[Lex] Reject standalone dollars as identifiers

[CodaFi]

What's in this pull request?

Fixes a case where we accidentally accepted a single dollar as an identifier. This patch supersedes #3004.

Resolved bug number: (SR-1661)

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[CodaFi]

@swift-ci please smoke test.

[CodaFi]

@swift-ci please smoke test.

[CodaFi]

@swift-ci please smoke test Linux platform.

[CodaFi]

Let's see if we can't get Linux green again?

@swift-ci please smoke test Linux platform.

[CodaFi]

I want to land this clean

@swift-ci please smoke test Linux platform.

[jtbandes]

Should this be included in the Swift 3 branch? cc @tkremenek

[lattner]

This is source breaking, right?

[lattner]

Source breaking changes should wait until we have a framework to support them. Also, $ is commonly used by dollar.swift, so I'd suggest a targeted fixme that rewrites the $ to a valid identifier, e.g. "💲" (yes, I'm serious). This will allow the migrator Swift 3->4 migrator to handle the framework.

[jtbandes]

Related proposal that's been waiting for a long time to be merged/closed: swiftlang/swift-evolution#354

[jtbandes]

(I'm working with others on a proposal to refine the operator/identifier characters — there's been some interest in including $ as an operator, too, but I think our proposal will leave it as is: invalid except as part of $0 or LLDB variables $R1)

[jtbandes]

What does "a framework to support them" mean? Aren't source-breaking changes encouraged to happen ASAP (3.0.1 or 3.1) so we can avoid them in the future?

[jrose-apple]

We already passed that deadline: it was 3.0. At this point source-breaking changes need to be justified and will be handled on a case-by-case basis.

[jrose-apple]

…but the process for doing this hasn't been decided and written up yet.

[jtbandes]

I see. And is the swift 3 branch still being used for 3.1? (Sorry for hijacking this PR thread...)

[jrose-apple]

Most likely no, we will rebranch there. (Which may mean downgrading this to a warning.)

[lattner]

We discussed this topic today in the core team. Here are some specific requests:

1. Please change this to reject $ with a fixit to $ instead of using the emoji I suggested above.
2. Please conditionalize rejection on Swift 4 mode. This is now a thing, you can see an example of it being used here: 12fb0ba

As to deciding the fate of single $ identifiers, I'm working with a swift-evolution proposal to run this topic by the community. That could possibly lead to backtracking on our rejection of $, but in the meantime we should still reject it the right way. Thanks @CodaFi!

[lattner]

Sigh, please fixit this from $ to $... i.e. a dollar sign in backticks. :-)

[lattner]

Maybe this is how you spell it in markdown?? $

[lattner]

OMG, I win! :-)

[rintaro]

$ spelled as $ spelled as `$` spelled as `` `$` `` ...