Add unused diagnostic subtype (#49646)

* Add unused diagnostic subtype

Fixes #15710

Adds a new `DiagnosticTag` class that provide additional information about a diagnostic. Introduce `DiagnosticTag.Unnecessary` to mark when a diagnostic is for unused / unnecessary code

The design comes from Rosyln's diagnostic object and allows us to modify how a diagnostic is rendered without changing its serverity.

Hooks up JS and TS to use this new tag. This is controlled by the `javascript.showUnused.enabled` setting which is enabled by default

- Introduce a new diagnostic severity for unused.

    However, using this approach, if a user sets `noUnusedLocals` in their `tsconfig.json`, the resulting diagnostic could only show the squiggly OR be grayed out. Using `customTags` allows us to support both graying out and showing the squiggly

- Custom JS/TS implementation using decorators

    Not themable. We want a standard experience across languages.

* - Move to proposed
- Use numeric enum

src/vs/workbench/api/node/extHost.api.impl.ts

@@ -677,6 +677,7 @@ export function createApiFactory(
 			DebugAdapterExecutable: extHostTypes.DebugAdapterExecutable,
 			Diagnostic: extHostTypes.Diagnostic,
 			DiagnosticRelatedInformation: extHostTypes.DiagnosticRelatedInformation,
+			DiagnosticTag: extHostTypes.DiagnosticTag,
 			DiagnosticSeverity: extHostTypes.DiagnosticSeverity,
 			Disposable: extHostTypes.Disposable,
 			DocumentHighlight: extHostTypes.DocumentHighlight,

src/vs/workbench/api/node/extHostTypeConverters.ts

@@ -20,7 +20,7 @@ import * as htmlContent from 'vs/base/common/htmlContent';
 import { IRelativePattern } from 'vs/base/common/glob';
 import * as languageSelector from 'vs/editor/common/modes/languageSelector';
 import { WorkspaceEditDto, ResourceTextEditDto } from 'vs/workbench/api/node/extHost.protocol';
-import { MarkerSeverity, IRelatedInformation, IMarkerData } from 'vs/platform/markers/common/markers';
+import { MarkerSeverity, IRelatedInformation, IMarkerData, MarkerTag } from 'vs/platform/markers/common/markers';
 
 export interface PositionLike {
 	line: number;
@@ -88,6 +88,16 @@ export namespace Position {
 	}
 }
 
+export namespace DiagnosticTag {
+	export function from(value: vscode.DiagnosticTag): MarkerTag {
+		switch (value) {
+			case types.DiagnosticTag.Unnecessary:
+				return MarkerTag.Unnecessary;
+		}
+		return undefined;
+	}
+}
+
 export namespace Diagnostic {
 	export function from(value: vscode.Diagnostic): IMarkerData {
 		return {
@@ -96,7 +106,8 @@ export namespace Diagnostic {
 			source: value.source,
 			code: String(value.code),
 			severity: DiagnosticSeverity.from(value.severity),
-			relatedInformation: value.relatedInformation && value.relatedInformation.map(DiagnosticRelatedInformation.from)
+			relatedInformation: value.relatedInformation && value.relatedInformation.map(DiagnosticRelatedInformation.from),
+			customTags: Array.isArray(value.customTags) ? value.customTags.map(DiagnosticTag.from) : undefined,
 		};
 	}
 }

src/vs/workbench/api/node/extHostTypes.ts

@@ -673,6 +673,10 @@ export class SnippetString {
 	}
 }
 
+export enum DiagnosticTag {
+	Unnecessary = 1,
+}
+
 export enum DiagnosticSeverity {
 	Hint = 3,
 	Information = 2,
@@ -747,6 +751,7 @@ export class Diagnostic {
 	code: string | number;
 	severity: DiagnosticSeverity;
 	relatedInformation: DiagnosticRelatedInformation[];
+	customTags?: DiagnosticTag[];
 
 	constructor(range: Range, message: string, severity: DiagnosticSeverity = DiagnosticSeverity.Error) {
 		this.range = range;