Rollup merge of rust-lang#129426 - notriddle:smaller-index-2024-08-22, r=GuillaumeGomez

rustdoc-search: use tighter json for names and parents

File size
---------

```console
$ du -hs doc.old/search-index1.82.0.js doc/search-index1.82.0.js
3.2M    doc.old/search-index1.82.0.js
2.8M    doc/search-index1.82.0.js
$ gzip doc/search-index1.82.0.js
$ gzip doc.old/search-index1.82.0.js
$ du -hs doc.old/search-index1.82.0.js.gz doc/search-index1.82.0.js.gz
464K    doc.old/search-index1.82.0.js.gz
456K    doc/search-index1.82.0.js.gz
$ du -hs compiler-doc.old/search-index.js compiler-doc/search-index.js
8.5M    compiler-doc.old/search-index.js
6.5M    compiler-doc/search-index.js
$ gzip compiler-doc/search-index1.82.0.js
$ gzip compiler-doc.old/search-index1.82.0.js
$ du -hs compiler-doc.old/search-index.js.gz compiler-doc/search-index.js.gz
1.4M    compiler-doc.old/search-index.js.gz
1.4M    compiler-doc/search-index.js.gz
```

Performance
-----------

Firefox profile: [before](https://profiler.firefox.com/public/jf1741wycma0n38asdf7kdtw8egs0pqakbr03jg/calltree/?globalTrackOrder=0w3&implementation=js&thread=3&v=10), [after](https://profiler.firefox.com/public/p4fptad7vncsfgrgk9a18yx7m6w8kdpgfy15f8r/calltree/?globalTrackOrder=0w3&implementation=js&thread=3&v=10)

CLI profiler scripts comparison: https://notriddle.com/rustdoc-html-demo-9/smaller-index-2024-08-22/index.html

| Benchmark | Before     | After      | % Diff
| --------- | ----------:| ----------:| ------:
| arti      | 225692 KiB | 218744 KiB | 3%
| cortex-m  | 58276 KiB  | 57852 KiB  | 0%
| sqlx      | 123132 KiB | 125448 KiB | -2%
| stm32f4   | 556828 KiB | 548996 KiB | 1%
| ripgrep   | 86964 KiB  | 86180 KiB  | 1%

Author: GuillaumeGomez

src/librustdoc/html/render/search_index.rs

@@ -579,12 +579,14 @@ pub(crate) fn build_index<'tcx>(
             let mut names = Vec::with_capacity(self.items.len());
             let mut types = String::with_capacity(self.items.len());
             let mut full_paths = Vec::with_capacity(self.items.len());
-            let mut parents = Vec::with_capacity(self.items.len());
+            let mut parents = String::with_capacity(self.items.len());
+            let mut parents_backref_queue = VecDeque::new();
             let mut functions = String::with_capacity(self.items.len());
             let mut deprecated = Vec::with_capacity(self.items.len());
 
-            let mut backref_queue = VecDeque::new();
+            let mut type_backref_queue = VecDeque::new();
 
+            let mut last_name = None;
             for (index, item) in self.items.iter().enumerate() {
                 let n = item.ty as u8;
                 let c = char::try_from(n + b'A').expect("item types must fit in ASCII");
@@ -597,17 +599,39 @@ pub(crate) fn build_index<'tcx>(
                     "`{}` is missing idx",
                     item.name
                 );
-                // 0 is a sentinel, everything else is one-indexed
-                parents.push(item.parent_idx.map(|x| x + 1).unwrap_or(0));
+                assert!(
+                    parents_backref_queue.len() <= 16,
+                    "the string encoding only supports 16 slots of lookback"
+                );
+                let parent: i32 = item.parent_idx.map(|x| x + 1).unwrap_or(0).try_into().unwrap();
+                if let Some(idx) = parents_backref_queue.iter().position(|p: &i32| *p == parent) {
+                    parents.push(
+                        char::try_from('0' as u32 + u32::try_from(idx).unwrap())
+                            .expect("last possible value is '?'"),
+                    );
+                } else if parent == 0 {
+                    write_vlqhex_to_string(parent, &mut parents);
+                } else {
+                    parents_backref_queue.push_front(parent);
+                    write_vlqhex_to_string(parent, &mut parents);
+                    if parents_backref_queue.len() > 16 {
+                        parents_backref_queue.pop_back();
+                    }
+                }
 
-                names.push(item.name.as_str());
+                if Some(item.name.as_str()) == last_name {
+                    names.push("");
+                } else {
+                    names.push(item.name.as_str());
+                    last_name = Some(item.name.as_str());
+                }
 
                 if !item.path.is_empty() {
                     full_paths.push((index, &item.path));
                 }
 
                 match &item.search_type {
-                    Some(ty) => ty.write_to_string(&mut functions, &mut backref_queue),
+                    Some(ty) => ty.write_to_string(&mut functions, &mut type_backref_queue),
                     None => functions.push('`'),
                 }
 

src/librustdoc/html/static/js/search.js

@@ -3546,7 +3546,7 @@ ${item.displayPath}<span class="${type}">${name}</span>\
             // Used to de-duplicate inlined and re-exported stuff
             const itemReexports = new Map(crateCorpus.r);
             // an array of (Number) the parent path index + 1 to `paths`, or 0 if none
-            const itemParentIdxs = crateCorpus.i;
+            const itemParentIdxDecoder = new VlqHexDecoder(crateCorpus.i, noop => noop);
             // a map Number, string for impl disambiguators
             const implDisambiguator = new Map(crateCorpus.b);
             // an array of [(Number) item type,
@@ -3593,6 +3593,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
             // faster analysis operations
             lastPath = "";
             len = itemTypes.length;
+            let lastName = "";
+            let lastWord = "";
             for (let i = 0; i < len; ++i) {
                 const bitIndex = i + 1;
                 if (descIndex >= descShard.len &&
@@ -3608,10 +3610,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                     descIndex = 0;
                     descShardList.push(descShard);
                 }
-                let word = "";
-                if (typeof itemNames[i] === "string") {
-                    word = itemNames[i].toLowerCase();
-                }
+                const name = itemNames[i] === "" ? lastName : itemNames[i];
+                const word = itemNames[i] === "" ? lastWord : itemNames[i].toLowerCase();
                 const path = itemPaths.has(i) ? itemPaths.get(i) : lastPath;
                 const type = itemFunctionDecoder.next();
                 if (type !== null) {
@@ -3633,15 +3633,16 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 }
                 // This object should have exactly the same set of fields as the "crateRow"
                 // object defined above.
+                const itemParentIdx = itemParentIdxDecoder.next();
                 const row = {
                     crate,
                     ty: itemTypes.charCodeAt(i) - 65, // 65 = "A"
-                    name: itemNames[i],
+                    name,
                     path,
                     descShard,
                     descIndex,
                     exactPath: itemReexports.has(i) ? itemPaths.get(itemReexports.get(i)) : path,
-                    parent: itemParentIdxs[i] > 0 ? paths[itemParentIdxs[i] - 1] : undefined,
+                    parent: itemParentIdx > 0 ? paths[itemParentIdx - 1] : undefined,
                     type,
                     id,
                     word,
@@ -3655,6 +3656,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 if (!searchIndexEmptyDesc.get(crate).contains(bitIndex)) {
                     descIndex += 1;
                 }
+                lastName = name;
+                lastWord = word;
             }
 
             if (aliases) {