Diagnostics and Fix-Its
In order to show diagnostics in the editor, you need to implemenet an Ide.DiagnosticProvider and override two methods: do_diagnose_async and do_diagnose_finish.
The do_diagnose_async is an asynchronous method that will be called with a callback as the fifth parameter. The callback can be passed to a Gio.Task for easy handling. When the task is done, do_diagnose_finish will be called with the Gio.Task object and is expected to return an Ide.Diagnostics object.
# my_plugin.py
import gi
from gi.repository import GLib, Gio, Ide
class MyDiagnosticProvider(Ide.Object, Ide.DiagnosticProvider):
    def do_diagnose_async(self,
                          file: Ide.File,
                          contents: bytes,
                          lang_id: str,
                          cancellable: Gio.Cancellable,
                          callback: Gio.AsyncReadyCallback,
                          user_data):
        task = Gio.Task.new(self, cancellable, callback)
        task.diagnostics_list = []
        start = Ide.SourceLocation.new(file, 0, 0, 0)
        severity = Ide.DiagnosticSeverity.WARNING
        error_message = 'Diagnostic example'
        diagnostic = Ide.Diagnostic.new(severity, error_message, start)
        task.diagnostics_list.append(diagnostic)
        task.return_boolean(True)
    def do_diagnose_finish(self, result: Gio.Task) -> Ide.Diagnostics:
        if result.propagate_boolean():
            return Ide.Diagnostics.new(result.diagnostics_list)
You also need to register the plugin as a diagnostic provider in the .plugin file. There is a X-Diagnostic-Provider-Languages field which specify the supported languages and X-Diagnostic-Provider-Languages-Priority which specify the diagnostic priority.
For example, a C diagnostic plugin will have a plugin file that look similar to this:
# my_plugin.plugin
[Plugin]
Module=my_plugin
Name=my_plugin
Loader=python3
Description=Provides C diagnostics
Authors=Author Name <authorname@mailprovider.com>
Copyright=Copyright © 2017 Author Name <authorname@mailprovider.com>
X-Diagnostic-Provider-Languages=c
X-Diagnostic-Provider-Languages-Priority=100
X-Builder-ABI=3.32