Add support for let chains

[c410-f3r]

See rust-lang/rust#88642.

[ChayimFriedman2]

I would like to work on that. How should I synchronize the changes to ungrammer and rust-analyzer?

[ChayimFriedman2]

Parsing turns out to be quite complex. The problem is that the let syntax can appear inside parentheses (if (let pat = expr)). This is to facilitate for macros, as stated in the RFC.

The problem is especially tricky since we cannot know if let is allowed in this position until we finish parsing it, becauses parentheses can be also tuples: if (let true = true, false) {} is invalid, but if (let true = true) {} is valid.

rustc chose to implement this as an expression kind and later verify that let is indeed valid there.

We don't have a validation pass to the AST. But it may be necessary to introduce it.

What do you think? Do you have an other way?

[Veykril]

Macros strike once more, way to go making the language a bit more inconsistent yet again 🙄

We did have a validation pass but disabled it for the time being(I don't recall the reason). If we don't reject the let expression for now where its disallowed that is fine.

[TennyZhuang]

Is it possible to implement let without parentheses first? It’s the main usecase, and the macro is corner case.

[ChayimFriedman2]

@TennyZhuang This is quite useless since if you want to implement it using the exisiting infra you will need to rewrite it anyway and if you want to create let expressions, like I did, it is almost no work to support parentheses. I'm also pretty close to closing this, hope for today.

[ChayimFriedman2]

I've finished the work (I think), but I cannot send a PR until rust-analyzer/ungrammar#45 is merged and published since Cargo can't update Cargo.lock until then.

[c410-f3r]

Thank you @ChayimFriedman2

[ChayimFriedman2]

After I implemented all of this, now the Rust developers want to forbid parentheses in let chains! 😭

At least we can issue a better error... Though I am not sure parsing it is required to do that.