Add tmt about, command showing things about tmt itself

[happz]

Pull Request Checklist

• implement the feature
• write the documentation
• extend the test coverage
• include a release note

[psss]

This sounds like a very nice idea! Are there any limitations about the usage? I tried the new command within the venv but I get no output.

[happz]

This sounds like a very nice idea! Are there any limitations about the usage? I tried the new command within the venv but I get no output.

$ tmt about plugins --how rest
Export plugins for story

* `dict`
* `json`
* `template`
* `rst`
* `yaml`

----

Export plugins for plan

* `dict`
* `json`
* `template`
* `yaml`

----
...

$ tmt about plugins --how json | jq .
{
  "export.story": [
    "dict",
    "json",
    "template",
    "rst",
    "yaml"
  ],
  "export.plan": [
    "dict",
    "json",
    "template",
    "yaml"
  ],
  "export.test": [
    "dict",
    ...

The current limitation is, it's just listing plugins, nothing more. I hope to get it to share more info, maybe render plugin docs, specs docs, or explanation of various errors, just like rustc --explain does:

$ PAGER= rustc --explain E0277
You tried to use a type which doesn't implement some trait in a place which expected that trait.

Erroneous code example:

// here we declare the Foo trait with a bar method
trait Foo {
    fn bar(&self);
}

// we now declare a function which takes an object implementing the Foo trait
fn some_func<T: Foo>(foo: T) {
    foo.bar();
}

fn main() {
    // we now call the method with the i32 type, which doesn't implement
    // the Foo trait
    some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
}

In order to fix this error, verify that the type you're using does implement the trait. Example:

trait Foo {
    fn bar(&self);
}

// we implement the trait on the i32 type
impl Foo for i32 {
    fn bar(&self) {}
}

fn some_func<T: Foo>(foo: T) {
    foo.bar(); // we can now use this method since i32 implements the
               // Foo trait
}

fn main() {
    some_func(5i32); // ok!
}

...