resolve: improve diagnostics for a single super at the crate root

[jseyfried]

For example,

fn main() {
    let super = 0;
    //  ^^^^^ Ideally, the error here would be something like:
    // - "the root module has not parent"
    // - "`super` is not allowed in the crate root"
}

[jseyfried]

cc @petrochenkov

[BGR360]

For reference, this is the message currently produced:

error[E0433]: failed to resolve: there are too many leading `super` keywords
 --> src/main.rs:2:9
  |
2 |     let super = 0;
  |         ^^^^^ there are too many leading `super` keywords

Ideally, the error here would be something like:

• "the root module has not parent"
• "super is not allowed in the crate root"

I'm not a Rust expert, but why should the error message say anything remotely related to path resolution? Is it valid to have a path after let? Here's what I would expect the message to be:

error: expected identifier, found keyword `super`
 --> src/main.rs:2:9
  |
2 |     let super = 0;
  |         ^^^^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
2 |     let r#super = 0;
  |         ~~~~~~~

[MolotovCherry]

Turns out that super is not a valid raw identifier either (along with a small few others), so the help suggestion (if any) would need to say to rename it.

[kadiwa4]

@BGR360

I'm not a Rust expert, but why should the error message say anything remotely related to path resolution?

You can use patterns on the left side of let statements.

let bar = core::num::Wrapping(3);
let core::num::Wrapping(foo) = bar;

That pattern is also allowed to consist of a single constant, therefore Rust has to check if the thing on the left-hand side is a path.

const MY_CONST: () = ();
// does not define any new variables:
let MY_CONST = ();