- Start Date: 2015-01-20
- RFC PR: rust-lang/rfcs#601
- Rust Issue: rust-lang/rust#22141
be reserved keyword to
A keyword needs to be reserved to support guaranteed tail calls in a backward-compatible way. Currently the keyword reserved for this purpose is
be, but the
become alternative was proposed in
the old RFC for guaranteed tail calls, which is now postponed and tracked in PR#271.
Some advantages of the
become keyword are:
- it provides a clearer indication of its meaning ("this function becomes that function")
- its syntax results in better code alignment (
becomeis exactly as long as
The expected result is that users will be unable to use
become as identifier, ensuring that it will be available for future language extensions.
This RFC is not about implementing tail call elimination, only on whether the
be keyword should be replaced with
be reserved word to
become. This is a very simple find-and-replace.
Some code might be using
become as an identifier.
The main alternative is to do nothing, i.e. to keep the
be keyword reserved for supporting guaranteed tail calls in a backward-compatible way. Using
become as the keyword for tail calls would not be backward-compatible because it would introduce a new keyword, which might have been used in valid code.
Another option is to add the
become keyword, without removing
be. This would have the same drawbacks as the current proposal (might break existing code), but it would also guarantee that the
become keyword is available in the future.