Add a summary of lints caught in `cargo clippy`

[ericwu17]

Problem

When a relatively medium/large project migrates to clippy, there might be a large number of lint diagnostics. It can be difficult to get a clear picture of the category with the most problems.

@dclong raised an issue here describing this feature request in the clippy github repo.

Proposed Solution

It would help if running cargo clippy automatically shows counts of lints whenever a large number of diagnostics (> 10) are generated. For example, cargo clippy should show something like

`my_project` (bin "my_project") generated 28 warnings

Summary of diagnostics:
clippy::collapsible_if: 16
clippy::collapsible_else_if: 4
clippy::approx_constant: 3
dead_code: 3
clippy::single_match: 1
clippy::new_without_default: 1

Although the default behavior is to show this summary whenever the total amount of diagnostics is greater than 10, we should also allow users to override whether the summary is shown by running cargo clippy --summary or cargo clippy --no-summary.

Notes

Note that the summary includes both warnings generated from clippy and rustc.

[weihanglo]

Thank you for proposing this.

cargo-clippy is an external subcommand provided by clippy's main.rs. Cargo usually just hands over argument parsing to subcommands, so --summary/--no-summary might not be feasible at this moment.

Apart from that, the idea of a summary report sounds good to me.. Cargo already has experiments on auto-fix suggestions (#10989, #11399) on nightly. This I guess could be a part of that. And we could tweak the style to fit with the current output style of cargo clippy. @Muscraft has previously worked on those experiments. We might hear great feedback from there :)

[ericwu17]

@rustbot claim