Add new lint doc_include_without_cfg (#13625)

It's becoming more and more common to see people including markdown
files in their code using `doc = include_str!("...")`, which is great.
However, often there is no condition on this include, which is not great
because it slows down compilation and might trigger recompilation if
these files are updated.

This lint aims at fixing this situation.

changelog: Add new lint `doc_include_without_cfg`

tests/ui/doc/doc_include_without_cfg.fixed

@@ -0,0 +1,40 @@
+#![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
+
+macro_rules! man_link {
+    ($a:literal, $b:literal) => {
+        concat!($a, $b)
+    };
+}
+
+// Should not lint!
+macro_rules! tst {
+    ($(#[$attr:meta])*) => {
+        $(#[$attr])*
+        fn blue() {
+            println!("Hello, world!");
+        }
+    }
+}
+
+tst! {
+    /// This is a test with no included file
+}
+
+#[cfg_attr(doc, doc = include_str!("../approx_const.rs"))] //~ doc_include_without_cfg
+// Should not lint.
+#[doc = man_link!("bla", "blob")]
+#[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 +1,40 @@
+#![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
+
+macro_rules! man_link {
+    ($a:literal, $b:literal) => {
+        concat!($a, $b)
+    };
+}
+
+// Should not lint!
+macro_rules! tst {
+    ($(#[$attr:meta])*) => {
+        $(#[$attr])*
+        fn blue() {
+            println!("Hello, world!");
+        }
+    }
+}
+
+tst! {
+    /// This is a test with no included file
+}
+
+#[doc = include_str!("../approx_const.rs")] //~ doc_include_without_cfg
+// Should not lint.
+#[doc = man_link!("bla", "blob")]
+#[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:31:1
+   |
+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() {}