Merge ref '4fa824bb7831' from rust-lang/rust

Pull recent changes from https://github.com/rust-lang/rust via Josh.

Upstream ref: 4fa824bb78318a3cba8c7339d5754b4909922547
Filtered ref: b8d3c3c
Upstream diff: rust-lang/rust@9f32ccf...4fa824b

This merge was created using https://github.com/rust-lang/josh-sync.

Author: The rustc-josh-sync Cronjob Bot

src/tests/directives.md

@@ -262,6 +262,20 @@ Consider writing the test as a proper incremental test instead.
 
 </div>
 
+#### The edition directive
+
+The `//@ edition` directive can take an exact edition, a bounded half-open range of editions or a left-bounded half-open range of editions, this affects which edition is used by `./x test` to run the test. For example:
+
+- A test with the `//@ edition: 2018` directive will only run under the 2018 edition.
+- A test with the `//@ edition: 2015..2021` directive can be run under both the 2015 and 2018 editions. However, CI will only run the test with the lowest edition possible (2015 in this case).
+- A test with the `//@ edition: 2018..` directive will run under any edition greater or equal than 2018. However, CI will only run the test with the lowest edition possible (2018 in this case).
+
+You can also force `./x test` to use a specific edition by passing the `-- --edition=` argument. However, tests with the `//@ edition` directive will clamp the value passed to the argument. For example, if we run `./x test -- --edition=2015`:
+
+- A test with the `//@ edition: 2018` will run with the 2018 edition. 
+- A test with the `//@ edition: 2015..2021` will be run with the 2015 edition. 
+- A test with the `//@ edition: 2018..` will run with the 2018 edition. 
+
 ### Rustdoc
 
 | Directive   | Explanation                                                  | Supported test suites                   | Possible values           |