Document what `Default` does for each type in libstd

[bluss]

Document what Default does for each type in libstd.

For example, for the Default impl for i32, mention that it returns 0. For slice, mention it returns an empty slice.

[cbreeden]

I'm thinking about where a good place would be to document this behavior. I'm leaning towards documenting these behaviors in the documents for each individual type, as opposed to in the docs for Default.

[durka]

It would be nice if the comments were near the Default impl so they don't become stale (obviously they won't for primitive types, but this would be a good convention for user types as well). It could be a good addition to the "built-in traits section" that's been occasionally suggested for Rustdoc. Somewhere near the top it would say, e.g.:

• This type is not Copy.
• This type is Clone-able.
• This type is not Send nor Sync.
• This type has a Default and it is 0.

And it would somehow gather this information from comments on the impl.

[bluss]

@cbreeden I was thinking on the Default-impl for each type. For example for a slice, it would show up on this impl or on this method (depending on what works right now). https://doc.rust-lang.org/std/primitive.slice.html#method.default

You can see that the Index impls for slice have specific documentation for the slice itself.

[cbreeden]

Interesting. I gave it a little test run to see how it would look:

The fact that we are missing the [Read More] originally present doesn't bother me much since it's the same link as if you were to click the default function itself. I like it (modulo a rewording if other people felt as such).

[athulappadan]

I would like to take up this task and would like to get an explanation of how to go about it.

[GuillaumeGomez]

@athulappadan: Find all the Default implementations and explain in the doc comments what it's doing. For example:

impl Default for String {
    /// Create an empty `String`.
    fn default() -> String {
        String::new() // I guess it's doing this
    }
}

So in the example above, the thing you need to add is "Create an empty String."

[bluss]

@athulappadan Don't miss this document when starting out. And feel free to ask questions https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md

[matthew-piziak]

@athulappadan Any update on this issue? If not I might like to work on it.

[athulappadan]

I was a bit busy. Will submit a PR tomorrow.