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[docs] Document _unsafelyUnwrappedUnchecked

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Michael Ilseman
2017-08-07 13:22:20 -07:00
parent a078f7f715
commit c6da08a384
1 changed files with 8 additions and 3 deletions
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11
docs/StandardLibraryProgrammersManual.md
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@@ -22,7 +22,12 @@ TBD
## Internals ## Internals
TBD ### Internal Functionality and SPI
#### Optionals
Optionals can be unwrapped with `!`, which triggers a trap on nil. Alternatively, they can be `.unsafelyUnwrapped()`, which will check and trap in debug builds of user code. Internal to the standard library is `._unsafelyUnwrappedUnchecked()` which will only check and trap in debug builds of the standard library itself. These correspond directly with `_precondition`, `_debugPrecondition`, and `_sanityCheck`. See [that section](#precondition) for details.
### Builtins ### Builtins
@@ -65,7 +70,7 @@ return
This should be rarely used. It informs the SIL optimizer that any code dominated by it should be treated as the innermost loop of a performance critical section of code. It cranks optimizer heuristics to 11. Injudicious use of this will degrade performance and bloat binary size. This should be rarely used. It informs the SIL optimizer that any code dominated by it should be treated as the innermost loop of a performance critical section of code. It cranks optimizer heuristics to 11. Injudicious use of this will degrade performance and bloat binary size.
#### `_precondition`, `_debugPrecondition`, and `_sanityCheck` #### <a name="precondition"></a>`_precondition`, `_debugPrecondition`, and `_sanityCheck`
These three functions are assertions that will trigger a run time trap if violated. These three functions are assertions that will trigger a run time trap if violated.
@@ -81,7 +86,7 @@ A call to `_fixLifetime` is considered a use of its argument, meaning that the a
```swift ```swift
var x = ... var x = ...
defer { _fixLifetime(x) } // Guarentee at least lexical lifetime for x defer { _fixLifetime(x) } // Guarantee at least lexical lifetime for x
let theBits = unsafeBitCast(&x, ...) let theBits = unsafeBitCast(&x, ...)
... // use of theBits in ways that may outlive x if it weren't for the _fixLifetime call ... // use of theBits in ways that may outlive x if it weren't for the _fixLifetime call
``` ```