Add new too_long_first_doc_paragraph first paragraph lint

tests/ui/too_long_first_doc_paragraph-fix.fixed

@@ -0,0 +1,8 @@
+#![warn(clippy::too_long_first_doc_paragraph)]
+
+/// A very short summary.
+///
+/// A much longer explanation that goes into a lot more detail about
+/// how the thing works, possibly with doclinks and so one,
+/// and probably spanning a many rows.
+struct Foo;

tests/ui/too_long_first_doc_paragraph-fix.rs

@@ -0,0 +1,7 @@
+#![warn(clippy::too_long_first_doc_paragraph)]
+
+/// A very short summary.
+/// A much longer explanation that goes into a lot more detail about
+/// how the thing works, possibly with doclinks and so one,
+/// and probably spanning a many rows.
+struct Foo;

tests/ui/too_long_first_doc_paragraph-fix.stderr

@@ -0,0 +1,19 @@
+error: first doc comment paragraph is too long
+  --> tests/ui/too_long_first_doc_paragraph-fix.rs:3:1
+   |
+LL | / /// A very short summary.
+LL | | /// A much longer explanation that goes into a lot more detail about
+LL | | /// how the thing works, possibly with doclinks and so one,
+LL | | /// and probably spanning a many rows.
+   | |_
+   |
+   = note: `-D clippy::too-long-first-doc-paragraph` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::too_long_first_doc_paragraph)]`
+help: add an empty line
+   |
+LL ~ /// A very short summary.
+LL + ///
+   |
+
+error: aborting due to 1 previous error
+

tests/ui/too_long_first_doc_paragraph.rs

@@ -0,0 +1,47 @@
+//@no-rustfix
+
+#![warn(clippy::too_long_first_doc_paragraph)]
+
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+/// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+/// gravida non lacinia at, rhoncus eu lacus.
+pub struct Bar;
+
+// Should not warn! (not an item visible on mod page)
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+/// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+/// gravida non lacinia at, rhoncus eu lacus.
+impl Bar {}
+
+// Should not warn! (less than 80 characters)
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit.
+///
+/// Nunc turpis nunc, lacinia
+/// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+/// gravida non lacinia at, rhoncus eu lacus.
+enum Enum {
+    A,
+}
+
+/// Lorem
+/// ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+/// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+/// gravida non lacinia at, rhoncus eu lacus.
+union Union {
+    a: u8,
+    b: u8,
+}
+
+// Should not warn! (title)
+/// # bla
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+/// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+/// gravida non lacinia at, rhoncus eu lacus.
+union Union2 {
+    a: u8,
+    b: u8,
+}
+
+fn main() {
+    // test code goes here
+}

tests/ui/too_long_first_doc_paragraph.stderr

@@ -0,0 +1,22 @@
+error: first doc comment paragraph is too long
+  --> tests/ui/too_long_first_doc_paragraph.rs:5:1
+   |
+LL | / /// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+LL | | /// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+LL | | /// gravida non lacinia at, rhoncus eu lacus.
+   | |_
+   |
+   = note: `-D clippy::too-long-first-doc-paragraph` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::too_long_first_doc_paragraph)]`
+
+error: first doc comment paragraph is too long
+  --> tests/ui/too_long_first_doc_paragraph.rs:26:1
+   |
+LL | / /// Lorem
+LL | | /// ipsum dolor sit amet, consectetur adipiscing elit. Nunc turpis nunc, lacinia
+LL | | /// a dolor in, pellentesque aliquet enim. Cras nec maximus sem. Mauris arcu libero,
+LL | | /// gravida non lacinia at, rhoncus eu lacus.
+   | |_
+
+error: aborting due to 2 previous errors
+