use keyword notification and quick fix

[Tony-Proum]

Environment

• Intellij-Rust plugin version: v0.2.97.2123-191
• Rust toolchain version: rustup 1.18.2 (a0bf3c9cb 2019-05-02)
• IDE name and version: IntelliJ IDEA 2019.1.2 (Ultimate Edition) IU-191.7141.44
• Operating system: Fedora

Problem description

Writing a .rs file, I try to use a crate that is not present in deps of the current project and I don't have any notification that this import will fail until I'd compiled the code.

Steps to reproduce

create a new file and tip use serde;
There is no visual notification or helper but, when I run cargo build this error message will be displayed :

error[E0432]: unresolved import `serde`
 --> cli/src/project/github.rs:1:5
  |
1 | use serde;
  |     ^^^^^ no `serde` in the root

I think that it could be a good thing to have quick feedback in this case

[Kobzol]

@vlad20012 I suppose that this has been historically disabled to avoid a lot of false positives. Now that there is an advanced macro expansion engine and modules generated by macros/build scripts are taken into account, maybe we can start emitting this error?

[Kobzol]

@vlad20012 is it worth it to try it or do you think that there will still be too many false positives?

[vlad20012]

build.rs is not evaluated by default, the new name resolution engine is in experimental status, and the work on proc macro is not started yet (but I'm going!), and cfg is not fully supported (but I'm on the finish line, I guess). These items block the unresolved reference inspection from being enabled by default.

[vlad20012]

build.rs and proc macro, in their turn, are partially blocked on #6104 and rust-lang/cargo#7178 (see #1786 (comment))