rustdoc: simplify JS search routine by not messing with lev distance

Since the sorting function accounts for an `index` field, there's not much
reason to also be applying changes to the levenshtein distance. Instead,
we can just not treat `lev` as a filter if there's already a non-sentinel
value for `index`.

This change gives slightly more weight to the index and path part, as
search criteria, than it used to. This changes some of the test cases,
but not in any obviously-"worse" way, and, in particular, substring matches
are a bigger deal than levenshtein distances (we're assuming that a typo
is less likely than someone just not typing the entire name).

Based on
#103710 (comment)

Author: notriddle

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

@@ -781,7 +781,29 @@ function initSearch(rawSearchIndex) {
                     return a - b;
                 }
 
-                // Sort by non levenshtein results and then levenshtein results by the distance
+                // sort by index of keyword in item name (no literal occurrence goes later)
+                a = (aaa.index < 0);
+                b = (bbb.index < 0);
+                if (a !== b) {
+                    return a - b;
+                }
+
+                // Sort by distance in the path part, if specified
+                // (less changes required to match means higher rankings)
+                a = aaa.path_lev;
+                b = bbb.path_lev;
+                if (a !== b) {
+                    return a - b;
+                }
+
+                // (later literal occurrence, if any, goes later)
+                a = aaa.index;
+                b = bbb.index;
+                if (a !== b) {
+                    return a - b;
+                }
+
+                // Sort by distance in the name part, the last part of the path
                 // (less changes required to match means higher rankings)
                 a = (aaa.lev);
                 b = (bbb.lev);
@@ -810,19 +832,6 @@ function initSearch(rawSearchIndex) {
                     return (a > b ? +1 : -1);
                 }
 
-                // sort by index of keyword in item name (no literal occurrence goes later)
-                a = (aaa.index < 0);
-                b = (bbb.index < 0);
-                if (a !== b) {
-                    return a - b;
-                }
-                // (later literal occurrence, if any, goes later)
-                a = aaa.index;
-                b = bbb.index;
-                if (a !== b) {
-                    return a - b;
-                }
-
                 // special precedence for primitive and keyword pages
                 if ((aaa.item.ty === TY_PRIMITIVE && bbb.item.ty !== TY_KEYWORD) ||
                     (aaa.item.ty === TY_KEYWORD && bbb.item.ty !== TY_PRIMITIVE)) {
@@ -1230,15 +1239,19 @@ function initSearch(rawSearchIndex) {
          * * `id` is the index in both `searchWords` and `searchIndex` arrays for this element.
          * * `index` is an `integer`` used to sort by the position of the word in the item's name.
          * * `lev` is the main metric used to sort the search results.
+         * * `path_lev` is zero if a single-component search query is used, otherwise it's the
+         *   distance computed for everything other than the last path component.
          *
          * @param {Results} results
          * @param {string} fullId
          * @param {integer} id
          * @param {integer} index
          * @param {integer} lev
+         * @param {integer} path_lev
          */
-        function addIntoResults(results, fullId, id, index, lev) {
-            if (lev === 0 || (!parsedQuery.literalSearch && lev <= MAX_LEV_DISTANCE)) {
+        function addIntoResults(results, fullId, id, index, lev, path_lev) {
+            const inBounds = lev <= MAX_LEV_DISTANCE || index !== -1;
+            if (lev === 0 || (!parsedQuery.literalSearch && inBounds)) {
                 if (results[fullId] !== undefined) {
                     const result = results[fullId];
                     if (result.dontValidate || result.lev <= lev) {
@@ -1250,6 +1263,7 @@ function initSearch(rawSearchIndex) {
                     index: index,
                     dontValidate: parsedQuery.literalSearch,
                     lev: lev,
+                    path_lev: path_lev,
                 };
             }
         }
@@ -1280,68 +1294,68 @@ function initSearch(rawSearchIndex) {
             if (!row || (filterCrates !== null && row.crate !== filterCrates)) {
                 return;
             }
-            let lev, lev_add = 0, index = -1;
+            let lev, index = -1, path_lev = 0;
             const fullId = row.id;
+            const searchWord = searchWords[pos];
 
             const in_args = findArg(row, elem, parsedQuery.typeFilter);
             const returned = checkReturned(row, elem, parsedQuery.typeFilter);
 
-            addIntoResults(results_in_args, fullId, pos, index, in_args);
-            addIntoResults(results_returned, fullId, pos, index, returned);
+            // path_lev is 0 because no parent path information is currently stored
+            // in the search index
+            addIntoResults(results_in_args, fullId, pos, -1, in_args, 0);
+            addIntoResults(results_returned, fullId, pos, -1, returned, 0);
 
             if (!typePassesFilter(parsedQuery.typeFilter, row.ty)) {
                 return;
             }
-            const searchWord = searchWords[pos];
 
-            if (parsedQuery.literalSearch) {
-                if (searchWord === elem.name) {
-                    addIntoResults(results_others, fullId, pos, -1, 0);
-                }
-                return;
+            const row_index = row.normalizedName.indexOf(elem.pathLast);
+            const word_index = searchWord.indexOf(elem.pathLast);
+
+            // lower indexes are "better" matches
+            // rank based on the "best" match
+            if (row_index === -1) {
+                index = word_index;
+            } else if (word_index === -1) {
+                index = row_index;
+            } else if (word_index < row_index) {
+                index = word_index;
+            } else {
+                index = row_index;
             }
 
             // No need to check anything else if it's a "pure" generics search.
             if (elem.name.length === 0) {
                 if (row.type !== null) {
                     lev = checkGenerics(row.type, elem, MAX_LEV_DISTANCE + 1);
-                    addIntoResults(results_others, fullId, pos, index, lev);
+                    // path_lev is 0 because we know it's empty
+                    addIntoResults(results_others, fullId, pos, index, lev, 0);
                 }
                 return;
             }
 
             if (elem.fullPath.length > 1) {
-                lev = checkPath(elem.pathWithoutLast, row);
-                if (lev > MAX_LEV_DISTANCE || (parsedQuery.literalSearch && lev !== 0)) {
+                path_lev = checkPath(elem.pathWithoutLast, row);
+                if (path_lev > MAX_LEV_DISTANCE) {
                     return;
-                } else if (lev > 0) {
-                    lev_add = lev / 10;
                 }
             }
 
-            if (searchWord.indexOf(elem.pathLast) > -1 ||
-                row.normalizedName.indexOf(elem.pathLast) > -1
-            ) {
-                index = row.normalizedName.indexOf(elem.pathLast);
-            }
-            lev = levenshtein(searchWord, elem.pathLast);
-            if (lev > 0 && elem.pathLast.length > 2 && searchWord.indexOf(elem.pathLast) > -1) {
-                if (elem.pathLast.length < 6) {
-                    lev = 1;
-                } else {
-                    lev = 0;
+            if (parsedQuery.literalSearch) {
+                if (searchWord === elem.name) {
+                    addIntoResults(results_others, fullId, pos, index, 0, path_lev);
                 }
-            }
-            lev += lev_add;
-            if (lev > MAX_LEV_DISTANCE) {
                 return;
-            } else if (index !== -1 && elem.fullPath.length < 2) {
-                lev -= 1;
             }
-            if (lev < 0) {
-                lev = 0;
+
+            lev = levenshtein(searchWord, elem.pathLast);
+
+            if (index === -1 && lev + path_lev > MAX_LEV_DISTANCE) {
+                return;
             }
-            addIntoResults(results_others, fullId, pos, index, lev);
+
+            addIntoResults(results_others, fullId, pos, index, lev, path_lev);
         }
 
         /**
@@ -1386,7 +1400,7 @@ function initSearch(rawSearchIndex) {
                 return;
             }
             const lev = Math.round(totalLev / nbLev);
-            addIntoResults(results, row.id, pos, 0, lev);
+            addIntoResults(results, row.id, pos, 0, lev, 0);
         }
 
         function innerRunQuery() {