…slice 4d) (pnpm#494)

* feat(package-manager): prev_graph diff from current lockfile (pnpm#438 slice 4d)

`lockfile_to_hoisted_dep_graph` now takes an optional
`current_lockfile: Option<&Lockfile>` and populates
`result.prev_graph` from a second walk over that lockfile.
Ports upstream's wrapper at
installing/deps-restorer/src/lockfileToHoistedDepGraph.ts.

When the current lockfile is `Some(lf)` with a non-empty
`packages:` map, the second walk runs with `force: true` and
`skipped: BTreeSet::new()`. Force matters: an orphan that
landed under the previous install but would now fail
installability (e.g., the host changed platforms) still surfaces
in `prev_graph` so Slice 5's linker can find and rimraf the
stale directory. Empty skipped matters: the previous install's
own filter set is unrelated to "which directories still exist
on disk."

API change

- The public `lockfile_to_hoisted_dep_graph(lockfile, opts)`
  signature gains a middle `current_lockfile: Option<&Lockfile>`
  argument. Single existing caller (the tests) updated to pass
  `None`. Splits the previous body into a private
  `build_dep_graph` helper that the wrapper calls once or twice
  depending on whether a current lockfile is present.

prev_graph shape

- `None` when no current lockfile is supplied, or when the
  supplied lockfile has no `packages:` map (a brand-new install
  in progress). Mirrors upstream's `prevGraph = {}` fallback —
  pacquet uses `None` rather than an empty map, but the linker
  treats both the same.
- `Some(graph)` otherwise, keyed by directory just like the
  wanted-lockfile graph.

Tests

- `prev_graph_none_when_current_lockfile_absent` — no current
  lockfile → `prev_graph` is `None`, wanted graph still produced.
- `prev_graph_none_when_current_lockfile_has_no_packages` —
  current lockfile with `packages: None` → `prev_graph` is
  `None`.
- `prev_graph_contains_orphan_from_current_only_lockfile` —
  package in current but not wanted appears in `prev_graph`,
  not in `graph`.
- `prev_graph_includes_orphan_even_when_now_incompatible` —
  darwin-only orphan in the current lockfile, host is linux,
  wanted lockfile is empty: `prev_graph` still contains it
  (proves `force: true` overrides the installability check),
  and the wanted-walk's `skipped` stays empty (proves the
  prev-walk's skipped set is independent).

All 4 new tests pass alongside the 15 walker tests from
4a-4c.

* fix(package-manager): collapse empty current packages to None for prev_graph (pnpm#494 review)

`current.packages.is_some()` matched `Some(empty_map)` too,
causing 4d to do an unnecessary second walk and return
`Some(empty_graph)` for a case the doc-comment described as
`None`. Tighten the guard to require a non-empty `packages` map.

Pacquet uses `Option<DependenciesGraph>` for `prev_graph`
(upstream uses an always-present `DependenciesGraph` with `{}`
for the no-current case). The no-packages and empty-packages
cases both produce the same observable behavior — "no orphans
to consider" — so they should share one representation rather
than be inconsistent. The doc-comment's stated contract was
`None`; the code now matches.

Added `prev_graph_none_when_current_lockfile_has_empty_packages`
to pin the empty-map case.

Caught by Coderabbit on pnpm#494.

* style(package-manager): rustfmt the 4d follow-up

…slice 4d) (pnpm#494)

* feat(package-manager): prev_graph diff from current lockfile (pnpm#438 slice 4d)

`lockfile_to_hoisted_dep_graph` now takes an optional
`current_lockfile: Option<&Lockfile>` and populates
`result.prev_graph` from a second walk over that lockfile.
Ports upstream's wrapper at
installing/deps-restorer/src/lockfileToHoistedDepGraph.ts.

When the current lockfile is `Some(lf)` with a non-empty
`packages:` map, the second walk runs with `force: true` and
`skipped: BTreeSet::new()`. Force matters: an orphan that
landed under the previous install but would now fail
installability (e.g., the host changed platforms) still surfaces
in `prev_graph` so Slice 5's linker can find and rimraf the
stale directory. Empty skipped matters: the previous install's
own filter set is unrelated to "which directories still exist
on disk."

API change

- The public `lockfile_to_hoisted_dep_graph(lockfile, opts)`
  signature gains a middle `current_lockfile: Option<&Lockfile>`
  argument. Single existing caller (the tests) updated to pass
  `None`. Splits the previous body into a private
  `build_dep_graph` helper that the wrapper calls once or twice
  depending on whether a current lockfile is present.

prev_graph shape

- `None` when no current lockfile is supplied, or when the
  supplied lockfile has no `packages:` map (a brand-new install
  in progress). Mirrors upstream's `prevGraph = {}` fallback —
  pacquet uses `None` rather than an empty map, but the linker
  treats both the same.
- `Some(graph)` otherwise, keyed by directory just like the
  wanted-lockfile graph.

Tests

- `prev_graph_none_when_current_lockfile_absent` — no current
  lockfile → `prev_graph` is `None`, wanted graph still produced.
- `prev_graph_none_when_current_lockfile_has_no_packages` —
  current lockfile with `packages: None` → `prev_graph` is
  `None`.
- `prev_graph_contains_orphan_from_current_only_lockfile` —
  package in current but not wanted appears in `prev_graph`,
  not in `graph`.
- `prev_graph_includes_orphan_even_when_now_incompatible` —
  darwin-only orphan in the current lockfile, host is linux,
  wanted lockfile is empty: `prev_graph` still contains it
  (proves `force: true` overrides the installability check),
  and the wanted-walk's `skipped` stays empty (proves the
  prev-walk's skipped set is independent).

All 4 new tests pass alongside the 15 walker tests from
4a-4c.

* fix(package-manager): collapse empty current packages to None for prev_graph (pnpm#494 review)

`current.packages.is_some()` matched `Some(empty_map)` too,
causing 4d to do an unnecessary second walk and return
`Some(empty_graph)` for a case the doc-comment described as
`None`. Tighten the guard to require a non-empty `packages` map.

Pacquet uses `Option<DependenciesGraph>` for `prev_graph`
(upstream uses an always-present `DependenciesGraph` with `{}`
for the no-current case). The no-packages and empty-packages
cases both produce the same observable behavior — "no orphans
to consider" — so they should share one representation rather
than be inconsistent. The doc-comment's stated contract was
`None`; the code now matches.

Added `prev_graph_none_when_current_lockfile_has_empty_packages`
to pin the empty-map case.

Caught by Coderabbit on pnpm#494.

* style(package-manager): rustfmt the 4d follow-up