Add become keyword and expressions

[laelath]

Adds support for the upcoming become expressions for guaranteed tail recursion.

[wetneb]

Thanks for working on this, but this doesn't seem sufficient to me. I would expect changes to the grammar.js to make it possible to parse expressions involving the keyword. At least one test case (in test/corpus) should be provided.

[laelath]

That's an embarrassing mistake to have edited the auto-generated file instead of the actual source file. I fixed that and added some tests taken from the rust doc page on become. I made the choice to have the sub-expression be allowed to be any arbitrary expression, even though strictly speaking it only needs to accept call expressions, though I checked that the rustc parser also follows this behavior, rejecting non-call becomes at a later stage.

[wetneb]

Thanks a lot! It's not embarrassing at all, auto-generated files shouldn't be checked into Git in the first place, so it's not your fault…
The reasoning about enabling become for all expressions sounds good to me. It could be worth checking that "become" can still be used as a regular identifier (such as let become = 4 or pub fn become() {}), especially since this is an unstable feature.

By the way I'm not a maintainer here but I'll happily integrate your changes into the tree-sitter-rust-orchard fork.

[laelath]

I don't think those test cases are valid rust syntax, since while become is an unstable feature, it is a reserved keyword in current stable versions of rust.

Feel free to merge this into your fork!

[wetneb]

My bad, you are clearly right!
Then I'm porting and releasing this into https://codeberg.org/grammar-orchard/tree-sitter-rust-orchard.
Thank you!