new lint: use_crate_prefix_for_self_imports

[lengyijun]

changelog: [use_crate_prefix_for_self_imports]: new lint

fix #13645

Only check main.rs and lib.rs

Known problem

Duplicate warning

TODO

• rm tests/ui/use_crate_prefix_for_self_imports.rs
• squash commits

Question

Although pass all tests, but if copy lintcheck to /tmp, clippy will report two warnings (expected)

warning: this import is not clear
  --> src/main.rs:42:5
   |
42 | use input::read_crates;
   |     ^^^^^ help: prefix with `crate::`: `crate::input`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_crate_prefix_for_self_imports
   = note: `#[warn(clippy::use_crate_prefix_for_self_imports)]` on by default

warning: this import is not clear
  --> src/main.rs:43:5
   |
43 | use output::{ClippyCheckOutput, ClippyWarning, RustcIce};
   |     ^^^^^^ help: prefix with `crate::`: `crate::output`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_crate_prefix_for_self_imports

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[llogiq]

While I understand that clearing up possible path confusion is a worthy cause, I'm afraid that introducing the lint as warn-by-default will lead to a lot of noise and broken CI jobs.

I'll discuss this on zulip.

[llogiq]

So, as you know we discussed this on zulip a while ago. The upshot is that we have two acceptable options:

• Put the lint in the nursery group and improve it until we deem it ready for style
• Add more checks to the lint to reduce the number of warnings that aren't too helpful. For example, if we have the respective mod near the use (where near could be measured in line number differences or in no other items but mods and uses in between), we could avoid linting, as the reader of the code will know that the item comes from the mod specified nearby (this could be done by walking the sibling items until a non-mod-or-use item is found).

Do you intend to continue working on this?