rustdoc: use a string with one-character codes for search index types

    $ wc -c search-index.old.js search-index.new.js
    3940530 search-index.old.js
    3843222 search-index.new.js

((3940530-3843222)/3940530)*100 = 2.47%

    $ wc -c search-index.old.js.gz search-index.new.js.gz
    380251 search-index.old.js.gz
    379434 search-index.new.js.gz

((380251-379434)/380251)*100 = 0.214%

1 files changed, 10 insertions, 1 deletions

diff --git a/src/librustdoc/html/render/search_index.rs b/src/librustdoc/html/render/search_index.rs
index 5b0caac099b..090ea2cb157 100644
--- a/src/librustdoc/html/render/search_index.rs
+++ b/src/librustdoc/html/render/search_index.rs
@@ -236,7 +236,16 @@ pub(crate) fn build_index<'tcx>(
             crate_data.serialize_field("doc", &self.doc)?;
             crate_data.serialize_field(
                 "t",
-                &self.items.iter().map(|item| &item.ty).collect::<Vec<_>>(),
+                &self
+                    .items
+                    .iter()
+                    .map(|item| {
+                        let n = item.ty as u8;
+                        let c = char::try_from(n + b'A').expect("item types must fit in ASCII");
+                        assert!(c <= 'z', "item types must fit within ASCII printables");
+                        c
+                    })
+                    .collect::<String>(),
             )?;
             crate_data.serialize_field(
                 "n",