Add regression test for doc_include_without_cfg lint

tests/ui/doc/doc_include_without_cfg.fixed

@@ -0,0 +1,19 @@
+#![warn(clippy::doc_include_without_cfg)]
+// Should not lint.
+#![doc(html_playground_url = "https://playground.example.com/")]
+#![cfg_attr(doc, doc = include_str!("../approx_const.rs"))] //~ doc_include_without_cfg
+// Should not lint.
+#![cfg_attr(feature = "whatever", doc = include_str!("../approx_const.rs"))]
+#![cfg_attr(doc, doc = include_str!("../approx_const.rs"))]
+#![doc = "some doc"]
+//! more doc
+
+#[cfg_attr(doc, doc = include_str!("../approx_const.rs"))] //~ doc_include_without_cfg
+// Should not lint.
+#[cfg_attr(feature = "whatever", doc = include_str!("../approx_const.rs"))]
+#[cfg_attr(doc, doc = include_str!("../approx_const.rs"))]
+#[doc = "some doc"]
+/// more doc
+fn main() {
+    // test code goes here
+}

tests/ui/doc/doc_include_without_cfg.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::doc_include_without_cfg)]
+// Should not lint.
+#![doc(html_playground_url = "https://playground.example.com/")]
+#![doc = include_str!("../approx_const.rs")] //~ doc_include_without_cfg
+// Should not lint.
+#![cfg_attr(feature = "whatever", doc = include_str!("../approx_const.rs"))]
+#![cfg_attr(doc, doc = include_str!("../approx_const.rs"))]
+#![doc = "some doc"]
+//! more doc
+
+#[doc = include_str!("../approx_const.rs")] //~ doc_include_without_cfg
+// Should not lint.
+#[cfg_attr(feature = "whatever", doc = include_str!("../approx_const.rs"))]
+#[cfg_attr(doc, doc = include_str!("../approx_const.rs"))]
+#[doc = "some doc"]
+/// more doc
+fn main() {
+    // test code goes here
+}

tests/ui/doc/doc_include_without_cfg.stderr

@@ -0,0 +1,17 @@
+error: included a file in documentation unconditionally
+  --> tests/ui/doc/doc_include_without_cfg.rs:4:1
+   |
+LL | #![doc = include_str!("../approx_const.rs")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `cfg_attr(doc, doc = "...")`: `#![cfg_attr(doc, doc = include_str!("../approx_const.rs"))]`
+   |
+   = note: `-D clippy::doc-include-without-cfg` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::doc_include_without_cfg)]`
+
+error: included a file in documentation unconditionally
+  --> tests/ui/doc/doc_include_without_cfg.rs:11:1
+   |
+LL | #[doc = include_str!("../approx_const.rs")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `cfg_attr(doc, doc = "...")`: `#[cfg_attr(doc, doc = include_str!("../approx_const.rs"))]`
+
+error: aborting due to 2 previous errors
+

tests/ui/missing_doc_crate.rs

@@ -1,4 +1,5 @@
 #![warn(clippy::missing_docs_in_private_items)]
+#![allow(clippy::doc_include_without_cfg)]
 #![doc = include_str!("../../README.md")]
 
 fn main() {}