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Artem Chikin
44 lines
1.5 KiB
Markdown
44 lines
1.5 KiB
Markdown
# Performance Hints (PerformanceHints)
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## Overview
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The `PerformanceHints` diagnostic group provides opt-in guidance for identifying
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language constructs and code patterns that carry non-obvious runtime performance
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costs. The diagnostics are intended to help developers writing
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performance-critical code make informed decisions about the abstractions they
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use and to surface hidden costs that may not be apparent from the source code
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alone to a non-expert Swift programmer.
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This diagnostic group is **off by default**. It can be enabled as warnings with
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`-Wwarning PerformanceHints`, or escalated to errors with `-Werror
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PerformanceHints`.
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## Sub-groups
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The `PerformanceHints` group contains the following sub-groups, each of which
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can also be enabled or controlled independently:
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- <doc:existential-type>: Warns on use of existential types (`any Protocol`),
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which incur heap allocation, reference counting, exclusivity enforcement, and
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dynamic dispatch overhead.
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- <doc:return-type-implicit-copy>: Warns when functions or closures return
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`Array` or `Dictionary` values, which leads to implicit heap-allocated copies
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of the collections.
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- <doc:untyped-throws>: Warns on untyped `throws` declarations, where the use of
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`any Error` incurs a heap allocation on each `throw`.
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For example, to enable only the `UntypedThrows` checks as warnings:
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```
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-Wwarning UntypedThrows
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```
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Or to enable all performance hints as errors but keep `UntypedThrows` as a
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warning:
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```
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-Werror PerformanceHints -Wwarning UntypedThrows
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```
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