ManualOwnership: introduce 'SemanticCopies' diagnostic group

This includes documentation to explain how to understand
these diagnostics.

Author: kavon

include/swift/AST/DiagnosticGroups.def

@@ -52,6 +52,7 @@ GROUP(ErrorInFutureSwiftVersion, "error-in-future-swift-version")
 GROUP(ExclusivityViolation, "exclusivity-violation")
 GROUP(ExistentialAny, "existential-any")
 GROUP(ExistentialMemberAccess, "existential-member-access-limitations")
+GROUP(SemanticCopies, "semantic-copies")
 GROUP(ImplementationOnlyDeprecated, "implementation-only-deprecated")
 GROUP(IsolatedConformances, "isolated-conformances")
 GROUP(MemberImportVisibility, "member-import-visibility")

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 +1,9 @@
+# Semantic Copies
+
+Values are implicitly copied for you in Swift, wherever they are needed to satisfy the language semantics.
+The Swift optimizer goes to great lengths to eliminate these copies. 
+But for performance-oriented programmers, it's helpful to know where these copies happen so they can change their program's style to avoid copies entirely.
+
+## Variable Bindings
+
+TODO explain why variables require a copy.