[swift-evolution] SPM support for branches and commits

[Said Sikira]

Hi,

Our code is almost always developed and pushed in small incremental
changes. When we implement critical amount of changes in our code, we push
new version.

When adding dependencies to Package.swift file, we supply their repository
url and version we want to use. However, differentiating code only by it’s
version is not enough in some cases. When writing new code features, we use
branches. They enable all the mechanisms one needs to create, test and
deploy new features without polluting production environment.

I think that Swift Package Manager should have support for branches (and
commits). There are several reasons why this feature would greatly improve
developer workflow:

   1. Writing new features. Being able to specify branch in Package.swift
   would make creating and testing new features easier. You wouldn’t need to
   push new version to be able to use it in your Swift program. You would just
   specify the branch you’re working on.
   2. Differentiating between new Swift versions. This problem comes from
   the current Swift 2.2 -> Swift 3.0 migration. Many framework developers use
   specific branches (swift–3, swift3.0) to work on migration of their API’s
   to Swift 3. However, you can’t use them in your Swift projects because they
   don’t live in the master branch in the repository. I’m sure this will also
   happen when Swift 3 starts migration to the Swift 4, until ABI becomes
   stable.

SPM should also have support for specifying commits. Specifying which
commit you want to use in your project dependency is not always a good
idea, but it’s necessary in some cases.

This shouldn’t be very hard to implement. We would need to update
PackageDescription and Get source from swift-package-manager repository to
enable specifying branches or commits. Pulling the branch source would just
be another parameter in git instruction.

Example:

// Specifying branch
let package = Package(
  name: "SomePackage",
  dependencies: [
    .Package(url: "https://repo-source.git", branch: "new-feature")
  ]
)

// Specifying commits
let package = Package(
  name: "SomePackage",
  dependencies: [
    .Package(url: "https://repo-source.git", commit:
"c336664020v4f94ed78cbe7447a39ae5ca0b6c11")
  ]
)

What are your thoughts on this subject?
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