import #[macro_export] from crate root help broken by use trees

[CAD97]

Given the following code: [playground]

mod m {
    #[macro_export]
    macro_rules! nu {
        {} => {};
    }

    #[allow(nonstandard_style)]
    pub struct other_item;

    pub use self::{nu, other_item as _};
}

The current output is:

error[[E0432]](https://doc.rust-lang.org/nightly/error-index.html#E0432): unresolved import `self::nu`
  --> src/lib.rs:10:20
   |
10 |     pub use self::{nu, other_item as _};
   |                    ^^ no `nu` in `m`
   |
   = note: this could be because a macro annotated with `#[macro_export]` will be exported at the root of the crate instead of the module where it is defined
help: a macro with this name exists at the root of the crate
   |
10 -     pub use self::{nu, other_item as _};
10 +     pub use self::{nu, other_item as _};
   |

If the starting order is reversed, the suggestion puts the macro first in the tree

error[[E0432]](https://doc.rust-lang.org/nightly/error-index.html#E0432): unresolved import `self::nu`
  --> src/lib.rs:10:37
   |
10 |     pub use self::{other_item as _, nu};
   |                                     ^^ no `nu` in `m`
   |
   = note: this could be because a macro annotated with `#[macro_export]` will be exported at the root of the crate instead of the module where it is defined
help: a macro with this name exists at the root of the crate
   |
10 -     pub use self::{other_item as _, nu};
10 +     pub use self::{nu, other_item as _};
   |

This is suggesting to change the code to itself 🙃 It needs to move the import into a use crate:: tree; the easy way is to make a new use statement (and let rustfmt clean that up); the medium way is to find an existing use crate:: context; the hard way is when a use crate:: tree root doesn't already exist (or isn't immediately a tree at that point) determine what import granularity the code is using and match that.

Related: #99694

@rustbot label +D-confusing +D-incorrect

[CAD97]

In this case, the ideal solution is actually to write just pub use nu; and get the import from legacy-lexical-macro-scope into the namespace. I originally hit this with just multiple macros in the use tree, and removing the self::{} was the correct fix. I "simplified" for the error report to use a second non-macro item here (to reduce the number of diagnostics).

So bonus challenge: handle this case properly and only emit a single help:

error[[E0432]](https://doc.rust-lang.org/nightly/error-index.html#E0432): unresolved imports `self::nu`, `self::nu_parameters`, `self::nu_parameter`
  --> src/lib.rs:19:20
   |
19 |     pub use self::{nu, nu_parameters, nu_parameter};
   |                    ^^  ^^^^^^^^^^^^^  ^^^^^^^^^^^^ no `nu_parameter` in `m`
   |                    |   |
   |                    |   no `nu_parameters` in `m`
   |                    no `nu` in `m`
   |
   = note: this could be because a macro annotated with `#[macro_export]` will be exported at the root of the crate instead of the module where it is defined
help: a macro with this name exists at the root of the crate
   |
19 -     pub use self::{nu, nu_parameters, nu_parameter};
19 +     pub use self::{nu, nu_parameters, nu_parameter};
   |
help: a macro with this name exists at the root of the crate
   |
19 -     pub use self::{nu, nu_parameters, nu_parameter};
19 +     pub use self::{nu_parameters, nu, nu_parameter};
   |
help: a macro with this name exists at the root of the crate
   |
19 -     pub use self::{nu, nu_parameters, nu_parameter};
19 +     pub use self::{nu_parameter, nu, nu_parameters};
   |

[CAD97]

@rustbot label +D-invalid-suggestion

It's not producing invalid code, but it's surely suggesting a fix which will cause an error (the identical one).