swift-mirror/userdocs/diagnostics/performance-hints.md

Performance Hints (PerformanceHints)

Overview

The PerformanceHints diagnostic group provides opt-in guidance for identifying language constructs and code patterns that carry non-obvious runtime performance costs. The diagnostics are intended to help developers writing performance-critical code make informed decisions about the abstractions they use and to surface hidden costs that may not be apparent from the source code alone to a non-expert Swift programmer.

This diagnostic group is off by default. It can be enabled as warnings with -Wwarning PerformanceHints, or escalated to errors with -Werror PerformanceHints.

Sub-groups

The PerformanceHints group contains the following sub-groups, each of which can also be enabled or controlled independently:

• doc:existential-type: Warns on use of existential types (any Protocol), which incur heap allocation, reference counting, exclusivity enforcement, and dynamic dispatch overhead.

• doc:return-type-implicit-copy: Warns when functions or closures return Array or Dictionary values, which leads to implicit heap-allocated copies of the collections.

• doc:untyped-throws: Warns on untyped throws declarations, where the use of any Error incurs a heap allocation on each throw.

For example, to enable only the UntypedThrows checks as warnings:

-Wwarning UntypedThrows

Or to enable all performance hints as errors but keep UntypedThrows as a warning:

-Werror PerformanceHints -Wwarning UntypedThrows