Auto merge of #11872 - llogiq:test-attr-in-doctest, r=xFrednet

add lint against unit tests in doctests

During RustLab, Alice Ryhl brought to my attention that the Andoid team stumbled over the fact that if one attempts to write a unit test within a doctest, it will be summarily ignored. So this lint should help people wondering why their tests won't run.

---

changelog: New lint: [`test_attr_in_doctest`]
[#11872](https://github.com/rust-lang/rust-clippy/pull/11872)

tests/ui/test_attr_in_doctest.rs

@@ -0,0 +1,51 @@
+/// This is a test for `#[test]` in doctests
+///
+/// # Examples
+///
+/// ```
+/// #[test]
+/// fn should_be_linted() {
+///     assert_eq!(1, 1);
+/// }
+/// ```
+///
+/// Make sure we catch multiple tests in one example,
+/// and show that we really parse the attr:
+///
+/// ```
+/// #[test]
+/// fn should_also_be_linted() {
+///     #[cfg(test)]
+///     assert!(true);
+/// }
+///
+/// #[test]
+/// fn should_be_linted_too() {
+///     assert_eq!("#[test]", "
+///     #[test]
+///     ");
+/// }
+/// ```
+///
+/// We don't catch examples that aren't run:
+///
+/// ```ignore
+/// #[test]
+/// fn ignored() { todo!() }
+/// ```
+///
+/// ```no_run
+/// #[test]
+/// fn ignored() { todo!() }
+/// ```
+///
+/// ```compile_fail
+/// #[test]
+/// fn ignored() { Err(()) }
+/// ```
+///
+/// ```txt
+/// #[test]
+/// fn not_even_rust() { panic!("Ouch") }
+/// ```
+fn test_attr_in_doctests() {}

tests/ui/test_attr_in_doctest.stderr

@@ -0,0 +1,29 @@
+error: unit tests in doctest are not executed
+  --> $DIR/test_attr_in_doctest.rs:6:5
+   |
+LL |   /// #[test]
+   |  _____^
+LL | | /// fn should_be_linted() {
+   | |_______________________^
+   |
+   = note: `-D clippy::test-attr-in-doctest` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::test_attr_in_doctest)]`
+
+error: unit tests in doctest are not executed
+  --> $DIR/test_attr_in_doctest.rs:16:5
+   |
+LL |   /// #[test]
+   |  _____^
+LL | | /// fn should_also_be_linted() {
+   | |____________________________^
+
+error: unit tests in doctest are not executed
+  --> $DIR/test_attr_in_doctest.rs:22:5
+   |
+LL |   /// #[test]
+   |  _____^
+LL | | /// fn should_be_linted_too() {
+   | |___________________________^
+
+error: aborting due to 3 previous errors
+