User-created 'core' crate confusingly shadows std 'core' crate

[RobJellinghaus]

This user reported a confusing error about core namespace not implementing an expected trait: https://twitter.com/Luminoth82/status/1459602117803450368

error[E0433]: failed to resolve: could not find `marker` in `core`
 --> runner-gui/src/app.rs:6:41
  |
6 | #[derive(Debug, Copy, Clone, PartialEq, EnumIter)]
  |                                         ^^^^^^^^ could not find `marker` in `core`
  |
  = note: this error originates in the derive macro `EnumIter` (in Nightly builds, run with -Z macro-backtrace for more info)

The root issue turned out to be that they had defined their own core crate which was shadowing the standard core crate. But the confusing error seems like it could be improved.

Ideally the output should look like:

error[E0433]: failed to resolve: could not find `marker` in `core`
 --> runner-gui/src/app.rs:6:41
  |
6 | #[derive(Debug, Copy, Clone, PartialEq, EnumIter)]
  |                                         ^^^^^^^^ could not find `marker` in `core`
  |
  = note: this error originates in the derive macro `EnumIter` (in Nightly builds, run with -Z macro-backtrace for more info)
  = note: the `core` crate is defined in `runner-gui\Cargo.toml`

Or some other additional note clarifying exactly which core is being built, or mentioning that there are shadowed core crates in scope, etc.