[SE-NNNN] Allow Property Wrappers on Let Declarations

[amritpan]

This is the formal proposal for allowing let declared property wrappers in Swift.

Implementation: [Sema/SILGen] Allow property wrappers on let declarations #62342
Review: Pitch

[xedin]

@amritpan I think the proposal needs to address nonmutating set discussion that happened during the pitch either in Detailed Design or in Alternatives Considered.

[xwu]

I've got a bunch of editorial suggestions below, but also a number of substantive comments.

The big one is that the examples in the "Detailed design" section don't seem to fulfill what the proposal template is asking for in that section, but fortunately you've got a great example in SE-0258 as to how to approach this, and I think a lot of the questions you answered in the pitch thread supply the material you would need to fill out that section.

In reviewing review thread comments, did you ever get around to looking into @ktoso's question about @TaskLocal?

[xwu]

Hi @amritpan, just wanted to check in about how things are going. I'm hoping my feedback is straightforward enough that it shouldn't take much to incorporate, but please let me know if there's something I can do to help facilitate.

[amritpan]

Thank you @xwu! I got sidetracked with unrelated work but am prioritizing this for this week. I'll likely have some questions!

[xwu]

Sure thing!

[xedin]

@xwu I think we are finally ready!

[xwu]

Initial editorial comments; I have some thoughts on the overall structure (what text goes in what section) which I'll suggest after these changes.

[xwu]

Suggested change

	[SE-0258 Property Wrappers](https://github.com/apple/swift-evolution/blob/main/proposals/0258-property-wrappers.md) models both mutable and immutable variants of a property wrapper but does not allow wrapped properties explicitly marked as a `let`. This proposal builds on the original vision by expanding property wrapper usage to include let declared properties with a wrapper attribute.
	[SE-0258: Property Wrappers](https://github.com/apple/swift-evolution/blob/main/proposals/0258-property-wrappers.md) does not allow wrapped properties declared with `let`. This proposal builds on the original vision by expanding property wrapper usage to include such properties.

I'm not sure I understand what's meant by "model[ing] both mutable and immutable variants" here, but it doesn't seem necessary to understand that in order to understand the proposal, so my suggestion is to take it out.

I didn't think there were such things as implicit let declarations, but if there were I didn't think SE-0258 would allow property wrappers there either, so my suggestion is to strike "explicitly"; the rest are edits for clarity and brevity.

[amritpan]

SE-0258 models immutable and mutable variants here. I wanted to make the distinction that we could manually set up property wrappers to behave like let wrapped declarations but this proposal would rid the need for it.

[xedin]

@xwu Is this new in review process?

[xwu]

Hi @amritpan, thanks for all your hard work and patience! From an editorial standpoint, this looks pretty much ready to go.

Now that WWDC is over, we discussed in the language steering group and folks had some thoughts about other features on let declarations that may need to be considered in parallel, so they'll comment with their thoughts over the in the forums. The conclusion of that discussion may well be that this proposal goes forward as-is into review and not too long from now, but it was felt that there's some exploration there to be had.

[amritpan]

@xwu Thank you! Very curious about those other features so I am looking forward to the review discussion!