cmLinkItem: Convert to a "sum type" over a string and target pointer

Avoid exposing the item name implicitly as std::string.  When the item
is a target, avoid storing a second copy of its name.

Most link item construction is paired with calls to `FindTargetToLink`
to get the possible target pointer.  Rename these methods to
`ResolveLinkItem` and refactor them to construct the entire item.

1 files changed, 72 insertions, 0 deletions

diff --git a/Source/cmLinkItem.cxx b/Source/cmLinkItem.cxx
new file mode 100644
index 0000000..69b6821
--- /dev/null
+++ b/Source/cmLinkItem.cxx
@@ -0,0 +1,72 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#include "cmLinkItem.h"
+
+#include "cmGeneratorTarget.h"
+
+#include <utility> // IWYU pragma: keep
+
+cmLinkItem::cmLinkItem()
+  : String()
+  , Target(nullptr)
+{
+}
+
+cmLinkItem::cmLinkItem(std::string const& n)
+  : String(n)
+  , Target(nullptr)
+{
+}
+
+cmLinkItem::cmLinkItem(cmGeneratorTarget const* t)
+  : String()
+  , Target(t)
+{
+}
+
+std::string const& cmLinkItem::AsStr() const
+{
+  return this->Target ? this->Target->GetName() : this->String;
+}
+
+bool operator<(cmLinkItem const& l, cmLinkItem const& r)
+{
+  // Order among targets.
+  if (l.Target && r.Target) {
+    return l.Target < r.Target;
+  }
+  // Order targets before strings.
+  if (l.Target) {
+    return true;
+  }
+  if (r.Target) {
+    return false;
+  }
+  // Order among strings.
+  return l.String < r.String;
+}
+
+bool operator==(cmLinkItem const& l, cmLinkItem const& r)
+{
+  return l.Target == r.Target && l.String == r.String;
+}
+
+std::ostream& operator<<(std::ostream& os, cmLinkItem const& item)
+{
+  return os << item.AsStr();
+}
+
+cmLinkImplItem::cmLinkImplItem()
+  : cmLinkItem()
+  , Backtrace()
+  , FromGenex(false)
+{
+}
+
+cmLinkImplItem::cmLinkImplItem(cmLinkItem item, cmListFileBacktrace const& bt,
+                               bool fromGenex)
+  : cmLinkItem(std::move(item))
+  , Backtrace(bt)
+  , FromGenex(fromGenex)
+{
+}