rustdoc-search: shard the search result descriptions

The descriptions are, on almost all crates[^1], the majority
of the size of the search index, even though they aren't really
used for searching. This makes it relatively easy to separate
them into their own files.

This commit also bumps us to ES8. Out of the browsers we support,
all of them support async functions according to caniuse.

https://caniuse.com/async-functions

[^1]:
    <https://microsoft.github.io/windows-docs-rs/>, a crate with
    44MiB of pure names and no descriptions for them, is an outlier
    and should not be counted.

1 files changed, 24 insertions, 4 deletions

diff --git a/src/librustdoc/html/static/js/main.js b/src/librustdoc/html/static/js/main.js
index b9a769a7c6d..2c1330614b8 100644
--- a/src/librustdoc/html/static/js/main.js
+++ b/src/librustdoc/html/static/js/main.js
@@ -329,6 +329,26 @@ function preLoadCss(cssUrl) {
             search.innerHTML = "<h3 class=\"search-loading\">" + searchState.loadingText + "</h3>";
             searchState.showResults(search);
         },
+        descShards: new Map(),
+        loadDesc: async function({descShard, descIndex}) {
+            if (descShard.promise === null) {
+                descShard.promise = new Promise((resolve, reject) => {
+                    descShard.resolve = resolve;
+                    const ds = descShard;
+                    const fname = `${ds.crate}-desc-${ds.shard}-`;
+                    const url = resourcePath(
+                        `search.desc/${descShard.crate}/${fname}`,
+                        ".js",
+                    );
+                    loadScript(url, reject);
+                });
+            }
+            const list = await descShard.promise;
+            return list[descIndex];
+        },
+        loadedDescShard: function (crate, shard, data) {
+            this.descShards.get(crate)[shard].resolve(data.split("\n"));
+        },
     };
 
     const toggleAllDocsId = "toggle-all-docs";
@@ -381,7 +401,7 @@ function preLoadCss(cssUrl) {
                                     window.location.replace("#" + item.id);
                                 }, 0);
                             }
-                        }
+                        },
                     );
                 }
             }
@@ -585,7 +605,7 @@ function preLoadCss(cssUrl) {
         const script = document
             .querySelector("script[data-ignore-extern-crates]");
         const ignoreExternCrates = new Set(
-            (script ? script.getAttribute("data-ignore-extern-crates") : "").split(",")
+            (script ? script.getAttribute("data-ignore-extern-crates") : "").split(","),
         );
         for (const lib of libs) {
             if (lib === window.currentCrate || ignoreExternCrates.has(lib)) {
@@ -1098,7 +1118,7 @@ function preLoadCss(cssUrl) {
         } else {
             wrapper.style.setProperty(
                 "--popover-arrow-offset",
-                (wrapperPos.right - pos.right + 4) + "px"
+                (wrapperPos.right - pos.right + 4) + "px",
             );
         }
         wrapper.style.visibility = "";
@@ -1680,7 +1700,7 @@ href="https://doc.rust-lang.org/${channel}/rustdoc/read-documentation/search.htm
                 pendingSidebarResizingFrame = false;
                 document.documentElement.style.setProperty(
                     "--resizing-sidebar-width",
-                    desiredSidebarSize + "px"
+                    desiredSidebarSize + "px",
                 );
             }, 100);
         }