Design: Support rules_swift_package_manager usage in non-root modules

[luispadron]

Problem

Depending on non-root bzlmod modules that use rules_spm / rules_swift_package_manager currently causes collisions when multiple modules declare the same swiftpkg_* repositories. This blocks transitive composition and prevents SwiftPM-style single-version resolution across modules.

Goals

• Run SwiftPM resolution once for the combined graph of all participating bzlmod modules.
• Eliminate swiftpkg_* collisions by generating each resolved package repo exactly once.
• Match SwiftPM’s model: single-version resolution with standard conflict detection.
• Prefer relying on bzlmod extension_metadata for resolution persistence; keep Package.resolved as an optional/debug fallback.

Non-goals (initial)

• Multi-version resolution (per-module independent resolution with namespacing).
• Forcing module authors to change their Package.swift layout beyond what rules already require.
• Advanced checkout dedup beyond SwiftPM’s built-in behavior.

---

Approach: merge by aggregating package roots (not dependency lists)

Instead of merging dependency requirements from each module, the extension merges package roots.

For each participating bzlmod module M, the extension locates a SwiftPM package root directory:

• pkg_root(M) contains a Package.swift.

The extension then synthesizes a single "merged" Package.swift that depends on each root via .package(path:). SwiftPM reads the dependency constraints from each local root’s manifest and computes a unified solution.

Why this works

• The merged manifest acts as a single SwiftPM package.
• Local root manifests contribute their constraints naturally (URLs, versions, branches).
• SwiftPM resolves the union of constraints into one set of pinned versions (or fails).

---

Inputs

Per module M participating in the extension:

• pkg_root(M): path to a directory containing Package.swift.
  • May be authored by the module or generated by the extension (e.g., gazelle-assisted flows).
• origin(M): metadata used for diagnostics (module name/version, repo label).

---

Generated artifacts

The extension creates a single resolver repository (e.g. @rules_spm_resolver) containing:

1. Merged manifest: Package.swift

   • Lists all module roots as local packages using .package(path:).
   • Paths should be relative when feasible for better cacheability.

2. Resolution representation

   • Preferred: bzlmod extension_metadata stores the resolved mapping:
     • identity -> {source_url, version|revision, checksum/digest (if applicable)}
     • plus a solution digest for change detection.
   • Fallback / debug: emit Package.resolved (configurable), primarily for troubleshooting.

---

Merged Package.swift shape

Deterministic ordering and minimal structure:

• Sort roots by stable key (module name/version or canonical label).
• Keep a single dummy target; the goal is resolution, not building.

Example of a "merged" package:

import PackageDescription

let package = Package(
  name: "swiftpkg",
  dependencies: [
    .package(path: "../External"),
    .package(path: "../Client"),
  ],
)

[cgrindel]

Prefer relying on bzlmod extension_metadata for resolution persistence; keep Package.resolved as an optional/debug fallback.

Can you elaborate on this goal? I don't follow.

[cgrindel]

@luispadron Thank you for taking on this effort!