vscode-extension-samples/multi-diagnostics-sample/src/extension.ts

'use strict';
import * as vscode from 'vscode';
import * as path from 'path';

export function activate(context: vscode.ExtensionContext) {

    const collection = vscode.languages.createDiagnosticCollection('test');
    if (vscode.window.activeTextEditor) {
        updateDiagnostics(vscode.window.activeTextEditor.document, collection);
    }
    context.subscriptions.push(vscode.window.onDidChangeActiveTextEditor(e => updateDiagnostics(e.document, collection)));

}

function updateDiagnostics(document: vscode.TextDocument, collection: vscode.DiagnosticCollection): void {
    if (document && path.basename(document.uri.fsPath) === 'sample-demo.rs') {
        collection.set(document.uri, [{
            code: '',
            message: 'cannot assign twice to immutable variable `x`',
            range: new vscode.Range(new vscode.Position(3, 4), new vscode.Position(3, 10)),
            severity: vscode.DiagnosticSeverity.Error,
            source: '',
            relatedInformation: [
                new vscode.DiagnosticRelatedInformation(new vscode.Location(document.uri, new vscode.Range(new vscode.Position(1, 8), new vscode.Position(1, 9))), 'first assignment to `x`')
            ]
        }]);
    } else {
        collection.clear();
    }
}

// this method is called when your extension is deactivated
export function deactivate() {
}