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Add an alternative spelling: lifetime vs. dependsOn. #8

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Jul 11, 2024
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Add an alternative spelling: lifetime vs. dependsOn. #8

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12 changes: 12 additions & 0 deletions proposals/NNNN-lifetime-dependency.md
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Expand Up @@ -896,6 +896,18 @@ func f(arg1: borrow ContiguousArray<Int>) -> borrow(arg1) Span<Int>
```
This was changed after we realized that there was in practice almost always a single viable semantic for any given situation, so the additional refinement seemed unnecessary.

The currently proposed `dependsOn` spelling was chosen to convey the direction of the dependence, as in:

func foo(a: A, b: B) -> dependsOn(a) R

This does, however, introduce compound keyword. Alternatively, we could use a simpler `lifetime` keyword, which better matches the feature description. The general syntax would then be:

> **lifetime**(*target*: [scoped] *source*)

APIs with ambiguous depenencies would then typically be spelled:

func foo(a: A, b: B) -> lifetime(a) R

### dependsOn(unchecked) to disable lifetime dependence checking

A `dependsOn(unchecked)` annotation could allow programmers to disable lifetime dependence checking for a function result or argument. For example, the programmer may want to compose a nonescapable result from an immortal value that isn't visible to the compiler:
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