ManualOwnership: introduce 'SemanticCopies' diagnostic group

This includes documentation to explain how to understand
these diagnostics.

Author: kavon

include/swift/AST/DiagnosticGroups.def

@@ -73,6 +73,7 @@ GROUP(ProtocolTypeNonConformance, "protocol-type-non-conformance")
 GROUP(RegionIsolation, "region-isolation")
 GROUP(ResultBuilderMethods, "result-builder-methods")
 GROUP(ReturnTypeImplicitCopy, "return-type-implicit-copy")
+GROUP(SemanticCopies, "semantic-copies")
 GROUP(SendableClosureCaptures, "sendable-closure-captures")
 GROUP(SendableMetatypes, "sendable-metatypes")
 GROUP(SendingClosureRisksDataRace, "sending-closure-risks-data-race")

include/swift/AST/DiagnosticsSIL.def

@@ -433,13 +433,13 @@ ERROR(wrong_linkage_for_serialized_function,none,
       "function has wrong linkage to be called from %0", (StringRef))
 NOTE(performance_called_from,none,
       "called from here", ())
-ERROR(manualownership_copy,none,
-      "explicit 'copy' required here; please report this vague diagnostic as a bug", ())
-ERROR(manualownership_copy_happened,none,
+GROUPED_WARNING(manualownership_copy,SemanticCopies,none,
+      "implicit 'copy' happens here; please report this vague diagnostic as a bug", ())
+GROUPED_WARNING(manualownership_copy_happened,SemanticCopies,none,
       "accessing %0 may produce a copy; write 'copy' to acknowledge or 'consume' to elide", (Identifier))
-ERROR(manualownership_copy_demanded,none,
+GROUPED_WARNING(manualownership_copy_demanded,SemanticCopies,none,
       "independent copy of %0 is required here; write 'copy' to acknowledge or 'consume' to elide", (Identifier))
-ERROR(manualownership_copy_captured,none,
+GROUPED_WARNING(manualownership_copy_captured,SemanticCopies,none,
       "closure capture of '%0' requires independent copy of it; write [%0 = copy %0] in the closure's capture list to acknowledge", (StringRef))
 
 // 'transparent' diagnostics

test/SIL/manual_ownership.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend %s -emit-sil -verify \
+// RUN:   -Werror SemanticCopies \
 // RUN:   -enable-experimental-feature ManualOwnership
 
 // REQUIRES: swift_feature_ManualOwnership

userdocs/diagnostics/semantic-copies.md

@@ -0,0 +1,3 @@
+# Semantic Copies (Experimental Diagnostics)
+
+TODO explain