[neotest] support test file for files without a single test module

[mrcjkb]

Feature description

For files without a test module, or with multiple test modules, testFile could potentially be supported by

• Filtering the rust-analyzer testables buy entries that are in the file
  • check if this is doable without too much complexity
• Construct a test command for all the entries

[roycrippen4]

I found that running cargo +nightly test -- --list -Zunstable-options --format=json yields a json table that has exactly the data needed for this.

// $ cargo +nightly test -- --list -Zunstable-options --format=json
[
  {
    "type": "test",
    "event": "discovered",
    "name": "tests::it_works",
    "ignore": false,
    "ignore_message": "",
    "source_path": "src/lib.rs",
    "start_line": 40,
    "start_col": 8,
    "end_line": 40,
    "end_col": 16
  }
]

Benefits:

• Solves the test discovery process in a simple way
• Provides a useful json response to stdout
• Filters out all files that do not contain tests
• Json response includes documentation testing
• Json response includes integration testing

Caveats:

• Requires the nightly rust compiler to be installed on the system
• Nightly (probably) would need to be recompiled each time there is an update to rustc's master branch

Note that nightly is only used transiently and does not become your default compiler

It would be trivial to close out this issue using this command, however I'm not sure if the trade-off is worth it.
Here's a repo I made for testing.

[mrcjkb]

@roycrippen4 thanks for the input.
That sounds like a good approach. But I would definitely want to wait for the feature to stabilize.
Perhaps the extra info will be added to the rust-analyzer experimental/runnables response schema then.