QEP 1: Implicitly-Scoped Qubit Allocation

[bamarsha]

This issue tracks the implementation of QEP 1: Implicitly-Scoped Qubit Allocation. The feature branch is feature/inline-qubit-scope.

• Deprecate using and borrowing keywords and replace with use and borrow (Add "use" and "borrow" keywords #804)
• Allow use and borrow statements to be followed by either a block { ... } or a semicolon ; (Allow using and borrowing statements to use the current scope #803)
• Deprecate parentheses around block statement headers (Make optional and deprecate parentheses in block statements #791)

[cgranade]

Thinking about using → use and borrowing → borrow a bit more, it feels a bit awkward to use present participles to represent an action taken by an operation, given that we elsewhere use finite verb phrases to directly indicate the tense and subject of that verb. Concretely, the gerund "using" on its own doesn't have a tense to make it clear when something is used. Moving to "use" makes that tense explicit, which strikes me as a very good thing for readability.

[tcNickolas]

When this change becomes breaking, will we offer the users a tool to mass-migrate to the new syntax automatically, like we did in the past (I think it was around 0.3 version)? I've seen a mention of a code action in the proposal, but migrating a sizeable code base would require a lot of effort without such tool.

[bamarsha]

We've been discussing using QsFmt to automatically update deprecated syntax across an entire folder of Q# files. It needs some more work to be able to do that, though.