Provide a way to generate files that only import `FoundationEssentials` and conditionalize all `Foundation` imports

[FranzBusch]

FoundationEssentials allows to import just a small subset of Foundation on non-Darwin platforms. This has a great impact on binary size. The current recommended way to do this is this pattern:

#if canImport(FoundationEssentials)
import FoundationEssentials
#else
import Foundation
#endif

We need to update our generator to support this and audit all of our current Foundation imports. As a first step it would be good to understand if we rely on anything from Foundation that is not in FoundationEssentials.

[thomasvl]

Does a Swift version also come into play? Or does FoundationEssentials exist back to 5.8?

[FranzBusch]

The canImport should be enough here. Since the module doesn't exist in previous versions.

[thomasvl]

Is there a good listing of what is in/out of FoundationEssentials? https://github.com/swiftlang/swift-foundation?tab=readme-ov-file#swift-foundation just lists a few and ends with and more which isn't that helpful.

My initial thought is we probably only make use of Data generally (and ContinuousBytes). Then Duration and Timestamp might make use of things for some of their bridging. So maybe it wouldn't be much to move the runtime and generated code over?

[FranzBusch]

I just landed this PR swiftlang/swift-foundation#1178 . So hopefully the documentation shows up on the Swift Package Index soon. This should help you see what types are part of FoundationEssentials.

[thomasvl]

Looks like docs are now live: https://swiftpackageindex.com/swiftlang/swift-foundation/main/documentation/foundationessentials

[thomasvl]

Will this count as a breaking change since someone importing SwiftProtobuf or a module that had generated proto code, indirectly was getting Foundation?

[thomasvl]

Did a quick search/replace on the runtime to try the saw, and a few things trip it up, so I think that means, we'd have to find a way to replace those things, otherwise, there wouldn't be any real wins. Failure: https://github.com/apple/swift-protobuf/actions/runs/13506421799/job/37736916317?pr=1758

Issues:

• DoubleParser.swift : strtod
• BinaryDelimited.swift : InputStream, OutputStream