1. Why does it say where T: Sized for references &T? A reference can definitely point to an unsized type, e.g. &str.

I think the point being made is that the layout shown only applies for Sized T. Layouts for &[T] and &dyn Trait are shown elsewhere on the sheet. &str is noted under &[T].

Edit: although, similar considerations would apply to other pointer types, but that isn’t noted on the sheet except for Box<[T]>

I’m glad my points we’re helpful!

There is some documentation on examples in the Cargo book. The basic procedure is to put it in an examples directory alongside the src directory (and in its own subfolder if it has multiple files), and you can add an entry for it in the Cargo.toml (although it should automatically detect it if you put it in the examples directory, so that is only strictly necessary if you want to change the default settings).

A few things I noticed:

• In http::request::parse(), do you actually need a BufReader? It would be better to make it generic over something implementing BufRead, that allows what you have but also makes tests and examples easier since you wouldn’t have to open a TCP connection just to do something that is essentially string parsing.
• In http::response::Response::to_string(), that match on lines 78-85 makes me uneasy, because you are silently changing the status code if it isn’t one you recognise. It would be better to signal an error. It would be even better to just check when the status code is set (perhaps with a status code enum to list the ones you support, since what you have isn’t all the defined codes) so that you can’t fail when converting to a string.
• Consider whether you need a special to_string() method at all, or whether you can just implement Display (which gives you to_string() for free via the ToString trait).
• You are using String as an error type pretty much everywhere. The better approach is to create an enum representing all the possible errors, so that a user of your library can match against them. Make the enum implement Error and Display and it will fit fine into the rest of the error handling infrastructure. There are crates like thiserror that can reduce the boilerplate around this.
• You have an io.rs that doesn’t appear to be connected to anything.
• You have a main.rs, which seems off in something that sounds like it should be purely a library crate. You probably want that to be an example or an integration test instead.

That’s all I could see with a quick look. In terms of general advice: remember to look at warnings, run cargo clippy, and look at the API guidelines.

The Rust book is the official standard resource for learning Rust. It assumes some experience in another programming language, but it doesn’t matter which one (that is, it isn’t aimed at teaching Rust as a first programming language), so there’s no reason why you couldn’t read it with some experience in C.

Depending on what sort of C programmer you are, you may find Learn Rust the Dangerous Way of interest as well.