Add string_set buildfile value type

This exposes the std::set<std::string> type to buildfiles.

New functions:

  $size(<string-set>)

Subscript returns true if the value is present and false otherwise (so
it is mapped to std::set::contains()). For example:

set = [string_set] a b c

if ($set[b])
  ...

Note that append (+=) and prepend (=+) have the same semantics
(std::set::insert()). For example:

set = [string_set] a b
set += c b              # a b c
set =+ d b              # a b c d

Example of iteration:

set = [string_set] a b c
for k: $set
  ...

1 files changed, 38 insertions, 0 deletions

diff --git a/libbuild2/variable.ixx b/libbuild2/variable.ixx
index b8f80e3..a448cd8 100644
--- a/libbuild2/variable.ixx
+++ b/libbuild2/variable.ixx
@@ -853,6 +853,44 @@ namespace build2
       new (&v.data_) vector<pair<K, V>> (move (x));
   }
 
+  // set<T> value
+  //
+  template <typename T>
+  inline void value_traits<set<T>>::
+  assign (value& v, set<T>&& x)
+  {
+    if (v)
+      v.as<set<T>> () = move (x);
+    else
+      new (&v.data_) set<T> (move (x));
+  }
+
+  template <typename T>
+  inline void value_traits<set<T>>::
+  append (value& v, set<T>&& x)
+  {
+    if (v)
+    {
+      set<T>& p (v.as<set<T>> ());
+
+      if (p.empty ())
+        p.swap (x);
+      else
+        // Keys (being const) can only be copied.
+        //
+        p.insert (x.begin (), x.end ());
+    }
+    else
+      new (&v.data_) set<T> (move (x));
+  }
+
+  template <typename T>
+  inline void value_traits<set<T>>::
+  prepend (value& v, set<T>&& x)
+  {
+    append (v, move (x));
+  }
+
   // map<K, V> value
   //
   template <typename K, typename V>