Better diagnostics for `let = (let =`

[steveklabnik]

This code isn't valid Rust:

let x = (let y = 6);

We have an okay, but not great, diagnostic here:

src/main.rs:2:14: 2:17 error: expected identifier, found keyword `let`
src/main.rs:2     let x = (let y = 6);
                           ^~~
src/main.rs:2:18: 2:19 error: expected one of `!`, `)`, `,`, `.`, `::`, `{`, or an operator, found `y`
src/main.rs:2     let x = (let y = 6);
                               ^

It might be nice to add a note when the keyword found is let that mentions that let is not an expression, and so cannot be used in this way.

[Manishearth]

@Wafflespeanut want to work on this? It's a similar task to the other doc comment one, but much simpler since it's a very specific situation.

I'd be happy to help someone :)

So, the parser already has its ways to differentiate keywords from identifiers. All we have to do is find the right place where that error is thrown, check whether it's a let and add a note along with the error. The parser has all sorts of code, and adding a note's also around there somewhere :)

Code: src/libsyntax/parse/parser.rs

[dsprenkels]

I would like to try this. :)

[Manishearth]

Awesome! Feel free to ask waffles or me for help (you can find us both on IRC on #rust-internals), or ask questions here.

(self-assigning since I can't assign to you -- just to mark it as "being taken")