Merge pull request swiftlang#83873 from owenv/owenv/testable-docs

Document the 'module not testable' diagnostic

Author: owenv

include/swift/AST/DiagnosticGroups.def

@@ -53,6 +53,7 @@ GROUP(ImplementationOnlyDeprecated, "implementation-only-deprecated")
 GROUP(IsolatedConformances, "isolated-conformances")
 GROUP(MemberImportVisibility, "member-import-visibility")
 GROUP(MissingModuleOnKnownPaths, "missing-module-on-known-paths")
+GROUP(ModuleNotTestable, "module-not-testable")
 GROUP(ModuleVersionMissing, "module-version-missing")
 GROUP(MultipleInheritance, "multiple-inheritance")
 GROUP(MutableGlobalVariable, "mutable-global-variable")

include/swift/AST/DiagnosticsSema.def

@@ -1229,7 +1229,7 @@ ERROR(imported_decl_is_wrong_kind_typealias,none,
 ERROR(ambiguous_decl_in_module,none,
       "ambiguous name %0 in module %1", (Identifier, Identifier))
 
-ERROR(module_not_testable,Fatal,
+GROUPED_ERROR(module_not_testable,ModuleNotTestable,Fatal,
       "module %0 was not compiled for testing", (Identifier))
 
 ERROR(module_not_compiled_for_private_import,none,

userdocs/diagnostics/diagnostic-descriptions.md

@@ -24,6 +24,7 @@ presented specially within your IDE of choice. See below for the full list of th
 - <doc:string-interpolation-conformance>
 - <doc:isolated-conformances>
 - <doc:error-in-future-swift-version>
+- <doc:module-not-testable>
 - <doc:multiple-inheritance>
 - <doc:nominal-types>
 - <doc:property-wrapper-requirements>

userdocs/diagnostics/module-not-testable.md

@@ -0,0 +1,5 @@
+# Module not testable
+
+Modules imported using `@testable import` must have been compiled using the `-enable-testing` compiler flag.
+
+`@testable import` allows clients of a module to access `internal` API when writing tests. A `@testable import` will report an error if the imported module was not compiled with the `-enable-testing` compiler flag. Use of `-enable-testing` is recommended only in debug builds or other builds not intended for production use.