Use the first paragraph, instead of cookie-cutter text, for rustdoc descriptions

src/librustdoc/html/markdown.rs

@@ -1124,6 +1124,7 @@ crate fn plain_text_summary(md: &str) -> String {
             Event::HardBreak | Event::SoftBreak => s.push(' '),
             Event::Start(Tag::CodeBlock(..)) => break,
             Event::End(Tag::Paragraph) => break,
+            Event::End(Tag::Heading(..)) => break,
             _ => (),
         }
     }

src/librustdoc/html/markdown/tests.rs

@@ -230,6 +230,7 @@ fn test_plain_text_summary() {
     t("code `let x = i32;` ...", "code `let x = i32;` ...");
     t("type `Type<'static>` ...", "type `Type<'static>` ...");
     t("# top header", "top header");
+    t("# top header\n\nfollowed by some text", "top header");
     t("## header", "header");
     t("first paragraph\n\nsecond paragraph", "first paragraph");
     t("```\nfn main() {}\n```", "");

src/librustdoc/html/render/mod.rs

@@ -1548,7 +1548,10 @@ impl Context<'_> {
         }
         title.push_str(" - Rust");
         let tyname = it.type_();
-        let desc = if it.is_crate() {
+        let desc = it.doc_value().as_ref().map(|doc| plain_text_summary(&doc));
+        let desc = if let Some(desc) = desc {
+            desc
+        } else if it.is_crate() {
             format!("API documentation for the Rust `{}` crate.", self.shared.layout.krate)
         } else {
             format!(

src/test/rustdoc/description.rs

@@ -0,0 +1,19 @@
+#![crate_name = "foo"]
+//! # Description test crate
+//!
+//! This is the contents of the test crate docstring. It should not show up in the description.
+
+// @matches 'foo/index.html' '//meta[@name="description"]/@content' 'Description test crate'
+
+// @matches 'foo/foo_mod/index.html' '//meta[@name="description"]/@content' 'First paragraph description.'
+/// First paragraph description.
+///
+/// Second paragraph should not show up.
+pub mod foo_mod {
+    pub struct __Thing {}
+}
+
+// @matches 'foo/fn.foo_fn.html' '//meta[@name="description"]/@content' 'Only paragraph.'
+/// Only paragraph.
+pub fn foo_fn() {}
+

src/test/rustdoc/description_default.rs

@@ -0,0 +1,12 @@
+#![crate_name = "foo"]
+
+// @matches 'foo/index.html' '//meta[@name="description"]/@content' 'API documentation for the Rust `foo` crate.'
+
+// @matches 'foo/foo_mod/index.html' '//meta[@name="description"]/@content' 'API documentation for the Rust `foo_mod` mod in crate `foo`.'
+pub mod foo_mod {
+    pub struct __Thing {}
+}
+
+// @matches 'foo/fn.foo_fn.html' '//meta[@name="description"]/@content' 'API documentation for the Rust `foo_fn` fn in crate `foo`.'
+pub fn foo_fn() {}
+