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Rollup merge of #75984 - kornelski:typeormodule, r=matthewjasper

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Improve unresolved use error message

"use of undeclared type or module `foo`" doesn't mention that it could be a crate.

This error can happen when users forget to add a dependency to `Cargo.toml`, so I think it's important to mention that it could be a missing crate.

I've used a heuristic based on Rust's naming conventions. It complains about an unknown type if the ident starts with an upper-case letter, and crate or module otherwise. It seems to work very well. The expanded error help covers both an unknown type and a missing crate case.
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Tyler Mandry 2020-09-09 15:05:45 -07:00 • committed by GitHub
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16
compiler/rustc_error_codes/src/error_codes/E0433.md
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@ -1,17 +1,27 @@
An undeclared type or module was used.
An undeclared crate, module, or type was used.
Erroneous code example:
```compile_fail,E0433
let map = HashMap::new();
// error: failed to resolve: use of undeclared type or module `HashMap`
// error: failed to resolve: use of undeclared type `HashMap`
```
Please verify you didn't misspell the type/module's name or that you didn't
forget to import it:
```
use std::collections::HashMap; // HashMap has been imported.
let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!
```
If you've expected to use a crate name:
```compile_fail
use ferris_wheel::BigO;
// error: failed to resolve: use of undeclared crate or module `ferris_wheel`
```
Make sure the crate has been added as a dependency in `Cargo.toml`.
To use a module from your current crate, add the `crate::` prefix to the path.