rustdoc: --show-coverage should show per-item, not per-file

[jyn514]

Currently, --show-coverage looks like this:

+-------------------------------------+------------+------------+------------+
| File                                | Documented |      Total | Percentage |
+-------------------------------------+------------+------------+------------+
| ...target-lexicon-0.10.0/src/lib.rs |          1 |          1 |     100.0% |
| ...exicon-0.10.0/src/parse_error.rs |          1 |          7 |      14.3% |
| ...et-lexicon-0.10.0/src/targets.rs |          9 |        154 |       5.8% |
| ...get-lexicon-0.10.0/src/triple.rs |         13 |         18 |      72.2% |
| ...con-03b8c8c76fdbfbc7/out/host.rs |          1 |          1 |     100.0% |
+-------------------------------------+------------+------------+------------+
| Total                               |         25 |        181 |      13.8% |
+-------------------------------------+------------+------------+------------+

However, this isn't particularly helpful to the crate author - they know some docs are missing, but not which ones. So it's hard to add the missing docs. It would be great if instead rustdoc showed this info per-item instead of per-file so you could tell which items were missing docs.

Originally reported in rust-lang/docs.rs#939 (comment).

[jyn514]

Since this is nightly only there's no concern about backwards-compatibility, but it might also be nice to make this toggle-able with --show-coverage=files or --show-coverage=items.

[GuillaumeGomez]

I was thinking about the same thing: adding a "coverage level". I'm still think that we should rely more on the rustdoc warnings for this though (there is the doc_missing lint after all).

[jyn514]

This could look something like this, with the items as a sub-section under the file. My ASCII art isn't particularly pretty, I'm not tied to exactly this:

+-------------------------------------+------------+------------+------------+
| File                                | Documented |      Total | Percentage |
+-------------------------------------+------------+------------+------------+
| ...target-lexicon-0.10.0/src/lib.rs |          1 |          1 |     100.0% |
|-+----------------------+------------+
| | Item                 | Documented |
|-+----------------------+------------+
| | struct@crate::TARGET | true       |
|-+----------------------+------------+
| ...exicon-0.10.0/src/parse_error.rs |          1 |          7 |      14.3% |
| ...et-lexicon-0.10.0/src/targets.rs |          9 |        154 |       5.8% |
| ...get-lexicon-0.10.0/src/triple.rs |         13 |         18 |      72.2% |
| ...con-03b8c8c76fdbfbc7/out/host.rs |          1 |          1 |     100.0% |
+-------------------------------------+------------+------------+------------+