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Zoltan Kochan c112b6106b feat(install): add --dry-run option (npm-style preview) (#12449 )

## Description

Adds a `--dry-run` option to `pnpm install` with **npm-style preview semantics**: it runs a full dependency resolution and reports what a real install **would** add/remove/update, but writes **nothing** to disk (no lockfile, no `node_modules`, no `.modules.yaml`, no workspace-state file) and **always exits 0**.

```
$ pnpm install --dry-run
Dry run complete. A real install would make the following changes (nothing was written to disk):

Importers
.
+ is-negative 1.0.0

Packages
+ is-negative@1.0.0
```

When the lockfile is already up to date it prints `Dry run complete. pnpm-lock.yaml is up to date; a real install would make no changes.`

Resolves https://github.com/pnpm/pnpm/issues/7340.

### Why this shape

An earlier attempt (#12270, now closed) implemented `--dry-run` as `--frozen-lockfile --lockfile-only` — i.e. a fail-on-drift *lockfile validator*. That collides with the well-established meaning of `--dry-run` across npm/yarn ("preview, never fail") and duplicated existing behaviour (`pnpm install --frozen-lockfile --lockfile-only` already does that). This PR implements the intuitive preview meaning instead.

### How it works (pnpm)

- Reuses the existing `lockfileCheck` callback (resolve fully, skip the lockfile write, hand back the before/after wanted lockfile) plus `lockfileOnly` (skip `node_modules`, the workspace-state file, and metadata-cache writes).
- The frozen/headless fast path is disabled whenever `lockfileCheck` is set, so a check-only install always resolves and never materialises anything.
- The before/after lockfiles are diffed (reusing the dedupe diff engine, now exported as `calcDedupeCheckIssues`) and rendered into the report.
- `--dry-run` with a configured pnpr server is rejected (that path resolves/links through the server).

### Pacquet

Ported in the second commit — `pacquet install --dry-run` forces the fresh-resolve path, skips every write (a new `dry_run` flag on `InstallWithFreshLockfile` skips the `pnpm-lock.yaml` save), and a new `dry_run` module diffs the existing lockfile against the freshly-resolved one and prints the same report.

2026-06-16 19:12:56 +02:00