Render Markdown in search results

Previously Markdown documentation was not rendered to HTML for search results,
which led to the output not being very readable, particularly for inline code.
This PR fixes that by rendering Markdown to HTML with the help of pulldown-cmark
(the library rustdoc uses to parse Markdown for the main text of documentation).
However, the text for the title attribute (the text shown when you hover over an
element) still uses the plain-text rendering since it is displayed in browsers
as plain-text.

Only these styles will be rendered; everything else is stripped away:

* *italics*
* **bold**
* `inline code`

2 files changed, 10 insertions, 37 deletions

diff --git a/src/librustdoc/html/render/cache.rs b/src/librustdoc/html/render/cache.rs
index 085ca01f58d..97f764517fa 100644
--- a/src/librustdoc/html/render/cache.rs
+++ b/src/librustdoc/html/render/cache.rs
@@ -9,7 +9,7 @@ use crate::clean::types::GetDefId;
 use crate::clean::{self, AttributesExt};
 use crate::formats::cache::Cache;
 use crate::formats::item_type::ItemType;
-use crate::html::render::{plain_text_summary, shorten};
+use crate::html::markdown::short_markdown_summary;
 use crate::html::render::{Generic, IndexItem, IndexItemFunctionType, RenderType, TypeWithKind};
 
 /// Indicates where an external crate can be found.
@@ -78,7 +78,7 @@ crate fn build_index(krate: &clean::Crate, cache: &mut Cache) -> String {
                 ty: item.type_(),
                 name: item.name.clone().unwrap(),
                 path: fqp[..fqp.len() - 1].join("::"),
-                desc: shorten(plain_text_summary(item.doc_value())),
+                desc: item.doc_value().map_or_else(|| String::new(), short_markdown_summary),
                 parent: Some(did),
                 parent_idx: None,
                 search_type: get_index_search_type(&item),
@@ -127,7 +127,7 @@ crate fn build_index(krate: &clean::Crate, cache: &mut Cache) -> String {
     let crate_doc = krate
         .module
         .as_ref()
-        .map(|module| shorten(plain_text_summary(module.doc_value())))
+        .map(|module| module.doc_value().map_or_else(|| String::new(), short_markdown_summary))
         .unwrap_or_default();
 
     #[derive(Serialize)]
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index de620a35c80..901f00b21da 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -76,7 +76,9 @@ use crate::html::format::fmt_impl_for_trait_page;
 use crate::html::format::Function;
 use crate::html::format::{href, print_default_space, print_generic_bounds, WhereClause};
 use crate::html::format::{print_abi_with_space, Buffer, PrintWithSpace};
-use crate::html::markdown::{self, ErrorCodes, IdMap, Markdown, MarkdownHtml, MarkdownSummaryLine};
+use crate::html::markdown::{
+    self, plain_text_summary, ErrorCodes, IdMap, Markdown, MarkdownHtml, MarkdownSummaryLine,
+};
 use crate::html::sources;
 use crate::html::{highlight, layout, static_files};
 use cache::{build_index, ExternalLocation};
@@ -1604,9 +1606,10 @@ impl Context {
                 Some(ref s) => s.to_string(),
             };
             let short = short.to_string();
-            map.entry(short)
-                .or_default()
-                .push((myname, Some(plain_text_summary(item.doc_value()))));
+            map.entry(short).or_default().push((
+                myname,
+                Some(item.doc_value().map_or_else(|| String::new(), plain_text_summary)),
+            ));
         }
 
         if self.shared.sort_modules_alphabetically {
@@ -1810,36 +1813,6 @@ fn full_path(cx: &Context, item: &clean::Item) -> String {
     s
 }
 
-/// Renders the first paragraph of the given markdown as plain text, making it suitable for
-/// contexts like alt-text or the search index.
-///
-/// If no markdown is supplied, the empty string is returned.
-///
-/// See [`markdown::plain_text_summary`] for further details.
-#[inline]
-crate fn plain_text_summary(s: Option<&str>) -> String {
-    s.map(markdown::plain_text_summary).unwrap_or_default()
-}
-
-crate fn shorten(s: String) -> String {
-    if s.chars().count() > 60 {
-        let mut len = 0;
-        let mut ret = s
-            .split_whitespace()
-            .take_while(|p| {
-                // + 1 for the added character after the word.
-                len += p.chars().count() + 1;
-                len < 60
-            })
-            .collect::<Vec<_>>()
-            .join(" ");
-        ret.push('…');
-        ret
-    } else {
-        s
-    }
-}
-
 fn document(w: &mut Buffer, cx: &Context, item: &clean::Item, parent: Option<&clean::Item>) {
     if let Some(ref name) = item.name {
         info!("Documenting {}", name);