* Adding scope mapping configuration manifest in `package.json`

* Loading configurable scope mappings from settings.
* Updating Readme with `rust-analyzer.scopeMappings`.
`rust-analyzer.scopeMappings` -- a scheme backed JSON object to tweak Rust Analyzer scopes to TextMate scopes.
   ```jsonc
    {
        //Will autocomplete keys to available RA scopes.
        "keyword.unsafe": ["keyword", "keyword.control"],
        //Values are string | TextMateScope | [string | TextMateScope]
        "comments": "comment.block"
    }
   ```

4 files changed, 77 insertions, 65 deletions

diff --git a/editors/code/src/config.ts b/editors/code/src/config.ts
index 12823f319..234a390ac 100644
--- a/editors/code/src/config.ts
+++ b/editors/code/src/config.ts
@@ -48,11 +48,13 @@ export class Config {
         const config = vscode.workspace.getConfiguration('rust-analyzer');
         
         Server.highlighter.removeHighlights();
-        scopes.load()
-        scopesMapper.load()
-        if (config.has('highlightingOn')) {
 
+        if (config.has('highlightingOn')) {
             this.highlightingOn = config.get('highlightingOn') as boolean;
+            if (this.highlightingOn) {
+                scopes.load();
+                scopesMapper.load();    
+            }
         }
 
         if (config.has('rainbowHighlightingOn')) {
@@ -61,9 +63,6 @@ export class Config {
             ) as boolean;
         }
 
-        if (!this.highlightingOn && Server) {
-            Server.highlighter.removeHighlights();
-        }
 
         if (config.has('enableEnhancedTyping')) {
             this.enableEnhancedTyping = config.get(
diff --git a/editors/code/src/highlighting.ts b/editors/code/src/highlighting.ts
index dad99254e..1c67e5dc3 100644
--- a/editors/code/src/highlighting.ts
+++ b/editors/code/src/highlighting.ts
@@ -1,7 +1,7 @@
 import seedrandom = require('seedrandom');
 import * as vscode from 'vscode';
 import * as lc from 'vscode-languageclient';
-import * as scopes from './scopes'
+import * as scopes from './scopes';
 import * as scopesMapper from './scopes_mapper';
 
 import { Server } from './server';
@@ -25,35 +25,35 @@ function fancify(seed: string, shade: 'light' | 'dark') {
     return `hsl(${h},${s}%,${l}%)`;
 }
 
+
 function createDecorationFromTextmate(themeStyle: scopes.TextMateRuleSettings): vscode.TextEditorDecorationType {
-    const options: vscode.DecorationRenderOptions = {}
-    options.rangeBehavior = vscode.DecorationRangeBehavior.OpenOpen
+    const options: vscode.DecorationRenderOptions = {};
+    options.rangeBehavior = vscode.DecorationRangeBehavior.OpenOpen;
     if (themeStyle.foreground) {
-        options.color = themeStyle.foreground
+        options.color = themeStyle.foreground;
     }
     if (themeStyle.background) {
-        options.backgroundColor = themeStyle.background
+        options.backgroundColor = themeStyle.background;
     }
     if (themeStyle.fontStyle) {
-        const parts: string[] = themeStyle.fontStyle.split(' ')
+        const parts: string[] = themeStyle.fontStyle.split(' ');
         parts.forEach((part) => {
             switch (part) {
                 case 'italic':
-                    options.fontStyle = 'italic'
-                    break
+                    options.fontStyle = 'italic';
+                    break;
                 case 'bold':
-                    options.fontWeight = 'bold'
-
-                    break
+                    options.fontWeight = 'bold';
+                    break;
                 case 'underline':
-                    options.textDecoration = 'underline'
-                    break
+                    options.textDecoration = 'underline';
+                    break;
                 default:
-                    break
+                    break;
             }
         })
     }
-    return vscode.window.createTextEditorDecorationType(options)
+    return vscode.window.createTextEditorDecorationType(options);
 }
 
 export class Highlighter {
@@ -66,7 +66,7 @@ export class Highlighter {
             textDecoration?: string
         ): [string, vscode.TextEditorDecorationType] => {
 
-            const rule = scopesMapper.toRule(tag, scopes.find)
+            const rule = scopesMapper.toRule(tag, scopes.find);
 
             if (rule) {
                 const decor = createDecorationFromTextmate(rule);
diff --git a/editors/code/src/scopes.ts b/editors/code/src/scopes.ts
index 5d4395930..a6138fad0 100644
--- a/editors/code/src/scopes.ts
+++ b/editors/code/src/scopes.ts
@@ -1,41 +1,41 @@
-import * as fs from 'fs'
-import * as path from 'path'
-import * as vscode from 'vscode'
+import * as fs from 'fs';
+import * as path from 'path';
+import * as vscode from 'vscode';
 
 
 
 export interface TextMateRule {
-    scope: string | string[]
-    settings: TextMateRuleSettings
+    scope: string | string[];
+    settings: TextMateRuleSettings;
 }
 
 export interface TextMateRuleSettings {
-    foreground: string | undefined
-    background: string | undefined
-    fontStyle: string | undefined
+    foreground: string | undefined;
+    background: string | undefined;
+    fontStyle: string | undefined;
 }
 
 // Current theme colors
-const rules = new Map<string, TextMateRuleSettings>()
+const rules = new Map<string, TextMateRuleSettings>();
 
 export function find(scope: string): TextMateRuleSettings | undefined {
-    return rules.get(scope)
+    return rules.get(scope);
 }
 
 // Load all textmate scopes in the currently active theme
 export function load() {
     // Remove any previous theme
-    rules.clear()
+    rules.clear();
     // Find out current color theme
-    const themeName = vscode.workspace.getConfiguration('workbench').get('colorTheme')
+    const themeName = vscode.workspace.getConfiguration('workbench').get('colorTheme');
 
     if (typeof themeName !== 'string') {
         // console.warn('workbench.colorTheme is', themeName)
-        return
+        return;
     }
     // Try to load colors from that theme
     try {
-        loadThemeNamed(themeName)
+        loadThemeNamed(themeName);
     } catch (e) {
         // console.warn('failed to load theme', themeName, e)
     }
@@ -44,7 +44,7 @@ export function load() {
 function filterThemeExtensions(extension: vscode.Extension<any>): boolean {
     return extension.extensionKind === vscode.ExtensionKind.UI &&
         extension.packageJSON.contributes &&
-        extension.packageJSON.contributes.themes
+        extension.packageJSON.contributes.themes;
 }
 
 
@@ -59,17 +59,17 @@ function loadThemeNamed(themeName: string) {
                 .filter((element: any) => (element.id || element.label) === themeName)
                 .map((element: any) => path.join(extension.extensionPath, element.path))
                 .concat(list)
-        }, Array<string>())
+        }, Array<string>());
 
 
-    themePaths.forEach(loadThemeFile)
+    themePaths.forEach(loadThemeFile);
 
     const tokenColorCustomizations: [any] = [vscode.workspace.getConfiguration('editor').get('tokenColorCustomizations')]
 
     tokenColorCustomizations
         .filter(custom => custom && custom.textMateRules)
         .map(custom => custom.textMateRules)
-        .forEach(loadColors)
+        .forEach(loadColors);
 
 }
 
@@ -79,26 +79,26 @@ function loadThemeFile(themePath: string) {
         .filter(isFile)
         .map(readFileText)
         .map(parseJSON)
-        .filter(theme => theme)
+        .filter(theme => theme);
 
     themeContent
         .filter(theme => theme.tokenColors)
         .map(theme => theme.tokenColors)
-        .forEach(loadColors)
+        .forEach(loadColors);
 
     themeContent
         .filter(theme => theme.include)
         .map(theme => path.join(path.dirname(themePath), theme.include))
-        .forEach(loadThemeFile)
+        .forEach(loadThemeFile);
 }
 
 function mergeRuleSettings(defaultSetting: TextMateRuleSettings | undefined, override: TextMateRuleSettings): TextMateRuleSettings {
-    if (defaultSetting === undefined) { return override }
-    const mergedRule = defaultSetting
+    if (defaultSetting === undefined) { return override; }
+    const mergedRule = defaultSetting;
 
-    mergedRule.background = override.background || defaultSetting.background
-    mergedRule.foreground = override.foreground || defaultSetting.foreground
-    mergedRule.fontStyle = override.fontStyle || defaultSetting.foreground
+    mergedRule.background = override.background || defaultSetting.background;
+    mergedRule.foreground = override.foreground || defaultSetting.foreground;
+    mergedRule.fontStyle = override.fontStyle || defaultSetting.foreground;
 
     return mergedRule
 }
@@ -106,29 +106,29 @@ function mergeRuleSettings(defaultSetting: TextMateRuleSettings | undefined, ove
 function updateRules(scope: string, updatedSettings: TextMateRuleSettings): void {
     [rules.get(scope)]
         .map(settings => mergeRuleSettings(settings, updatedSettings))
-        .forEach(settings => rules.set(scope, settings))
+        .forEach(settings => rules.set(scope, settings));
 }
 
 function loadColors(textMateRules: TextMateRule[]): void {
     textMateRules.forEach(rule => {
         if (typeof rule.scope === 'string') {
-            updateRules(rule.scope, rule.settings)
+            updateRules(rule.scope, rule.settings);
         }
         else if (rule.scope instanceof Array) {
-            rule.scope.forEach(scope => updateRules(scope, rule.settings))
+            rule.scope.forEach(scope => updateRules(scope, rule.settings));
         }
     })
 }
 
 function isFile(filePath: string): boolean {
-    return [filePath].map(fs.statSync).every(stat => stat.isFile())
+    return [filePath].map(fs.statSync).every(stat => stat.isFile());
 }
 
 function readFileText(filePath: string): string {
-    return fs.readFileSync(filePath, 'utf8')
+    return fs.readFileSync(filePath, 'utf8');
 }
 
 // Might need to replace with JSONC if a theme contains comments. 
 function parseJSON(content: string): any {
-    return JSON.parse(content)
+    return JSON.parse(content);
 }
\ No newline at end of file
diff --git a/editors/code/src/scopes_mapper.ts b/editors/code/src/scopes_mapper.ts
index 19a4213d4..7056a8e24 100644
--- a/editors/code/src/scopes_mapper.ts
+++ b/editors/code/src/scopes_mapper.ts
@@ -1,10 +1,9 @@
-import * as vscode from 'vscode'
-import { TextMateRuleSettings } from './scopes'
+import * as vscode from 'vscode';
+import { TextMateRuleSettings } from './scopes';
 
 
 
-
-let mappings = new Map<string, string[]>()
+let mappings = new Map<string, string[]>();
 
 
 const defaultMapping = new Map<string, string[]>([
@@ -27,25 +26,39 @@ const defaultMapping = new Map<string, string[]>([
     ['field', ['variable.object.property', 'meta.field.declaration', 'meta.definition.property', 'variable.other',]],
     ['module', ['entity.name.section', 'entity.other']]
 ]
-)
+);
 
 // Temporary exported for debugging for now. 
 export function find(scope: string): string[] {
-    return mappings.get(scope) || []
+    return mappings.get(scope) || [];
 }
 
 export function toRule(scope: string, intoRule: (scope: string) => TextMateRuleSettings | undefined): TextMateRuleSettings | undefined {
-    return find(scope).map(intoRule).filter(rule => rule !== undefined)[0]
+    return find(scope).map(intoRule).filter(rule => rule !== undefined)[0];
+}
+
+
+function isString(value: any): value is string {
+    return typeof value === 'string';
+}
+
+function isArrayOfString(value: any): value is string[] {
+    return Array.isArray(value) && value.every(item => isString(item));
 }
 
 
 export function load() {
-    const configuration = vscode.workspace
+    const rawConfig: { [key: string]: any } = vscode.workspace
         .getConfiguration('rust-analyzer')
-        .get('scopeMappings') as Map<string, string[]> | undefined
-        || new Map()
+        .get('scopeMappings')
+        || {};
 
-    mappings = new Map([...Array.from(defaultMapping.entries()), ...Array.from(configuration.entries())])
+    mappings = Object
+        .entries(rawConfig)
+        .filter(([_, value]) => isString(value) || isArrayOfString(value))
+        .reduce((list, [key, value]: [string, string | string[]]) => {
+            return list.set(key, isString(value) ? [value] : value);
 
+        }, defaultMapping);
 
 }
\ No newline at end of file