Add documentation for --output-format=doctest

Author: GuillaumeGomez

src/doc/rustdoc/src/unstable-features.md

@@ -524,6 +524,8 @@ use `-o -`.
 
 ## `-w`/`--output-format`: output format
 
+### json
+
 `--output-format json` emits documentation in the experimental
 [JSON format](https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc_json_types/). `--output-format html` has no effect,
 and is also accepted on stable toolchains.
@@ -542,6 +544,65 @@ It can also be used with `--show-coverage`. Take a look at its
 [documentation](#--show-coverage-calculate-the-percentage-of-items-with-documentation) for more
 information.
 
+### doctest
+
+`--output-format doctest` emits JSON which gives you information about doctests in the provided crate.
+
+You can use this option like this:
+
+```bash
+rustdoc -Zunstable-options --output-format=doctest src/lib.rs
+```
+
+For this rust code:
+
+```rust
+/// ```
+/// let x = 12;
+/// ```
+pub trait Trait {}
+```
+
+The generated output will look like this:
+
+```json
+{
+  "format_version": 1,
+  "doctests": [
+    {
+      "file": "foo.rs",
+      "line": 1,
+      "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;",
+      "doctest_code": "#![allow(unused)]\nfn main() {\nlet x = 12;\n}",
+      "name": "foo.rs - Trait (line 1)"
+    }
+  ]
+}
+```
+
+ * `format_version` gives you the current version of the generated JSON. If we change the output in any way, the number will increase.
+ * `doctests` contains the list of doctests present in the crate.
+   * `file` is the file path where the doctest is located.
+   * `line` is the line where the doctest starts (so where the \`\`\` is located in the current code).
+   * `doctest_attributes` contains computed information about the attributes used on the doctests. For more information about doctest attributes, take a look [here](write-documentation/documentation-tests.html#attributes).
+   * `original_code` is the code as written in the source code before rustdoc modifies it.
+   * `doctest_code` is the code modified by rustdoc that will be run. If there is a syntax error, this field will not be present.
+   * `name` is the name generated by rustdoc which represents this doctest.
+
 ## `--enable-per-target-ignores`: allow `ignore-foo` style filters for doctests
 
  * Tracking issue: [#64245](https://github.com/rust-lang/rust/issues/64245)