How to suggest a feature to be added to rust compiler

What you should first do is write a detailed description of what you think the problem is, and why that's something that deserves to be addressed at all.

The default answer to all feature requests is "no, make a library", so the most important thing is to be persuasive that the problem is worth solving. If you get buy-in on that, then getting a fix for it is comparatively easy.

(The most common friction people hit is showing up with a solution, and then getting frustrated when most of the responses are "no, rust is fine, let's not" rather than engaging in details of the proposed solution.)