Comment by tialaramex

In a language like Rust where local variables are explicitly destroyed when scope ends a naive TCO is very annoying and `become` also helps fix that.

Suppose I have a recursive function f(n: u8) where f(0) is 0 and otherwise f(n) is n * bar(n) + f(n-1)

I might well write that with a local temporary to calculate bar(n) and then we do the sum, but this would inhibit TCO because that temporary should exist after we did the recursive calculation, even though it doesn't matter in practice.

A compiler could try to cleverly figure out whether it matters and destroy that local temporary earlier then apply TCO, but now your TCO is fragile because a seemingly minor code change might fool that "clever" logic, by ensuring it isn't correct to make this change and breaking your optimisation.

The `become` keyword is a claim by the programmer that we can drop all these locals and do TCO. So because the programmer claimed this should work they're giving the compiler permission to attempt the early drop and if it doesn't work and can't be TCO then complain that the program is wrong.