alt Hacker NewsI am not a Clang expert, but first, obviously that's a C++ attribute and so while Clang can decide what it means in Clang in the programming language itself it has no semantic weight because the ISO document says attributes are always ignorable.
Secondly however in these languages you often won't naively get TCO because you have at least one local variable which C++ would say has a "non-trivial destructor" or Rust would say "implements Drop". These both mean that naively the "tail call" wasn't actually the last thing to happen, the destructor / Drop::drop happen at the end of the function, after the tail call.
The proposed become keyword tries to core::mem::drop any such variables, if it succeeds now that tail call is last and we can do TCO, if it fails [e.g. because the variables it wants to drop are needed for the tail call] we can diagnose the problem. I believe the Clang attribute doesn't have this behaviour.
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Reordering destructors is not safe in C++, as it's fairly common to rely on objects being destroyed in reverse order and doing stuff like
A a;
B b(&a);
Clang tail-calls aren't guaranteed to work with all C++ code. If you have a non-trivial constructor, as you mention, it will tell you this and fail instead of silently letting you believe you have tail-calls when you don't.