/usr/lib/gcc/x86_64-linux-gnu/5/plugin/include/all-tree.def is in gcc-5-plugin-dev 5.5.0-12ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #include "tree.def"
END_OF_BASE_TREE_CODES
#include "c-family/c-common.def"
#include "ada/gcc-interface/ada-tree.def"
#include "cp/cp-tree.def"
#include "d/d-tree.def"
#include "java/java-tree.def"
#include "objc/objc-tree.def"
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