In addition to the other comment about the exit code, you might be interested in the exitcode crate, which offers up a BSD convention for those exit codes.
They are, essentially, just numbers on unixes and don't really have as much standardization as e.g. HTTP codes afaik. Various programs may have their own local conventions as to what an exit code means.
ExitCodeis a struct, therefore it behaves like a type with many fields which define the types contained in the struct.
That's a bit too off. structs in rust are product types. A struct may define zero or more fields. And fields can be named or not. if not, such structs are called tuple structs.
In the doc page, if you clicked on source, it would have taken you to the definition.
pub struct ExitCode(imp::ExitCode);
That's a public struct with one unnamed private field. The type of the private field also happens to be private/internal.
As for why, usually the purpose is providing type safety, a unified interface,... etc. Notice how for example a windows-only extension trait is implemented that allows converting raw u32 exit codes into ExitCode.
So now you have exit codes possibly sourced from u8 or u32 values depending on the platform. And you need a safe unified interface to represent them.
I hope that's an enough starting point.
Here's the source of the ExitCode implementation for Unix: https://doc.rust-lang.org/src/std/sys/pal/unix/process/process_common.rs.html#622
I'm a seasoned Rust dev and this wasn't easy for me to find. The std lib can be somewhat complex, because it needs to cover all possible use-cases...
this wasn’t easy for me to find
Huh! This is the internal ExitCode, and it's two jump-to-definition calls away. The first to get to the public type definition, and the second from the public type's struct field to the private type.
If I had downloaded the source code and opened it in my editor, then it probably would've lead me there pretty easily, yeah. But you won't normally download the source code for libraries you're looking to use. Trying to navigate there via the imports and module structure is certainly a lot more difficult, because they seem to do various re-exports, in particular for the different implementations for different operating systems...
Another huh from me. Or maybe I'm missing something, because this should all be obvious.
The source of the standard library is a rustup component that everyone should have installed. As for crates, you don't have to manually download any sources. Just add the crate as a dependency, and jump around definitions as much as you like. You can remove the dependency later if you decide to not use it.
Alright, yeah, I guess, I could've opened one of my Rust projects, written down use std::process::Termination; and jumped in that way. Admittedly, I didn't think of that, but it also sounded like OP wants to learn how to read the webpage documentation...