Introduce structured build variables in Crosstool

This cl adds a 3rd type of build variable - structs. Structs have fields, which
can hold any build variable type (including structs). In the CROSSTOOl, the
fields are accessed by the dot-notation, e.g.:

    flag_group {
      iterate_over: "libraries_to_link
      flag_group {
        iterate_over: "libraries_to_link.libraries"
        flag: "-L%{libraries_to_link.libraries.directory}"
      }
    }

As a memory optimization, we also add StructureSequences. These save us from
the overhead of individual StructureValue objects.

--
MOS_MIGRATED_REVID=138851774

1 files changed, 9 insertions, 0 deletions

diff --git a/src/main/protobuf/crosstool_config.proto b/src/main/protobuf/crosstool_config.proto
index daff025585..739ec3cf31 100644
--- a/src/main/protobuf/crosstool_config.proto
+++ b/src/main/protobuf/crosstool_config.proto
@@ -63,6 +63,15 @@ message CToolchain {
   // ... will get expanded to -I /to/path1 -I /to/path2 ... for each
   // include_path /to/pathN.
   //
+  // To expand a variable of structure type, use dot-notation, e.g.:
+  //    flag_group {
+  //      iterate_over: "libraries_to_link"
+  //      flag_group {
+  //        iterate_over: "libraries_to_link.libraries"
+  //        flag: "-L%{libraries_to_link.libraries.directory}"
+  //      }
+  //    }
+  //
   // Flag groups can be nested; if they are, the flag group must only contain
   // other flag groups (no flags) so the order is unambiguously specified.
   // In order to expand a variable of nested lists, 'iterate_over' can be used.