minor release, and breaking changes should only happen in major releases according to semver (which is why marked it as a bug fix, since it's fixing a regression from a semver point of view))

Tested in the following cases:

Ensure it invokes darwin-rebuild by default
Ensure it falls back to activate if darwin-rebuild doesn't exist
Ensure it also invokes activate-user if it's the non-deprecated version and it invoked activate

Sanity Checking

[x] I have updated the changelog as per my changes
[x] I have tested, and self-reviewed my code
Style and consistency
- [ ] I ran nix fmt to format my Nix code
- [x] I ran cargo fmt to format my Rust code
- [ ] I have added appropriate docunentation to new code
- [x] My changes are consistent with the rest of the codebase
Correctness
- [x] I ran cargo clippy and fixed any new linter warnings.
If new changes are particularly complex:
- [ ] My code includes comments in particularly complex areas to explain the logic
- [ ] I have documented the motive for those changes in the PR body or commit description.
Tested on platform(s)
- [ ] x86_64-linux
- [ ] aarch64-linux
- [ ] x86_64-darwin
- [x] aarch64-darwin

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Summary by CodeRabbit

Bug Fixes
- macOS activation is more resilient: if the standard rebuild tool isn’t available, activation falls back to direct activation and may perform a user-level activation step.
- Automatically selects appropriate activation method and elevation based on the environment.
- Provides clearer, more specific error messages for activation and user activation failures.
Documentation
- Added an Unreleased entry documenting the macOS activation fallback behavior.

Sep 10 '25 10:09 andre4ik3

Walkthrough

Detects darwin-rebuild presence and new vs legacy activation in src/darwin.rs, conditionally runs either darwin-rebuild activate or out_path/activate, optionally runs activate-user for legacy flow, adjusts elevation logic and error contexts. Updates CHANGELOG.md to document the Darwin fallback. No public API changes.

Changes

Cohort / File(s)	Summary
Documentation `CHANGELOG.md`	Added Unreleased -> Fixed entry documenting Darwin-specific fallback: when `darwin-rebuild` is absent, activation falls back to executing `activate` (and `activate-user` when applicable). Documentation-only.
Darwin activation logic `src/darwin.rs`	Added detection for `darwin-rebuild` (`has_darwin_rebuild`) and activation-style detection (`uses_new_activation`); select activation command conditionally (`darwin-rebuild activate` vs `out_path/activate`); compute `should_elevate` and apply elevation accordingly; run legacy `activate-user` when needed; improved error contexts and distinct failure messages. No public signatures changed.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  actor User
  participant CLI as CLI
  participant FS as Filesystem
  participant Sudo as Elevation
  participant Shell as System Shell

  User->>CLI: trigger activation
  CLI->>FS: stat `darwin-rebuild`
  alt darwin-rebuild exists
    CLI->>FS: inspect `activate-user` (detect new activation)
    CLI->>CLI: set uses_new_activation, should_elevate
    alt should_elevate
      CLI->>Sudo: run `darwin-rebuild activate`
      Sudo->>Shell: execute activation
    else
      CLI->>Shell: run `darwin-rebuild activate`
    end
  else darwin-rebuild missing
    CLI->>FS: inspect `out_path/activate` and `activate-user`
    CLI->>CLI: set uses_new_activation=false, should_elevate=true
    CLI->>Sudo: run `out_path/activate`
    Sudo->>Shell: execute activation
    opt legacy user activation (not uses_new_activation)
      CLI->>Shell: run `out_path/activate-user`
    end
  end

  note over CLI,Shell: Errors include contextual messages for activation vs user activation failures

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45–60 minutes

Pre-merge checks (2 passed, 1 warning)

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run `@coderabbitai generate docstrings` to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The title “darwin: fix activation without darwin-rebuild” accurately and concisely captures the primary change—restoring activation fallback on macOS when the darwin-rebuild tool is unavailable—without extraneous detail, making it clear to reviewers scanning the pull request history.

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Sep 10 '25 10:09 coderabbitai[bot]

Made the comment shorter so it passes the format CI check. Idk what all this AI stuff is I've never seen it before, not sure what I'm supposed to do with it exactly.

Sep 10 '25 10:09 andre4ik3

You may disregard the AI comments. I'm currently looking to see if it provides helpful comments that help guide my reviews (as the sole maintainer) or if it creates useless noise. Outlook not so good.

I'll take a review later today. Please feel free to remind me with a ping if I forget. Also CC @khaneliman as someone more aware of nix-darwin's internals.

Sep 10 '25 10:09 NotAShelf

I mean, it was a deliberate choice https://github.com/nix-community/nh/pull/238#discussion_r2003366687

Sep 15 '25 04:09 khaneliman

I see. So should I add a warning about it and mark that it will be removed in the future? (v5.0.0)

Sep 15 '25 06:09 andre4ik3