[rustdoc] Give more information into extracted doctest information

[GuillaumeGomez]

Follow-up of #134531.

This update fragment the doctest code into its sub-parts to give more control to the end users on how they want to use it.

The new JSON looks like this:

{
  "format_version":2,
  "doctests":[
    {
      "file":"$DIR/extract-doctests-result.rs",
      "line":8,
      "doctest_attributes":{
        "original":"",
        "should_panic":false,
        "no_run":false,
        "ignore":"None",
        "rust":true,
        "test_harness":false,
        "compile_fail":false,
        "standalone_crate":false,
        "error_codes":[],
        "edition":null,
        "added_css_classes":[],
        "unknown":[]
      },
      "original_code":"let x = 12;\nOk(())",
      "doctest_code":{
        "crate_level":"#![allow(unused)]\n",
        "code":"let x = 12;\nOk(())",
        "wrapper":{
          "before":"fn main() { fn _inner() -> core::result::Result<(), impl core::fmt::Debug> {\n",
          "after":"\n} _inner().unwrap() }",
          "returns_result":true
        }
      },
      "name":"$DIR/extract-doctests-result.rs - (line 8)"
    }
  ]
}

for this doctest:

let x = 12;
Ok(())

With this, I think it matches what you need @ojeda? If so, once merged I'll update the patch I sent to RfL.

r? @aDotInTheVoid

[GuillaumeGomez]

Fixed fmt.

[lolbinarycat]

Core logic looks correct (mostly just exposing existing variables), just have a few nits about naming to hopefully increase readability.

[lolbinarycat]

Nit: could we call this inner_code to disambiguate?

[lolbinarycat]

Suggested change

	SyntaxError(String),
	/// syntactically invalid code that should be passed through verbatim.
	SyntaxError(String),

It wasn't immediately obvious that the String here is source code and not a description of the error, as errstrs are a fairly common pattern in rust.

I assume we do this partially for compile_fail tests and also because rustc provides better diagnostics?

[lolbinarycat]

1. The name DocTestWrapper seems a bit confusing since we already have DocTest and WrapperInfo (and unlike WrapperInfo, this also includes the code itself), maybe this could be DocTestWrapResult as it is the result of wrapping the doctest, and it has an error variant like Result?
2. couldn't we avoid specifying all the fields twice by leveraging the existing struct? like so: Valid(DocTest)

[lolbinarycat]

If we're doing all this to estimate capacity, should we also do + 4 * code.lines().count(), or is that counterproductive? how much do we care about avoiding allocations?

[lolbinarycat]

Suggested change

	let (wrapper, _) = doctest.generate_unique_doctest(&test, false, &opts, krate);
	let test = wrapper.to_string();
	let (wrapped, _) = doctest.generate_unique_doctest(&test, false, &opts, krate);
	let test = wrapped.to_string();

Nit: it's not just a wrapper, but the wrapper in combination with the inner code that has been wrapped. we make this distinction in the JSON output, so maybe we should stick to that terminology here.