When you would want it

Otherwise the message lives in a panel at the bottom, or inside a tooltip you have to hover to see. Both mean looking away from the line you are fixing.

Where to find it

All Features → Inline error and warning lens
Turn on or off
Settings

What you get out of it

You find out a line is broken while your eyes are still on it.
No hover-hunting and no panel-diving for the one-line answer.

How to use it, step by step

Turn it on in Editing settings - it is off by default to keep the editor quiet.
Write code - errors and warnings render their message at the end of the offending line.
The text is dimmed and truncated so code always wins the space.
The lens hides on the line you are typing on, and returns when you move away.
Hover, or open the Problems panel, for the full detail.

Why this beats the usual way

A squiggle demands a hover; a panel demands a click. Putting the message at the end of the line costs zero interaction - the error announces itself where you will fix it.

Find it in Universal Search

Inline error and warning lens · Editing